Preventing a GFCI from tripping on transmitted RF

Figure 1:
The installed GFCI (and chokes) in the
utility room/laundry room.
Click on the image for a larger version.

Several months ago I had need to rewire an outlet in my kitchen:  Rather than having just two outlets with a multi-outlet adapter I decided to replace it with a pair of duplex outlets to accommodate everything that would be plugged in.

It turned out that the electrical circuit for this outlet came from my (semi-unfinished) utility room where it powered the washing machine and gas clothes dryer.  Because it is recommended that both laundry and kitchen areas be equipped with some sort of protection against electrical shock, I decided to install a GFCI (Ground Fault Circuit Interrupter) at that point (it's an older home) to protect both.  The installation wasn't all that difficult, the hardest bit was cutting away a bit of tile on the kitchen backsplash to allow the installation of a bigger box to house the two additional outlets.  In the semi-unfinished utility room I replaced what had been a single duplex outlet with that pictured in Figure 1.

Everything worked fine, except that when I transmitted on 40 meters, especially with the amplifier, I would find not only the outlet in the kitchen dead, but also the Ooma VOIP phone adapter, which was also plugged into the Utility room outlet served by this GFCI which showed as having been triggered.  Clearly, the GFCI was "seeing" some RF energy and "tripping out".

Why it happens:

The mere presence of RF at the GFCI isn't likely enough to cause it to trip - more likely, it was some RF current flowing through the GFCI - and the way that one typically wires a GFCI and uses it makes it more likely that this could happen.  Taking a look at Figure 2 helps to understand the situation.

Figure 2:
Interconnection of the GFCI to other circuits and loads.  As mentioned in the text, most of these connections are made via long cable runs - which will naturally act as antennas.  As can be seen from the drawing, the AC power from the circuit breaker (itself a long cable run) goes through the GFCI which then has wires that radiate out from there, acting as sort of a multi-wire dipole that permits RF current to flow through the GFCI.  The fact that these units are "grounded" means little at radio frequencies where about any length of wire can act as an antenna!
Click on the image for a larger version.

At first glance, this diagram shows nothing special - just normal interconnection.  But in terms of RF, there are a bunch of antennas here:

• AC Power from the circuit breaker - This line runs from the middle of the house to a corner, where the circuit breaker panel is located:  A lot of RF can be intercepted and flow through here!
• Interconnect to the kitchen - This goes up, through the floor and into the kitchen - yet another leg of the antenna.
• Items plugged in in the kitchen outlet - These further extend the "antenna" of the wiring to the kitchen.
• The washing machine - This is a large, metal box that can act as a sort of antenna.
• The clothes dryer - Like this washing machine, this is a large box - and it has a metal gas pipe attached to it that more or less goes to ground.
• The Ooma phone interface - There is an extension cord (also an antenna) that goes to this device, which is capacitively coupled (via isolation transformer windings) to both in-house phone wiring and the Ethernet cable connected to it.

The end result is that RF can flow through the GFCI, mimicking a current imbalance that can be detected by its internal circuitry as if it were a real current imbalance on the line and neutral wires, causing it to trip - and the susceptibility of it can depend on the amplitude and frequency of the RF energy.  As is the nature of anything that acts as antenna, the amount of RF energy that is picked up depends on many things, such as the length of the wire, where it is routed, what is connected to it, and the frequency of the RF energy and the transmitter power, all of these things working together in ways that are often inscrutible.

While keeping the RF outside the house (e.g. preventing it from coming into the shack by assuring that balanced feedlines are actually balanced and that coaxial cables are adequately decoupled to prevent current from flowing on their shields) is of great help - and highly recommended - but significant energy can still be picked up "over the air" by mains wiring.

Preventing false trips:

Regardless of the specific situation, the trick to preventing the GFCI from tripping out in the presence of RF is to keep RF from flowing through it!

Practically speaking, it is very difficult to fundamentally alter where wires go and what is connected to them as these are usually part of the wiring in the house, behind walls, making it inconvenient to change - and that assumes that one somehow knew exactly how RF pick-up might occur from an antenna onto these conductors.   The easiest way of preventing a problem is to keep RF from getting into the GFCI in the first place using series inductance to choke the RF currents.

To see what was done, consider Figure 3, which is an annotated version of Figure 2, above:

Figure 3:
An annotated version of Figure 2 showing the locations of the added ferrite devices.  Devices weren't placed on the cords to the washer or dryer as they were quite short compared to the other conductors - and because it wasn't required.
Click on the image for a larger version.

To figure out what I needed to do I first unplugged everything from the related outlets.  In my case, the GFCI no longer tripped out on RF so I started plugging things back in, noting when it started to trip out again:  If I'd already plugged something else in by this point I removed it to see if it still tripped, or if it simply contributed to the problem, tipping it over the edge.  In other words, you'll probably want to tray all possible combinations (within reason, of course) while noting the results.

Having done this, I determined that one main culprit was an extension cord to the Ooma device (see below) but it seemed that every appliance added just a bit to the problem.


The drawing above demonstrates several things:

Figure 4:
Three snap-on chokes on the
wires going to the box with the
GFCI.
Click on the image for
a larger version.

• Any long run (more than a couple of feet/meters) has on it a ferrite choke, located as close to the GFCI unit as possible.

By placing the choke as close as physically possible to the GFCI unit, there is less wire that can act as antenna.  This also reduced the amount of RF that can be coupled from one conductor into another from parallel runs of wire.

• In my installation, the conductor that comes from the circuit breaker and that which goes to the kitchen outlet are paralleled for several feet/meters.  Near the GFCI one can see that both conductors go through a single ferrite for common-mode decoupling while a bit farther away, we see that there is a ferrite device on each, individual cable as shown in Figure 4.

In Figure 4 we see the three snap-on chokes:  The one with both cables is just above the junction box (bottom of the picture), the one on the yellow cable is just above it and near the top of the picture is the one on the black cable.  These were so-placed because there was just enough slack in the wire to allow their installation at those locations.
 
The choke just above the box provides common-mode suppression for signals that might appear on both cables while the individual chokes suppress the different signals that might appear on each.  This is probably overkill, but it seems to be sufficient.

• There is a choke on the green extension cord (that goes to the Ooma box located a few feet away) that is located very near the plug (and GFCI). See Figure 5.

Figure 5
A multi-turn choke on the extension cord.
Click on the image for a larger version.

This choke actually consists of three turns in the core (as many as would fit!) to provide maximum reactance on this conductor.  It is this wire that seemed to be particularly problematic in coupling RF into the GFCI - possibly because this cord ran near some copper piping and furnace duct work, not to mention being connected to a device that has a degree of capacitive coupling into the house telephone wiring and, to a lesser extent, Ethernet cables.

I did not put similar chokes on the power cords connecting to the washing machine and clothes dryer, not finding it necessary to do so.  Being that they were right next to each other, they are likely to have very similar RF potential - and since we've effectively isolated all of the other RF paths to/from the GFCI with chokes, we've broken up possible paths for current flow, anyway.

Choke selection:

One of the problems with using snap-on ferrite chokes is that they are typically of little efficacy, the added reactance being quite small.  With the little reactance, lower frequencies - particularly 160-40 meters - may not be strongly attenuated.

The "typical" chokes that one finds on electronic devices are of a different material and have most of their efficacy at high HF and VHF/UHF frequencies where there is most concern when the device in question is facing EMC testing that may be required by regulatory agencies.  What we need is to use a ferrite device that is especially suited for lower HF frequencies.

