Making LED headlights RF-quiet

TL;DR

 If you have LED headlights that are causing RF interference, ferrites alone will probably not be enough to completely solve the problem:  You will probably need to put the offending switch-mode LED controller in "RF Jail" as described below.  While VHF/UHF is the target of the efforts noted below, they should be equally effective at HF.

* * * * *

We live in a world of RF - and an increasing amount of this is from devices that are not intended to produce radio-frequency energy.  These devices have proliferated in the past several decades and surround us at home, work and in our cars.

Figure 1:
The vehicle in question with LED headlights -
which are now RF-quiet on all bands.
Click on the image for a larger version.

Generally speaking, modern internal combustion vehicles are in RFI terms "pretty quiet" - especially compared to their electric and hybrid counterparts - and someone in such a vehicle will experience less RF noise when they are "out and about" in a rural-ish area than they will at home.  For amateur radio operators, this is a good thing as mobile operation often includes weak signals - whether this is on HF, or on VHF/UHF with weak signals from distant repeaters or during simplex operation, with intervening terrain.

Effective mobile operation is therefore contingent on a vehicle that is intrinsically "RF-quiet", but this also means that any accessories that you might add to this vehicle also be RF-quiet as well.  These days there are any number of things that you might throw in your car that can spoil an otherwise-clean RF environment and the short list includes USB chargers¹ , GPS receivers and extra lighting ², to name but a few.

Real-world case - Aftermarket LED headlights

A friend of mine recently installed aftermarket LED headlights in his older Honda CRV ³.  It took a while to correlate the cause, but he eventually noticed that when the headlights were turned on, he lost significant weak-signal sensitivity on 2 meters:  Around town and with stronger repeaters, the effect wasn't really noticeable, but when the repeater was distant - or when communicating simplex (e.g. direct - with no repeater) with a weak/distant station - turning on the headlights dramatically reduced received ⁴ signal quality and range.  As it happens, he has more than one VHF/UHF radio in his vehicle and although both were affected, the one with its antenna mounted to the front fender - much closer to the LED headlights - was more severely impacted.

LED headlights typically consist of two modules:  The LED module itself and the "controller" - which itself is a switch-mode power supply to regulate the current to the LEDs as well as switching arrays of LEDs for high/low beam - and even color temperature in some models.  Sometimes these units are combined, but in this case, they were separated with a short cable, the "controller" being separate from the LED module that is mounted in the headlight housing.

Even knowing that it was futile, we tried putting clamp/snap-on ferrites on the cables to/from the LED modules and its controller, but all that we could manage was a slight reduction in interference that was hard to quantify, taking the problem from being "terrible" to just "awful".  This was not unexpected:  Under the very best conditions ferrites alone may provide 15-20dB of reduction in conducted energy (2-3 "S" units) but at VHF/UHF getting anywhere near that much attenuation is very difficult - and measurements indicated that even if we did achieve 15dB reduction across the board, the "jamming" of weak signals by the headlights' switch-mode controllers would still be significant:  It would be like taking an "10-over S-9" interfering signal down to just "S-8" - still pretty bad!

Methods of filtering

As noted earlier, simply putting ferrite devices on the conductors can reduce the amount of conducted energy, but their effect is typically limited - likely 15-20dB in the best case when this is the only method employed and ferrites alone are likely to be effective only when the interference is very slight to begin with.  Ferrite devices - such as beads - simply add inductance (and thus loss or impedance) to RF energy while leaving DC and low-frequency signals alone, but these devices have limitations:  Properties such as self-resonance and the permeability of the magnetic material vary wildly with frequency and high levels of attenuation are difficult to attain - particularly at high frequencies (e.g. VHF/UHF) where even short conductors carrying RF currents can radiate with reasonable efficiency - particularly when the noise-generating device and the receive antenna are in close proximity.