Ferrite devices are available in a wide variety of "mixes" - the mix being a different formulation designed specifically to impede signals over certain ranges, the efficacy of most efficacy using being inversely related to the permeability of the mix:  In other words, materials with lower permeability often work adequately at higher frequencies while materials with high permeability are more effective at low frequencies - but often suffer somewhat at higher frequencies.

In general, on HF you are better off with the (less common, more expensive) higher permeability materials - and the higher the permeability, the better - particularly if you are only able to get a single turn (e.g. wire simply passing through) of the conductor through the device. 

A common material for snap-on chokes is "Mix 43" - a relatively inexpensive ferrite that is widely used and has best efficacy above 10 MHz.  Having a permeability of around 800, it will reasonably add a bit of extra reactance (resistance to RF) to the conductor over which it is installed.  In described case, I used two devices:

• Fair-Rite 0443800506, which has an inside diameter of 13.2mm (0.52") and is 15.6mm (0.61") long.
• Fair-Rite 0443806406, which has an inside diameter of 15.5mm (0.61") and is 16.2mm (0.63") long.

Note:  The above devices - and those below - are available from Mouser Electronics.

 The former will fit nicely over standard "12/2" "Romex" cable while the latter will fit over two of these cables:  The larger '6406 is what is used in Figure 4 right above the box's clamp and in Figure 5 over which three turns of the green electrical cord are wound while the upper two in Figure 4 are the '0506.

As mentioned before, these devices are typically used above 10 MHz and because of their nature, their efficacy decreasing with frequency.  Taking the '0406, it's rated impedance at 10 MHz is 24 ohms and 43 ohms at 25+ MHz, implying an inductance of approximately 0.33 microHenries.  To be sure, this isn't very much, and scaling this for other frequencies implies that at 40 meters its impedance would be on the order of 15 ohms, around 7 ohms at 80 meters and around 4 ohms at 160 meters.  Because the inductance increases with the square of the number of turns, the three turns wound around the green cable (in figure 5) considerably increased its efficacy - probably into the 30-50 ohm range on 40 meters.

This doesn't sound like much - and it isn't - but it is often enough to add just a bit of reactance to the connecting cables to reduce the amount of RF current flowing through the device - which is probably fairly well-protected in its own right - and also to slightly shift self-resonant frequencies in the wiring and the device to reduce the amount of intercepted RF.

What if it hadn't worked?


In my case, the Mix 43 devices were chosen because they were comparatively inexpensive, I had quite a few on hand and they did the job.  If the had not done the job, I would have used different devices - ones with a higher permeability so that the impedance of the conductor(s) over which it was used would also be higher.

The next logical step is to use Mix 31 which has nearly twice the permeability and, for whatever reason, are typically much longer in length.  Take, as an example, two devices of approximately the same inside diameter (e.g. able to accommodate one or two cables) as the two mentioned above:

• Fair-Rite 0431164181 - 13.05mm I.D. (0.51") and about 31mm (1.22") long.
• Fair-Rite 0431173551 - 18.8mm I.D. (0.77") and about 42mm (1.65") long.

Because of the size and length and material, these offer about twice the reactance as shorter versions - and cost about four times as much, too!

Even more impedance:

For those really difficult situations, there's yet another material - Mix 75 - and we can get devices similar to those above in that material as well:

• Fair-Rite 475164181 - 13.05mm I.D. (0.51") and about 39mm (1.54") long.
• Fair-Rite 475176451 - 18.7mm I.D. (0.74") and about 47mm (1.85") long.

Mix 75 isn't as effective at VHF, but that's not usually where the problem with RF susceptibility usually occurs.  They are about the best thing for HF operation, but at a cost - literally:  Because of the material - and the larger size of the above devices - they tend to cost nearly ten times as much as the Mix 43 material, but if you can only manage to get a single turn of a conductor through it (as would be the case for existing, in-situ wiring) it's the best thing to use.

For example, the Mix 31 '6406 device mention above provides only about 24 ohms of impedance at 10 MHz, but the Mix 75 '4181 device above has 100 ohms at the same frequency and close to 80 ohms at 40 meters.  To be sure, it's a bit of apples and oranges comparison because the '4181 device is about twice as long.

Final comments:

Similar techniques should work on AFCIs (Arc Fault Circuit Interruptors) as well as smoke/fire alarm cables that connect units together - just make sure everything still works when you are done!

In my case, the "semi-unfinished" state of the room allowed access to the "Romex" wires to the outlet, but this is likely not the case when wiring is concealed inside the wall - often with no additional slack in the conductor to permit installation of a ferrite.  If this is the case, the obvious first step toward diagnosing the issue would be to unplug anything on that circuit to see if it still tripped.

If the protector still trips with nothing plugged in, you'll have to get creative, perhaps replacing the box with a larger version (one that is deeper and/or has another "gang" position on it) or even adding a nearby box (with a blank cover plate) to which the wire could be pulled back and ferrite devices installed.

Whatever you do, be safe and sensible and make sure that everything still works as it should when  you are done!

[End]

This page stolen from ka7oei.blogspot.com

A limited attenuation high-pass filter for the KiwiSDR

NOTE:

Figure 1:
Inside the "limited attenuation" high pass filter, housed inside a small, die-
cast aluminum enclosure to which two BNC connectors were mounted.  Some
components were secured using small dabs of clear RTV sealant.
Click on the image for a larger version.

There is a follow-up articles to this one that describes a circuit that properly matches the source/load over a wide frequency range - See the article: "Revisiting the limited attenuation high-pass filter for the KiwiSDR" link and  "Revisiting the limited attenuation High Pass Filter - again".

 

Since the original posting of this blog entry I was made aware of an 1977 article on this very topic - you can read it HERE. (The article in question begins on page 3 of the PDF.)

* * *

One of the issues common with using a broad-band, direct-sampling SDR (software-defined radio) like the KiwiSDR is that of overload by strong, low-frequency signals, such as those on the AM (mediumwave) broadcast band - but there's another problem that should be considered as well:  The high generally-high signal levels at lower HF frequencies.  If one looks at an spectrum analyzer connected to a broad-band receive  antenna during the evening, one will immediately note that the lower the frequency, the higher the signals seem - particularly the background noise.

This becomes problematic if one is using an antenna with a relatively flat gain across the entire HF spectrum - and one wishes to make the receiver usable at both the top and bottom ends of this range.  As an example, I have a KiwiSDR connected to an antenna that is rated to cover from 3 to 30 MHz with roughly constant gain, but I noted that at the top end of the frequency range, around the 10 meter amateur band, the overall system gain was not quite sufficient to "hear" the background iononspheric noise.

The obvious solution to this gain deficit is to install an RF amplifier - which I did - but this had the effect of increasing the already-strong signals below 5-10 MHz even more, resulting in occasional "OV" indications on the KiwiSDR's S-meter signalling to me that the RF levels were high enough to "clip" the A/D converter.  While this wasn't too much of a problem during normal conditions, if the lower HF band were particularly noisy - as often occurs in the summer with thunderstorms on the same continent - reception across the entire HF spectrum was compromised when the loud static crashes would occasionally saturate the A/D converter.

It occurred to me that while I had about the right amount of system gain on 10 meters, I had far more than I needed at lower frequencies and could throw some of it away, so I set about designing a filter that would reduce signals at the low end of the HF spectrum, but have minimal effect at the upper end.

A "limited" high-pass filter:

Note:  The filter detailed below is NOT recommended as it does not match well to 50 ohms across Ll frequencies - see the follow-up article HERE for one that provides a good match/return loss.

 
The obvious solution to this would be the addition of a high-pass filter - but there's a problem:  Even a minimal high-pass filter would have increasingly-higher attenuation at lower and lower frequencies - potentially in the many 10s of dB - but we don't really want to get rid of the lowest frequencies.  What we need is a filter that will "knock down" signals by a significant amount - but not so much that they become inaudible.

In analyzing the signal levels, I determined that the goal of the design would be to leave signal levels above about 10 MHz unaffected, but reduce the signals below 8 MHz or so by 10-15dB.  This amount of attenuation (about 2 "S" units) would significantly reduce the amount of RF energy entering the A/D converter at the lower end of the frequency range (about 2 "bits" worth) but analysis of the noise floor and signal levels at these lower frequencies indicated that I would still be able to hear the noise floor.

The diagram of this filter is shown below:

Figure 2:  Diagram of the "limited attenuation" 10 MHz high-pass filter.
This filter attenuates by about 12dB (2 "S" units) below 8-10 MHz, reducing the overall signal power reaching the A/D converter of the KiwiSDR.
"R2" represents the 50 ohm input of the receiver and is not a resistor.
See text below for details of L1-L4.

The diagram above, as rendered by "LT Spice", depicts the load (the receiver) as R2, a 50 ohm resistor - and this is not an actual component that would be installed.

No real attempt was made to make this filter's input and output impedances "flat" across the entire HF spectrum - and to be sure, below about 14 MHz its input impedance a bit high, but this will have little practical effect on its operation - and we really don't need to be too precise, anyway.

As tested on a spectrum analyzer, the insertion loss is 12-13dB from DC to about 4 MHz at which point it gradually drops to about 2dB at 11 MHz and then dropping to less than 1dB by 30 MHz.  When doing an "A/B" comparison with and without the filter on the KiwiSDR, the waterfall above 10 MHz looked unchanged, but the signals below about 7 MHz were much less "bright" - and most importantly, the occasional "OV" indications on the S-meter pretty much stopped appearing altogether.

Comment:

In my opinion, the RF input on the KiwiSDR is slightly deaf, requiring a bit of gain (say, 6-10dB) to be able to reliably hear the background ionospheric noise on the higher HF bands - particularly when they are closed - a problem compounded by normal amplitude roll-off as one nears the Nyquist frequency.  To this end, the KiwiSDR at this location is preceded by a low-noise, high dynamic range RF amplifier that is flat from a few 10s of kHz to well over 30 MHz.

Update:

After this article was originally written it was determined by several testers using different KiwiSDRs that the absolute sensitivity of a KiwiSDR is on the order of -155dBm/Hz for 0dB S/N at 28 MHz.  This sensitivity level is about 6-8 dB below the expected noise at a "quiet" site using a unity gain antenna on the 10 meter band.

In the real world, it is likely that 10-12 dB of overall signal amplification should preceded a KiwiSDR to allow it to be sensitive enough to hear the noise on a "quiet" 10 meter band and the weakest signals.  If amplification is used, it should be placed as close to the antenna as possible in the signal path, but after a filter such as that described on this page (the filter will reduce the probability of overload by strong signals below 10 MHz and its loss at 10 meters is low and will have minimal effect), and before any splitter if you plan to feed more than one receiver from that signal path.  When the overall amount of amplification is calculated, be sure to include the loss of a splitter is taken into account.  For example, a 4-way splitter will incur about 7dB of loss, so if you wish the KiwiSDR to "see" 12dB of additional signal at 10 meters you will need an amplifier with a gain around 20dB.

The components for construction of this filter aren't critical:  The capacitors are high-stability NP0 (a.k.a. C0G) ceramic types while L1-L3 are wound using 30 AWG enameled wire with L1 and L2 having 15 turns and L3 having 12 turns on T37-2 toroidal cores, respectively.  L4 is a an inexpensive molded inductor and its value can be anything from 2.2 to 3.3 uH, or one could make it by winding 25 turns on the same type of T37-2 toroidal cores as used for L1-L3.

A look at the Kiwi's waterfall with the filter:

Figure 3, below, shows this filter in place on the KiwiSDR at the Northern Utah WebSDR site:

Figure 3:
A 1-18 MHz span of the waterfall from a KiwiSDR with the "limited attenuation" high-pass filter.  At the far left side a mass of signals from local AM broadcast stations can be seen.
The horizontal streaks are from wideband lightning static that is slightly "noising up" the A/D converter in the KiwiSDR.
Click on the image for a slightly larger version.

If you look at the waterfall above, there's no obvious evidence of the filter described on this page even being connected - and that's exactly the point!  The only evidence that something is a bit "strange" is the fact that the background noise between 8 and 11 MHz is slightly higher - and that's exactly where the filter rolls off:  At increasingly higher frequencies in this range, the natural noise drops off a bit - but the filter doesn't drop off at quite the same rate with the result being that the overall signal levels in this range are slightly elevated.

If I build another of these filters I'll push the "knee" up 1-2 MHz higher, starting the roll-off of signals below 11-12 MHz, instead.

Conclusion:

This filter seems to be very effective in reducing the total signal power from lower HF frequencies while having minimal effect at higher frequencies.  Because the signal+noise levels from a broadband antenna are much higher at the lower end of the spectrum, it is possible to reduce these signals by 2 "S" units or so without dropping the background noise - or the signals themselves - below the noise floor of the receiver.

For information on a filter system that is specifically designed to attenuate AM (MW) broadcast band signals, see the article "Managing HF signal dynamics on the RTL-SDR (and KiwiSDR) receivers", also on this blog.

Follow up:

There is a follow-up article to this one - "Revisiting the limited attenuation high-pass filter for the KiwiSDR" link where a variation of this filter is presented that passes the AM broadcast band and frequencies below it and is recommended for those installation where you wish to receive longwave signals.

[End]

This page stolen from ka7oei.blogspot.com
 

A "floaty thingie" for keeping NiHM cells topped off

A charge-state maintenance device for NiMH cells


PLEASE NOTE:  Messing about with batteries/cells can be hazardous:  Most cells contain hazardous materials and injury and/or damage can result from mishandling them.

Cells that are shorted, improperly charged or otherwise maltreated can pose an explosion/burn/chemical or other hazard.  It is entirely up to you to do research and provide the appropriate precautions to prevent damage and/or injury.


You have been warned!

The problem:

Table 1:  Comparison of self-discharge of various types of cells.

Comparison of self-discharge rates of various types of cells The table below shows the approximate amount of time that it takes to lose 10% of the cell's current charge capacity at different temperatures. Cell Type 0C (32F) 20C (68F) 40C (104F) 60C (140F) Alkaline >15 yrs. 4 yrs. 18 mo. 3 mo. NiCd 3 mo. 1 mo. 14 days 5 days (A) NiMH 1 mo. 10 days 5 days 1-2 days Zinc 6 yrs. 2 yrs. 10-12 mo. 2-3 mo. (A) These are typical values for new cells, published by various manufacturers.  Note that aging/mistreated cells will probably exhibit much higher self-discharge rates.  The NiMH information above is for "standard" cells, not the so-called "low-self-discharge" variety.

NiMH cells are ubiquitous these days - and for good reason:

• They have usable capacity comparable to that of an Alkaline cell of the same size.  A typical AA alkaline cell has 2.4-2.8 amp-hours of capacity whereas modern NiMH cells range in capacity from 1.8 to 2.8 amp-hours.