As discussed previously on this blog ⁵ one sure-fire way to quash such interference is to put the offending device in "jail" - that is, enclose it completely in a metal box and use both inductive and capacitive filtering on each and every wire to prevent RF energy from being conducted and the metal enclosure can prevent direct radiation from leads themselves - particularly important at VHF/UHF but less-so at HF.  Done properly, this method can be "completely" effective ⁶ in preventing interference.

Figure 2:
Simplified diagram of the method of filtering.  The inductance - provided by the ferrite beads ("L")
provide "choking" impedance to the RF currents being carried on the wires from the LED controller
(the "noisy device") before they connect to the feedthrough capacitors ("FT") in order to maximize
their efficacy.
Click on the image for a larger version.

Figure 2 shows a very effective method of dealing with this problem and it involves inductance ("L") and capacitance in the form of "feedthrough" capacitors (marked "FT").  The inductance is in the form of a ferrite bead installed on each of the conductors between the "noisy device" and the capacitors that increase the impedance at radio frequencies on that conductor.  The capacitors are then used to shunt the remaining RF energy to the local "ground" which, in this case, is the partition on which the feedthrough capacitors are mounted (this will be discussed shortly) and also the metal enclosure in which the noisy device is mounted.

The intent here is to prevent RF currents flowing through to the "external connections" where the wires themselves will act as antennas to radiate the RFI generated by the noisy device.  By shunting RF to the partition - and the metal box itself - the remaining RF energy will be minimal and confined within the enclosure.  The above configuration is easily capable of attenuating RF energy from HF through UHF by 30dB or better (more likely 50-60dB at some frequencies) - a value far higher than ferrite alone.

The metal box containing the electronics and filtering offer another important benefit:  As even short conductors a few inches/cm long can radiate at VHF/UHF, placing the noisy device and its conductors within the metal enclosure will prevent this.

An important feature of the design is that the inductances ("L") are located between the noisy device and the feedthrough capacitors.  As these inductances (ferrite beads) offer 10s to 100s of Ohms of impedance to the RF signal, this allows the very low impedance of the feedthrough capacitors at those same frequencies (which is likely an Ohm or less at higher frequencies) to more-effectively shunt that energy.  If the ferrite beads - or some sort of inductor - had not been installed, the shunting of the low-impedance RF energy from the noisy device would have been far less effective.

Figure 3:
An assortment of feedthrough capacitors.  The top two
rows are of the "screw-in" type, typically mounted to chassis-
walls and bulkheads while those on the bottom row are the
"solder-in" type as used in the partition in this project.
Click on the image for a larger version.

In this case, we were preventing RF from a noisy device from leaving the enclosure - but if we were trying to protect a sensitive device FROM RF energy from a nearby transmitter we would place the inductances on the conductors coming from the outside world as well to allow the capacitors to better-perform their function.

Feedthrough capacitors and the partition

A bit more needs to be said about "feedthrough capacitors".  Even if you are "into" electronics, you may not have seen these devices for the simple reason that they are a bit esoteric - and, perhaps, they are not quite as prominent as they have been in the past.  Figure 3 shows an assortment of feedthrough capacitors:  The top two rows are chassis-mounted types that are held in a pre-drilled hole by a nut while the three on the bottom row are of the "solder-in" type.

A feedthrough capacitor has a wire that passes through its center with the "capacitance" surrounding this wire over the length of the of its body and the other "plate" of this capacitor is the body of the feedthrough capacitor itself.  By being constructed this way, there are no wires or leads between the "capacitor" part  and either the signal or ground wires and as such, any series inductance - which would reduce the efficacy of the capacitor - is minimized.

Figure 4:
Solder-in type feedthrough capacitors soldered
to the brass partition.  This large sheet of metal
provides a low-impedance RF path to the
common "ground" (e.g. case) to contain RF
entirely within the metal case.
Click on the image for a larger version.