• They are relatively inexpensive.  If you shop around you can easily find AA NiMH cells for $2 each - often much less!  This means that if they are used just a half-dozen times, they may pay for themselves.

• They have low internal resistance compared to alkaline cells.  When you pull power from a battery, the output voltage sags - something that can make many devices such as digital cameras shut down before the battery is drained:  Alkaline cells typically have higher internal resistance than NiMH (or NiCd) cells which means that many devices cannot fully-utilize the energy of the cells - particularly when partially discharged.

• NiMH cells are more forgiving than NiCd and LiIon cells.  NiCd battery packs suffer from a problem called "cell reversal" in which when just one of the cells runs down before the others - an inevitability when several cells are connected together - the weakest cell ends up being charged backwards as the others pull power through it.  This causes an irreversible chemistry change that robs the NiCd cell of its power - making it more likely to run down first next time and become even more damaged than before!  NiMH cells are more tolerant of such abuse.  While NiMH cells can take a bit of abuse, LiIon cells can not, which is why they should always be connected using "protection" circuitry to guard against overcharge and overdischarge.

About "Ready-to-use" low self-discharge types.

There are some types of NiMH cells that are marketed as being "ready-to-use" that have significantly lower self-discharge rate than the standard cells.  It would seem that these cells - at least when new - do, live up to the claim, but I've yet to see information as to how much the self-discharge rate increases as they age.  I've also noted that these types of NiMH cells tend to have lower rated capacities than some other NiMH cells, ranging between 1500 and 1800mAh for these types versus 2100-2800 mAh for "normal" NiMH AA-size cells.  Such cells shouldn't be damaged if they are put in the "floaty-thingie.

Dealing with self-discharge:

As wonderful as NiMH cells are, the higher-capacity types and older, heavily-used cells do have a drawback:  Self discharge.

Referring to Table 1you'll notice something:  At ordinary room temperature, a good NiMH cell will lose 10% of its power after just 10 days - which means that after 6-8 weeks it's already half dead - and that's just from sitting there, doing nothing!  At higher temperatures things get far worse.  If you have a device with NiMH cells in it in a car on a hot, summer day you can expect it to be mostly dead in just a week or two.  Remember that the lower-capacity, "low discharge" types lose their charge slower than this, but I have yet to find specific information on these devices.

The data in Table 1 also assumes something else:  Typical, new cells.  As they age they tend to self-discharge even faster.

What does this mean, then?

• Don't leave NiMH cells around for "later use."  If you charge up your NiMH cells and the just leave them around, chances are they'll be mostly dead by the time you get around to using them - unless you have a system of cycling through them very quickly.

• Don't put NiMH cells away in your emergency box.  You should not rely on NiMH cells for emergency purposes unless you have a system by which you can guarantee that they are kept fully-charged.  For those devices that are put away for months at a time, Alkaline cells are a much better choice as long as they are stored outside the device to prevent possible damage from cell leakage and/or accidental discharge.
  

The challenge, then, is to have a system by which you can be reasonably assured that any NiMH cell you pick up is likely to have a full charge - but you don't want to do anything that is likely to damage them.

Maintenance charge:

In the case of NiMH cells (where the self-discharge rate is rather high - especially as the cell ages) it may be desirous to leave it on a "maintenance" (or "trickle") charge for very long periods of time.  Recent recommendations by some battery manufacturers suggest a "C/300" current for this while other manufacturers recommend a charging rate as high as C/40.  Following the C/300 example, our hypothetical 1 amp-hour cell above, this would be about 3.33 milliamps - that is, 1/300^th of the cell's rating.  I have not seen any specific recommendations for such a maintenance charge for NiCd cells, but I would expect that the same C/300 rate would be suitable.

It should go without saying that charging a "dead" battery at the maintenance charge rate may take weeks to accomplish!

Comment:

At this point in the article I would normally provide a link to the sites of several cell manufacturers - but I've observed that these links are constantly changing, so I'll forgo doing this:  I will leave it up to you to find the technical data for larger manufacturers such as Eveready, Ray-O-Vac, Duracell, etc. that give recommendations for long-term float charging.

A "Floaty Thingie" - A simple device to maintain NiMH cell charge during periods of non-use.

Because I extensively use NiMH cells - and because I'm aware of their tendency to self-discharge - I have built a simple device that does a maintenance charge for large numbers of cells.  This device, which I have called a "Floaty-Thingie" (a highly technical term, I know...) consists of several multi-cell battery holders with series resistors and LEDs to both limit current and indicate that a maintenance charge is occurring.  The battery holders are simply attached to a sheet of wood or plastic and powered by a 12 volt DC "Wall Wart" from my junk box.  Note that while I use mostly 4-cell holders, there is also one 2-cell and one single-cell holder so that I don't need exact multiples of 4 cells to fill a holder!

Figure 1:
Top:  The "Floaty-Thingie" used to maintain charged on NiMH cells.   (This version only does AA cells in groups of 4).  Even though there can be up to 48 cells being floated, a small 12 volt, 100mA wall-wart is all that it necessary.
Bottom:  The schematic of one section of the "Floaty-Thingie."
Click on either image for a larger version.

The circuitry is extremely simple:  A resistor and cell(s) in series with an LED - the latter being used to indicate current flow which allows you to be sure that the battery is connected.  All of this is powered by a 12 volt (nominal) voltage source.

Using a 12 volt (unregulated) DC "wall wart" supply (which ranges from 12-15 volts, depending on total battery load) a resistance was calculated, taking into account how many cells were used and what size.  My "Floaty-Thingie" handles only AA and AAA sizes as these are the most common, but using the information here and a simple application of Ohm's law, other values can be calculated.

For the maintenance charge I chose to follow the "C/300" float rate as this seemed to be adequately comparable to the self-discharge rate of the cell itself.  For typical AA NiMH cells, this would be about 8 milliamps - assuming a cell capacity of 2.5 amp/hours - and for AAA NiMH cells, this would be around 3 milliamps - assuming a cell capacity of 1.0 amp/hours.  These values are typical and are definitely not critical!   Do not worry if your AA cells have 1800 mAH or 2800 mAH capacity, for example!

At this point, a few assumptions are made:

• A supply of 13.5 volts.  This is a reasonable voltage to see from a "12 volt" unregulated "Wall Wart" under moderate load, but anything from 11 to 15 volts would be OK.

• About 1.5 volts per cell.  (We are assuming that our cells are already fully-charged.)

• Float currents:  The float current is 8 mA for AA cells and 3 mA for AAA cells - values that roughly correlate with C/300 for typical NiMH cells of those sizes.
  

The series resistance for various cell combination under the above conditions is as follows:

Table 1:

Typical values for different types and numbers of cells using the circuit in figure 1 with a supply voltage of 12-15 VDC

Number and type of cells	Resistance value (ohms) with 2 volt LEDs (standard-brightness red/yellow/green)	Resistance value (ohms) with 3.6 volt LEDs (high-brightness green/blue/white)
4 AA	680	470
2 AA	1000	820
1 AA	1200	1000
4 AAA	1800	1200
2 AAA	2700	2200
1 AAA	3300	2700

• The above values are not critical and variations of +-25% should not be of any concern
• 1/4 watt resistors or larger are suitable.

In Figure 1 may be seen the schematic of the "Floaty-Thingie."  As you can see it is very simple and there's nothing critical about it - except to say that any exposed wires should be insulated to prevent accidental shorting of any components:  Remember that NiMH cells can put out many amps under such conditions!