Compared to a "normal" capacitor with wires, such a capacitor is far more effective at bypassing RF energy to "ground" and it also suffers much less from parasitic issues like self-resonance - a property in which the capacitor and its internal inductance form a resonant circuit can cause it to practically "disappear" from the circuit (e.g. cease to be effective) at certain frequencies.  For this application - where it's particularly important to reduce RF interference at VHF and UHF - the use of feedthrough capacitors is - along with some inductance - a nearly foolproof method of attenuating such energy without resorting to surface-mount components and/or a specially-designed PC board. ⁷

Figure 4 shows nine feedthrough capacitors soldered to a brass partition (the soldering to the plate is on the opposite side) and as can be seen from the photo, these are capacitors that have a wire that runs through them.  As such, they have no "ground lead" aside from the outside of the body of the device and all of them are tied together on the same piece of metal.  This method assures a low impedance RF path between all of the capacitors and since the partition itself is bolted to the aluminum case (see Figure 5), it, too, is well-bonded.

Putting it in the box

Figure 5:
The LED controller in the box w/filtering.  By
containing RF currents within the box, both
common-mode and differential RF currents
on the leads are reduced to near zero.
Click on the image for a larger version.

To eliminate direct radiation from even the very short leads, the LED's controller which can cause problems at VHF/UHF, ferrite beads ⁸ and feedthrough capacitors are contained within a box as seen in Figure 5.

Toward the top of the image we see the switch-mode controller for the LED headlights, bolted inside the case (which also helps dissipate heat) and farther down we see that all nine wires (three for power, the remaining six to the LED module itself) connect to the feedthrough capacitors on the partition. Each of these wires has its own ferrite bead and these wires go directly to their respective feedthrough capacitors on the brass partition, which is held in the case with screws.

Below the partition (see Figure 5) are the wires that connect to the outside world:  On the right are the three wires that go to the power supply (e.g. the original connector to the headlights) while the gray cable on the left goes to the LED module.  The original LED retrofit had very short leads - on the order of 5" (13cm) for the headlight connector and another set of similar length to the LED module - and this made the installation a bit challenging as there was just enough wire to make the connection between the controller and the capacitors on the partition and install the rather large ferrite beads.

The box containing everything is die-cast aluminum and it's a bit larger than necessary - but it was the only size for which I had two identical cases and also large enough to accommodate the LED controller and the filtering.  As this box is quite a bit larger than the original controller, rather long wires had to be used to allow it to be placed where there was room in the rather crowded engine compartment, somewhat away from the headlights.

Figure 6:
As the original cables were very short (about 5", 13cm) longer
wires had to be spliced to allow placement of the large die-
cast box.  This shows the male headlight connector and the
LED module spliced to the cable and covered with nylon web.
Click on the image for a larger version.

At the opposite end of the wires it was necessary to splice the added cable to the LED unit's headlight connector and LED module and this was carefully done using soldered connections insulated with head-shrinkable tubing, all of which was covered by woven nylon braid for protection and a neater appearance.

The result

With the added length of the cables, there were nooks and crannies into which the die-cast box with the LED controller could be placed within the engine compartment of the Honda CRV.

The real test came when a distant 2-meter repeater was keyed up to cause it to send its ID:  The return signal was very weak and noisy - as hoped and expected - but there was no difference in the way that it sounded when the lights were switched on and off.  While admittedly unscientific, this test tells us pretty much everything that we needed to know:  Whatever RF interference there is that might be escaping the box and its filtering is well below the level at which it can be detected and the problem is considered to be solved!

* * * * *

Footnotes:

1. The topic of "very noisy" plug-in USB chargers effectively "jamming" VHF/UHF reception was discussed on this blog several years ago - see:  "How USB car power power adapters can ruin 2 meter mobile reception" - link and its follow-up article:  "A 'quiet' 5 volt USB car power supply" - link.
   
2. This same friend frequently volunteers in public service events involving runners and cyclists on roads where it is required that yellow/amber lights be used to minimize hazards.  Certain makes/models of these lights have been observed to produce tremendous amounts of RF energy that effectively quashed all 2 meter reception,so they were sent back to the seller until he found a unit that was "quiet".
3. Check your local regulations regarding retrofitting of headlights with equipment other than that of the type provided by the original manufacturer.
4. A low-level increase in the noise floor in the proximity of the vehicle would have absolutely no effect on the transmitted signals, but the result of this interference is that the station in his vehicle became an "alligator" - all mouth, no ears - meaning that he was able to "talk" much farther than he could hear.
   