On the schematic, "R" is a resistance from the table above, "D" is the LED, and "B" is the holder, containing 1, 2 or 4 cells.  When operating from a "12 volt" supply (which can be anything from 11 to 15 volts) it is not recommended that more than 4 cells be used as you need several of volts of drop across resistor "R" in order to limit current effectively and maintain fairly consistent current with minor voltage fluctuations.

Note that Table 1 shows different resistance values for "2 volt" LEDs and "3.6 volt" LEDs.  The older-style "normal brightness" red, yellow and green LEDs (but not blue or white!) are of the 2 volt variety while the newer "ultra bright" LEDs (most notably green, blue and white) are of the "3.6" volt type.  When you by the LEDs, a quick look at the "forward voltage" specifications will tell you what you wish to know - but don't be worried by slight variations.  For example, the "2-volt" types may vary from 1.7 to 2.2 volts while the "3.6 volt" types may say anything from 3.2 to 4.1 volts.

A note about the use of 3.6 volt LEDs:

• These types are usually the "ultra bright" (green, blue, white) LEDs.  If you use these - and you have a lot of holders - the total amount of light coming off the "floaty-thingie" may be surprisingly bright - even at just 8 or 3 milliamps.  If you build one of these, expect that they may still be painful to look at and also that at night, the entire assembly may be annoyingly bright!
  

Remember:  We aren't aiming for ultra-precise results here - just those that are "in the ballpark."

Using the "Floaty Thingie"

I've used this thing for several years now (over a decade!) - as have several friends who have seen it and made their own.  Here are a few observations and comments:

• Put ONLY fully-charged cells in the Floaty-Thingie.  It will take a very long time to charge a dead cell (several weeks, perhaps!) at the above currents.  Since the whole idea is to have fully-charged cells on hand for immediate use it would be a bad idea to put anything but fully-charged cells in it in the first place!
• Completely fill up the cell holder.  This should go without saying:  Unless every position in the cell holder is filled, you won't complete the circuit and do charge maintenance.  Because of this, I recommend having one single-cell holder and one two-cell holder - in addition to a larger number of four-cell holders for each cell size (e.g. AA and/or AAA.)  Doing this allows you to "float" any number of cells that you may have onhand.  Some people who have built it have used two-cell holders (and a single one-cell holder) instead of any four-cell holders, which works, too, but remember that since each holder takes the same amount of current, regardless of the number of cells, you'll be able to maintain fewer cells overall if your wall-wart is rather small.
• Make sure that you adequately size the wall-wart.  When you pick your "wall wart" supply to run this, consider how much current you will pull from it if you load cells into every holder.  To play it safe, assume that each AA holder will pull 10 milliamps and each AAA holder will pull 5 milliamps and simply add the total number of holders of each size - and make sure your supply can handle this.  
• Note that a one-cell holder pulls the same current as a two or four-cell holder of the same cell size:  The difference in power is "eaten" by the series resistor used to limit current.  Again, this means is that if you have a very small wall wart - of if you have a limited power budget (say, from a small solar panel) you can get better efficiency by using mostly four-cell holders rather than mostly two-cell holders.
• Yes, you can use a 12 volt solar panel for this.  Since the sun only shines part of the day, don't worry if the voltage goes well above 12 volts (as high as 18-20 volts) during bright sun as the "average" current will be in the general range of what it should be.
  
• This "maintenance" charge doesn't seem to have damaged the NiMH cells.  Over the past 5 10 years or so, neither I or others who have used a Floaty-Thingie have seen any evidence that its use causes loss of electrolyte due to overcharging, "Lazy Cell" syndrome (see below) or obviously shortens the life.  Nevertheless, it would be a good idea to rotate through and use all of the cells as this would reduce the possibility of "Lazy Cell" syndrome (if it is likely to occur in NiMH at this "maintenance" rate anyway) and it give you another chance to spot those cells that are going bad!  Even when treated well, cells won't last forever!
• The "Floaty-Thingie" doubles as a night light.  Since my Floaty-Thingie can hold over 30 cells, its LEDs give off a surprising amount of light when all holders are populated and if you happen to use a mixture of different colors you can get some pretty cool effects!  Remember, though:  The modern "ultra bright" LEDs put out a lot of light - enough to make looking at them painful and keeping a room annoyingly bright at night.  If you do use these newer, modern LEDs be aware that many of them (such as the blue, white and green) have higher voltages - between 3 and 4 volts as opposed to around 2 volts for the old-fashioned, dim red, yellow and green "indicator" type LEDs, so be sure to take that into account when selecting the resistor values.
  
• I try group group "like" cells together.  If you are like me, you have been acquiring NiMH cells for years so you not only have different brands, but different milliamp-hour capacities of cells - even of the same brand!  Grouping like-cells together will also assure that when you use them in a device that takes several cells, you'll get optimal performance.
• Note:  When I buy rechargeable cells, I always write the month and year of purchase on them with an indelible marker as this also makes it easier to group them together.
• DO NOT put alkaline cells in the "Floaty-Thingie."  When one attempts to recharge alkaline cells, they can do unpredictable things such as leak, so don't!
• Come up with a system for "rotating" stock.  It is best if you make sure that all cells get as equal use as possible.  One way to do this would be to leave at least one empty holder at all times, knowing that the next holder contains the cells to be used when previously-charged cells are to be installed in the now-blank one.  In this way one can help assure more even usage of cells over time.
  

Can you put NiCds in the "floaty thingie"?  Yeah, probably...  It probably won't hurt them to keep them in there for short periods such as days, but I'm not sure that I'd leave them in the device for weeks/months at a time!

Using "similar" cells:

As with other types of cells, it is recommended that you avoid, as much as possible, mixing different brands/capacities of cells.  While the chemistry of NiMH cells makes it less likely than with NiCds that they will be damaged by cell reversal, it never hurts to play it safe.

This is fairly easy to do, actually:  Simply group the same brand and same-capacity cells together and use them as such.  Personally, I write the month and year of acquisition on cells when I buy them with an indelible marker, making it even easier to match the cells into groups - plus, it lets me readily identify the oldest of the cells and keep track of how old they are and whether or not they deserve further scrutiny as they age.

Detecting apoptosis (e.g. "cell death"):

The "floaty-thingie" has another use:  To detect cells that are near the end of their useful life.

Inevitably, cells will lose their capacity and die - but how do you detect that fact before discovering that the device you put them in quit working sooner than expected?

In using the "floaty-thingie" there are some signs that an individual cell may be "sick" and might have lower-than-expected capacity.  To do this, you'll need a reasonably accurate digital voltmeter:  It needn't be expensive - I've found that even the $3-on-sale digital multimeters from places like Harbor Freight have more than adequate accuracy.

Here's the procedure:

• Charge the cell normally using your normal charger.
• Put it in the "floaty-thingie" and wait a week or so.  This wait time is required to allow the cell to equalize and "do its thing" - that is, if it's really bad, it may take a few days for the symptoms to show up.
• While in the holder, measure the cell voltage.  I have found that a normal room temperature that typical NiMH cells measure between 1.35 and 1.47 volts.  I've noticed that same-brand and same-vintage cells tend to stay very close to each other and that this voltage seems to slowly decrease over time as the cells age and self-discharge (leakage) currents increase.

If you find one cell that has radically different voltage from the others - especially if it was made at the same time and is of the same brand as the others - then be suspicious of that cell!  If the cell's voltage is unusually high after a week of being in the "floaty-thingie" (a reading above 1.5 volts should certainly set off alarm bells!) then it is very likely that there is something seriously wrong with that cell!

If the cell voltage is lower than it should be - say below 1.3 volts - mark it with a piece of tape (so you can tell it apart from the others) and then try charging it normally, re-install it in the "floaty-thingie" and wait another week or so - just to make sure that it is really sick.  If it tests OK this second time, chalk up the first "bad" results to, perhaps, accidentally putting a battery that was not fully charged into the "floaty-thingie" - but if it tests bad again, get rid of it!

Of course, it should go without saying that all batteries should be disposed of properly!

Disclaimer:


Again, messing about with batteries/cells can be hazardous:  Most cells contain hazardous materials and injury and/or damage can result from mishandling them.

Cells that are shorted, improperly charged or otherwise maltreated can pose an explosion/burn/chemical or other hazard.  It is entirely up to you to do research and provide the appropriate precautions to prevent damage and/or injury.

You have been warned!


This blog posting was adapted from an earlier article on my web site.


[End]

This page stolen from ka7oei.blogspot.com

Pine needle what? (Keeping your coniferous antenna supports alive!)

The afflicted tree:  The needles are more normal at
the top, but looking rather "thin" farther down.
Because of the number of pine cones, the tree
looks "browner" in this picture than it really
is - but it is definitely under stress!
Click on the image for a larger version.

Late last year I noticed something amiss with a fairly large (50 foot/15 meter tall) Scots Pine tree in my front yard:  The needles looked a bit "thin" and short.  In the past this tree would tend to shed needles and pine cones all over the place on a seasonal basis, so I kind of ignored it over the winter - but this spring, when I started yard work again, I became concerned.

Something was definitely wrong with this tree.  While the needles themselves seemed to be green and flexible, I noticed that at the top of the tree they looked normal-ish, but the bottom 2/3rds of the tree looked "skimpy", so I decided to investigate more closely.

If you've followed this web page you've probably figured out that while I'm probably not a yard and garden person, I do what I need to do to keep the yard in reasonable shape, asking my Dad or friends for advice when something wasn't quite right.   A couple of weeks ago I finally did what I should have done months ago:  Take a very close look at the tree.

On the lowest branches - where the problem seemed worst - I could tell that the limbs were very green and flexible - but the needles themselves, while green, were thinner than they should have been - and were covered with small spots.  Doing what many people do these days I resorted to Google and it "told" me about all sorts of possible fungal infections and other things - but I wasn't satisfied that it was describing what I was seeing, so I asked a friend of mine who'd had tree work done in the past year or so.  He referred me to the guy that did his tree work - a "semi-professional" who did this as a combination of a hobby (I gathered that he really likes trees!) and a second job.

After a couple weeks of phone tag, I was finally able to talk to him and I described what I was seeing.  Almost the first thing he asked was "Do the spots scrape off?"

"What?" I thought.  "Those are probably small bugs" he continued.  I must admit that it never once occurred to me to try scraping them off - and it then I realized that I should have looked at the needles with a magnifier.  He continued to explain that he suspected that the tree was suffering from "Pine Needle Scale" - an infestation of small insects that feed on the sugars in the needles - and this would probably kill the tree if left un-checked.  He then explained that he could probably drive out to my house and charge me a chunk of change to tell me this same thing in person - and then charge me more to treat it, or he could just save himself some time and me some money and have me treat it myself.

A close up of a bough on the affected tree.  As you can see, the needles are a bit shorter and thinner ("thin" like paper rather
than in number) and paler than they should be - and there are lots of spots!
Click on the image for a larger version.

After a few more minutes detailing the common treatments, we got off the phone and this time, armed with more definitive information, I did a bit of online research and what I saw in the pictures looked very much like what I'd seen.  It wasn't until I got home and plucked a few needles off the tree and looked through a magnifier that I saw that these spots were, in fact, small insects - looking exactly as he described and much like the pictures on the web.

A close-up of one of the needles showing the infestation
of what are probably Chionaspis pinifoia - little, hard-shelled
insects that literally suck the life out of the tree!  These will scrape off
with a fingernail revealing more-or-less normal looking needle
underneath - except that it's wet with weeping liquid from the tree.
Click on the image for a larger version.

I have two other pine trees in my yard that look healthy - so I inspected them, but the results were inconclusive:  I saw nothing obvious, but I didn't check everywhere within reach. If they were infested, it wasn't bad... yet...  A more thorough inspection has revealed that both of the trees in the back yard are infested - but it appears to be mild as I had to look a bit for them.


Why did the tree in my front yard get infested but the others not as bad?  I have a suspicion:

Two years ago, during the installation of a solar power system, it turned out to be necessary to upgrade the utility power connection to my house, so a narrow, 48" (122cm) deep trench was dug through my front yard - but the path took it very close to this same tree.  When the trench was open I could see that there were several rather large roots that had been cut - and this concerned me a bit.  What I suspect happened was that this weakened the tree a bit, making it more susceptible to an infestation - but then again, it could have just been bad luck!

Between talking to the tree guy and going online, I read about three common ways to control these critters - typically Chionaspis pinifoia (read about this insect at the "Tree Geek" web site).  These methods include:

1. Spraying with an insecticide.  This is the "kill them right now!" approach - but it may not get the entire tree (particularly if it is a tall tree that is difficult to spray in its entirety) and this is most detrimental to beneficial insects like bees, ladybugs and other things if they happen to be on the tree, downwind, or very nearby.
2. Ground soak.  A solution of insecticide and water is poured (usually in a small "moat" to confine it) around the base of the tree so that it is quickly absorbed into the root system.  This systemic treatment is slower to take effect, but is longer lasting and will protect the entire tree - and it is somewhat less harmful to beneficial insects since it is more or less confined to the tree.
3. By injection.  If the tree is in really bad shape, insecticide is injected directly into the tree where it can more-quickly be taken up.  The "tree guy" with whom I was speaking seemed to think that since I didn't (yet!) have large sections of die-off that this wouldn't be necessary.

Comment:

There are more ways to deal with these things that are less harmful to beneficial insects - but the general opinion seemed to be that these were best for preventing infestations, controlling those infestations that were minor, or in those situations where there was a need to minimize the effect on the "good" bugs (e.g. to protect pollinators, etc.)  For trees that were under significant stress, arborists seem to recommend the "strong" approach where it can be safely done.

Based on what I read, my tree wasn't in "really bad shape" since the needles - while getting a bit pale - weren't dying off in large quantities... yet - but it is under a fair amount of stress.  Apparently, it is about this time of year (April, May) around here that these bugs start to reproduce and become more active - so this is the time to do the treatment.  I decided on a combined approach:  Spraying where I could reach and doing a ground soak around the base.

To this end, I sprayed the tree on a wind-less day as far as I could reach with a solution of "Sevin" (a carbaryl inseciticide):  I was able to get the bottom 1/4-1/3 of the tree, which encompasses about 1/3-1/2 of the pine needles.  Because of the height of the tree, I couldn't really go much higher than I could reach via the spray while working from a free-standing ladder - but this would, at least, have an immediate effect on a significant part of the tree.

The second treatment is a ground soak (an imidacloprid-based insecticide) as described by the "tree guy", the guy at the local farm supplies distributor from whom I bought the stuff, and the online descriptions:  This latter application will take a couple of weeks to work its way through the entire tree - but it is, by all accounts, considered to be very effective.  While I'm at it, I'll also proactively treat the other two pine trees in my yard with the ground soak - just in case those critters managed to get around - however they do that.  Based on what I have been told and what I have read, this treatment will become an annual, spring ritual.  Since pine trees are less attractive to bees than other plants, I'm hoping that the effect on them will be minimal - although they may collect some components of propolis from them.