5. Whereas simple capacitor (shunt) or inductor (series - and this includes ferrite devices) may reliably attenuate an offending signal by 15-20dB or so at best (very generally speaking) combining both types of reactance - "L" (inductor) and "C" (capacitor) - appropriately can provide many 10s of dB of attenuation if done properly - easily 30-60dB for simple circuits.  This greater amount of attenuation is far more likely to be able to put the interference from the device well below the noise floor of the receive system.  This is the technique used in footnote #1 (above) and explained in some detail in the blog entry "Completely containing Switching Power Supply RFI" link.
6. "Completely" eliminating conducted RF is actually impossible, but reducing it by 30-60dB is likely to attenuation it below the level of detection.
7. The use of surface-mount components - like capacitors - with their lower parasitic reactance than their counterparts with leads - can be used very effectively to filter RF, but several cascaded stages of such capacitors and inductors - and careful layout of a PC board - are likely to be required to obtain sufficient attenuation.  "Feedthrough"-type surface-mount capacitors are also available - which have excellent performance - but these, too, require a properly-designed PC board.  With the solder-in feedthrough capacitors, a brass partition was used as it was easier to drill and solder to than one of copper.  If screw-in feedthrough capacitors were used, the partition could have been aluminum.
   
8. The ferrite beads used in this project were Fair-Rite 2643000801.  These use "43 mix" ferrite and are 0.295" O.D., 0.297" long and 0.094 I.D. (7.5x7.55x2.375mm) and are able to accommodate the wires + insulation of the conductors from the LED controller.  This material has a typical impedance of about 94 ohms at 100 MHz and cost about $0.24 each in single quantity at the time of writing.  I used them primarily because they were on hand.
   

 * * * * *

This page stolen from ka7oei.blogspot.com

[END]

Using TV (F-connector) 75 ohm splitters and taps in 50 ohm systems on the amateur HF, VHF and UHF bands

I recently posted several articles about using commercially-available splitters link - and making one's own splitters - link - particularly for the HF frequencies and below (e.g. 30 MHz, down to a few 10s of kHz).  A comment was posted asking about how useful inexpensive 75 ohm "TV and satellite" type splitters might be for amateur radio use.

Figure 1:
The assortment of 75 ohm TV and satellite splitters and
taps tested in this article.
Click on the image for a larger version.

Implied by this question is the use of these devices in receive-only or small signal applications:  They cannot be used for transmit purposes as putting even 100 milliwatts through one of these devices is likely pushing its power-handling capability.

I've used these devices in 50 ohm circuits before - typically for VHF and UHF (2 meters, 70cm) where, along with some attenuators, combined the outputs of multiple signal generators to do "multi-tone" testing of receivers - but the question seemed to be a good one.  Rummaging around, I gathered a bunch of devices of various manufacturers and decided to test them for insertion loss and port-to-port isolation.

Note:

Please do not ask questions like "How well does a 'brand X' splitter work over the [fill in the blank] frequency range?"

There have been thousands of makes and models of these devices sold around the world over the past several decades and I simply am not able to find, locate, and measure more than the tiniest fraction of devices that have been sold.  The information given here is expected to be generally representative of the devices available from reputable manufacturers and distributors - but your mileage may vary.

Limitations of the measurements taken:

Because my VNA (DG6SAQ WVNA) was constructed for use with 50 ohm systems (the changing of  both internal hardware components and software would be required for "proper" analysis of a 75 ohm system) I was able only to analyze them in that context - but because the question was about using them in amateur radio service - which presumes a nominal 50 ohm system - I believe that the results are still useful within the limits noted in this article.