So, the next few months will be telling.  I really do like my pine trees:  I think that they look nice, they offer a bit of cooling shade to the house - and they provide nice anchor points for my ham radio antenna!

* * *

Update - 6 June, 2018:

It's been about six weeks since treating the tree - and it is no worse off, but the bugs are still hanging on, so I went a step further as recommended by an expert:  Treating the tree with "Safari 20SG" - a Dinotefuran-based insecticide.  According to available information, this insecticide is taken up by the tree more quickly and is more effective at eradicating parasitic insects.  Hopefully, once these bugs are knocked down, the tree will recover and will be able to better-resist them in the future.

The instructions indicated that 1.0-2.2 oz were recommended for every 10 feet of tree height, so for a tree of this size (about 55 feet tall) the entire 12 oz container (which cost about US$110) was dissolved into several gallons of water and poured around the base using "soil drench" techniques.

Update - 27 June, 2018 - Success, I hope:

 About a week ago, I examined the tree again and saw that the bugs were rapidly dying off:  In fact, it was difficult to find a live one.  This die-off must have started happening a week or so before this as the needles are already looking "plumper" and slightly more green - a sign that fewer of these things are still trying to suck the life out of the tree!


There are still a lot of these things covering the needles even though they are dead, but at least some of them should weather off - and all of their carcasses will eventually fall with the needles as the tree replaces them in its normal cycle.

Update - August, 2019:

The bugs have remained dead and the tree is looking much more healthy, once again in the process of producing pine cones.  From what I have read, it takes about 3 years for this type of tree to completely replace its pine needles, but new needles are easy to spot - partly because they don't have the carcasses of bugs on them!

It looks as though a simple ground soak of an imidacloprid-based insecticide has halted the (mild) infestation of the trees in the back yard - but from what I have read and what I have been told it will be a constant vigilance going forward as one can never really get rid of them completely, mostly likely due to the fact that there is always the potential for re-infestation from wherever these things came from in the first place!

Update - October 2024:

All three trees are healthy and produce copious quantities of pine cones every year.  I did notice on a walk around my extended neighborhood a few weeks ago that my pine trees are some of the very few that are still alive - let alone healthy:  A few of them were still up, but dead - no doubt the owners dreading shelling out $$$$ to have them removed.

* * *

[End]

This page stolen from ka7oei.blogspot.com

Better frequency stability for the QRP Labs "ProgRock" synthesizer

NOTE:

The Progrock 2 from QRP-Labs has replaced the original ProgRock.  This device has a 0.25ppm TCXO, eliminating the problems noted below.

* * * * * * * *

Update:

It turns out that the newer (version 4) of the QRP Labs ProgRock board has pads for a TCXO.  See below for a link to the ProgRock web page.

The ProgRock:

The "ProgRock" synthesizer from QRP Labs is an inexpensive device based on the Si5131 "any frequency" synthesizer that may be used to produce up to three frequencies simultaneously - typically from around 8 kHz to around 200 MHz (with some limitations) but it may be coaxed to go down about 3.5 kHz as high as 290 MHz.

Figure 1:
The "synthesizer" portion of an unmodified Version 3 ProgRock.  The
27 MHz crystal, in the upper right quadrant, is a typical "computer grade"
device amd is typically stable to only a few 10s of PPM over a a wide temperature
range - OK for many applications, but not where you want really
good frequency stability.
Version 4 of the ProgRock has pads for an SMD TCXO on the
top of the board, although it's not the same device that I used - see text.
Click on the image for a larger version.

Typically programmable using a pushbutton and a DIP switch, newer versions of firmware may be programmed via a serial port as well.  These devices also have an input from a 1PPS (1 pulse per second) source, such as a GPS receiver, to allow precise setting/control of the frequency.

Unlike a VFO, the ProgRock produces only a set of fixed, pre-programmed frequencies:  Up to 8 "banks" of frequencies may be selected via three digital select lines.

What sort of things might this be used for?

• Arbitrary frequency sources for the workbench.
• Providing clocks for digital circuits.
• The local oscillator of a fixed-frequency receiver or transmitter.
• An internal local oscillator for a radio - such as a a frequency converter or BFO.

For casual use, the supplied crystal - a typical computer-grade unit - is adequate, but if you need the frequency to be held to fairly tight tolerance - say, a couple of parts-per-million - over a wide temperature range you will probably want something else.  QRP Labs does sell an "OCXO" version of the synthesizer which works well, but it is more complicated to build and adjust, it consumes several watts of power and produces extra heat.

You might ask:  "Why not just use the 1PPS input for frequency control?"

Figure 2:
The bottom side of the board after modification.  The tiny TCXO module
is affixed to the board and then connected to the circuit using flying leads.
In the picture above, pin "1" is in the lower right corner of the device - the
only one without a solder connection.  On the "label" side of the chip
pin 1 is identified by a very tiny dot.  Pin 2 ground (lower left, blue flying
lead) can be identified with an ohmmeter as it is also connected to the case.
Click on the image for a larger version.

While these devices can be "nailed down" to a precise frequency with the application of a 1pps input from a GPS receiver or other high-stability source, there is a problem with this option:  It tends to cause a "step" change in frequency on the order of 1-2 Hz.

Such step changes would probably go unnoticed on SSB or CW, but with certain narrow-band digital modes there might be a problem.  While modes like WSPR or JT-65 can deal with frequency drift, this would normally occur very gradually over the period of several symbols giving the decoder enough time to track, but if the frequency shift were very sudden, a few symbols would probably be lost.  While the occasional loss of data is normal, any loss caused intrinsic to the receive system - perhaps due to frequency steps of the local oscillator - would degrade the remaining error-correcting capability overall.

In other words:  If phase or very fine frequency changes will affect your communications, you might not want to use the 1PPS input.

Using a TCXO:


Another option is to replace the crystal with a TCXO (Temperature Controlled Crystal Oscillator).  These small, self-contained oscillators have on-board circuitry that counteracts the temperature-related drift, holding the frequency relatively constant over their design range.

A suitable device is a part made by Taitien and is readily available, being DigiKey part number 1664-1269-1-ND (Mfg. P/N TXETBLSANF-27.000000).  This device is tiny - only 3.2x2.5mm square so soldering to it is a bit of a challenge - but still manageable with a fine-tipped iron and some magnification.

Note:

As pointed out in the QRL Labs documentation, some TCXOs may have "stepped" frequency adjustments as part of their temperature compensation due to a built-in temperature sensor and D/A converter referencing a look-up table.  If sufficiently large (e.g. results in more than a few 10ths of Hz "step") these frequency discontinuities can disrupt/degrade modes such as WSPR that operate over very narrow bandwidths.  If a TXCO does this, the synthesizer being controlled by it will also exhibit the same frequency steps, proportional to the output frequency.

The Taitien TCXO units noted above were observed at 432 MHz (the 16th harmonic of the 27 MHz TCXO)  using signal analysis software to magnify possible frequency steps:  If such "step" behavior was happening, it was smaller than 0.34 Hz at 432 MHz (e.g. 0.02Hz at 27 MHz.)

The power requirements of this device are very low - only 1-2mA, far less than the 100-200mA of a warming crystal oven - and it may be powered directly from the existing 3.3 volt supply of the synthesizer board.  This TCXO produces about a volt pk-peak output which is in line with what the data sheets for the Si5351A suggest for a capacitively-coupled external signal being fed into the crystal input.  It is possible that the Si5351A would work just fine if this TCXO were directly-coupled, but I included the capacitor just to be safe.

Wiring the TCXO:

Comment: 

As noted above, later version of the ProgRock have pads for a TCXO, albeit one with a different footprint than above.  The device suggested by QRP Labs is the FOX924B-27.000 (Digi-Key P/N:  631-1075-1-ND) which is quite a bit larger than the Taitien device and has a rated stability of 2.5 ppm instead of 1 ppm.  This device has a higher output voltage swing which allows the omission of the coupling capacitor used with the Taitien device noted above.

I first removed the crystal and cleaned the holes of solder.  The TCXO module was then glued using cyanoacrylate adhesive (a.k.a. "Super Glue") "belly up" to the circuit board (after it was cleaned with denatured alcohol) at a location on the bottom side of the board between the synthesizer chip and the crystal position as shown with pin "1" in the lower right corner.  With the oscillator firmly in place, a small piece of 30 AWG wire was used to solder pin "4" (V+ - upper-right) to the nearby connection of C3, one of the V+ lines for the synthesizer chip.  Connected to the opposite corner (pin 2, lower left) another short piece of 30 AWG wire is connected to the other side of capacitor C3 to provide the ground.

A small, 1000pF disc ceramic capacitor was inserted into the bottom side of the board to connect to the crystal terminal closest to C3 and the "CLK 0" terminal with the other lead carefully formed and bent to be soldered to the upper-left pin, #3 - the output terminal of the TCXO.  Once the capacitor is soldered into place it is a good idea to re-heat the capacitor's other lead (the one soldered into the board) to relieve any mechanical stress that might have occurred from bending the lead to fit to the connection.

Before soldering to the TCXO - but after it has been glued to the board - it is recommended that a small amount of liquid flux be applied to the connections and that they be tinned using a hot iron with a very fine tip:  The ceramic package tends to draw away heat quickly, making it a bit difficult to solder and tinning it before-hand assures that a solid connection has been made.  Don't tin the unused pin as it's an easy way to identify the pins of the device while it is inverted.

Assuming that the connections are good and that the pins were properly identified, the synthesizer may be plugged into a ProgRock as normal.  If all went well, the output frequency will be pretty close to what it was before - but slightly low in frequency.  While the nominal frequency of the original crystal is 27.000 MHz, the frequency is usually 2-5 kHz high in this circuit so the "default" clock frequency of the ProgRock is set to about 27.003 kHz to compensate.  With the TCXO being within 1 PPM of its intended frequency, register 02 of the ProgRock will have to be set to the new frequency, hopefully within a few 10s of Hz of exactly 27.0 MHz:  If you have a means of precisely measuring the frequency, use that number for register 02, otherwise use 27.000000 MHz.  Once this is done the programmed, output frequencies will be quite close.

Once everything was checked out I put a few more dabs of adhesive on the capacitor and flying leads to make sure everything was held into place.

How well does it work?

I put together two of these TCXO-based ProgRocks and when compared to a GPS-referenced source, I found one to be 1 Hz high (e.g. 27.000001 MHz) and the other to be about 13 Hz low (26.999987 MHz) - both well within the 1PPM specification.  These frequencies were programmed into register 02 and CLK0 was set to precisely 10 MHz and I found the output to be within 1 Hz of the intended frequency.  I then heated and cooled the units and observed that the frequency stayed well within the 1PPM spec, indicating that all was as it should be.

Example applications:

Stable receiver local oscillator:

Figure 3:
An application of the ProgRock where two of the outputs are being used as
the local oscillators of two "SoftRock Lite II" receivers configured to cover
different portions of the 40 meter band.  Fitted with a TCXO, these
frequencies will be held to within 1ppm over any reasonable temperature
excursion.
Click on the image for a larger version.

An immediate need for a stable frequency source came about recently while I was putting together a module that is designed to cover the entire 40 meter amateur band in two segments using two "SoftRock Lite II" SDR receiver modules.  Normally these ship with crystal oscillators, but the use of a single ProgRock module allowed a pair of these receivers to collectively cover the entire 40 meter band with very good frequency stability - important if digital modes such as WSPR are to be considered.

Figure 3 shows the result.  Both receiver modules and the ProgRock were mounted in the lid of a Hammond 1590D die-cast enclosure and a simple 2-way splitter using a BN-43-2404 binocular core was constructed.  The end result - when coupled with good-quality 192 kHz sound cards - is a high-performance, stable receive system capable of covering the entire U.S. 40 meter amateur band - with a bit of overlap in the middle and extra coverage on the edges.

Replacement of a crystal in a phase-modulated VHF/UHF transceiver:

These days it is increasingly difficult to source custom quartz crystals for older "rockbound" commercial radio gear.  An example of this is the GE MastrII line of VHF (and UHF) transceivers that require a crystal for each transmit or receive frequency.  Even though this equipment is now quite old, it is still useful as it is quite rugged and has excellent filtering and when properly prepared, it has been proven to very reliable.

These radios use crystals in the 12-13 MHz area for transmit and 16-17 MHz area for receive so a ProgRock can be easily programmed to be used in lieu of a crystal with a slight modification of a GE "ICOM" channel element.  Because the MastrII transmitters use phase modulation, the signal source is never modulated - and this is an advantage if you happen to need to set several overlapping transmitters to the same frequency and you need their modulation to "track" precisely.  By using a TCXO (or QRP Labs' OCXO) rather than the 1PPS to maintain frequency stability, the possibility of occasional "clicks" in the audio due to frequency correction steps is eliminated.

Comment:  For "Direct FM" radios like many older Motorolas, a ProgRock cannot be used as they require that the oscillator itself be modulated, and this is not possible with a ProgRock.

With any synthesizer the concern is that it will produce spurious signals and/or additional phase noise that will degrade the transmit/receive performance, but preliminary testing has shown that even when multiplied to 70cm, the resulting spectra is quite clean - probably good enough to be used on a repeater.  If one does do this, there are a few things that should probably be kept in mind:

• Even though the ProgRock can output two frequencies at once, there is a small amount of crosstalk between them and when multiplied to the ultimate VHF/UHF frequency, these low-level spurs could end up on the output.  For this reason it would probably be a good idea to use two separate ProgRocks, located physically apart from each other, in a full-duplex radio.  For half-duplex, a single ProgRock could be used with the RX and TX frequencies being toggled by selecting a pre-programmed "bank".
• Its worth noting that in many of these radios the LO frequency of the receive frequency is immediately multiplied by the next stage.  Testing was done on a receiver showing that the ProgRock could be set to the output frequency of this multiplier stage.  This cannot be done for the transmitter as the oscillator's output is immediately phase-modulated at its operating frequency.
• It would probably be a good idea to place some high-Q band-pass filtering tuned to the synthesizer's output frequency to minimize any low-level spurs at frequencies removed from the main output frequency that might be present on the synthesizer.  Initial testing didn't show any obvious problems, but using such a filter would be a sensible precaution.
• A very small amount of added "hiss" - probably from low-level phase modulation - was observed at UHF.  In normal use, this would probably have not been noticeable unless one did an "A/B" test. With the phase noise being lower at VHF, this hiss would probably be unnoticeable.

[End]

This page stolen from ka7oei.blogspot.com