Because the emphasis of the question was interpreted as being for amateur-band frequencies likely to be encountered by the average user, the measurement range was limited to frequencies below 1 GHz - in some cases down to 100 kHz.  The nature of the equipment and methods (e.g. 50 ohm test equipment and cabling, the use of inter-series adapters, etc.) used to test the splitters and taps increasingly limits the usefulness and accuracy of these measurements at frequencies above that of the 70cm amateur band (above 450 MHz).

The variety of splitters and taps available:

There are literally thousands of brands and models of TV/Satellite splitters and taps available on this planet - some of them from recognizable names, but most not.  For those devices from sources that might be suspect (e.g. not "name" brands from reputable suppliers) you are on your own to determine the suitability of those devices for your purpose.

Although not intended as an endorsement per se, it has been observed that devices marketed by Holland Electronics appear to consistently meet their stated specifications and is one of the few brands that is likely available worldwide from a number or different sellers - including Amazon - and major suppliers of electronic components and TV/satellite supplies.

Over the years I have seen many dozens of brands and models of these devices - and the vast majority of them are what they are purported to be, but I have run across some devices that claimed to be splitters, but were simply a box with wires connecting the ports together.  In many cases, the casual user would not have noticed anything amiss, but using several of these faux devices in a larger system would certainly result in cumulative signal degradation (e.g. "ghosting" of analog signals, degrading of quality - but not necessarily signal strength - of digital signals).

General types of devices:

There seem to be three general types of these devices out there:

• "TV" and/or "VHF/FM/UHF" and/or "CATV" - These devices are typically designed to operate over the range of off-air TV stations across the world and the frequencies typically found on receive-only cable TV (with no Internet), encompassing the frequency range of about 40 MHz through 700 MHz, more or less.  While useful for use on the amateur bands from 6 meters through 70cm, inclusive, their usability on HF or above this range is limited as noted in the testing, below.

• "Satellite" splitters - These devices are typically designed to operate starting at about 900 MHz, often extending to 1500 or as high as 2500 MHz, depending on the vintage and intended use.  These devices are not usable on the 70cm amateur band frequencies and below.

• "TV/CATV/Satellite"- These devices are of a bit more recent vintage and are designed to accommodate a very wide range of frequencies - often from about 5 MHz through and above 2000 MHz - a band that includes off-air, cable and "L-Band" satellite signals - plus the "reverse" channels (sometimes called the "T" channels) often used by "cable Internet" modems that reside below 45 MHz.  These are the most useful to amateur service and can often be used on HF through 70cm.

If you do not see a specific frequency range noted on the device itself, assume the worst-case, smallest frequency range that covers that usage category - unless you can test them yourself.

* * *

General findings

For the TL;DR types, here is a summary of the results of the measurements described in more detail farther down the page.


Using 75 ohm devices in 50 ohm systems:

The most obvious issue is that TV-type consumer devices are almost universally equipped with type "F" connectors which means that one must use either an adapter or use a cable with an attached "F" connector.

Figure 2:
Left to right:  Two BNC female to male F connecitrs with an
F-type 75 ohm terminator on the right.
Click on the image for a larger version.

For receive-only systems, it's not too uncommon to simply use a 75 ohm cable like RG-6 - which is quite low loss and very inexpensive - to connect a 50 ohm antenna to a 50 ohm receiver.  The effects of this apparent "mismatch" are typically minimal as most receivers are only "approximately" 50 ohms, anyway.  In theory, the use of 75 ohm cable on 50 ohm devices will result in a 1.5:1 mismatch and commensurate losses, but this sort of mismatch is commonly observed on many antenna systems that are ostensibly designed to operate at 50 ohms and is usually of minor consequence.

When using an inexpensive cable like RG-6, it's worth noting that most of these cables use copper-coated steel (CCS) center conductors which may have implications for DC resistance of power is being sent on this cable (for a preamplifier, converter, controls) as this type of cable will have far more total resistance than one with a solid copper center conductor.  Copper-coated steel center conductors may also have implications in terms of skin effect at low frequencies (low HF and below) - but this is beyond the scope of this article - see, instead, this article by Owen Duffy from the Internet Archive.  There exist cables with copper-coated aluminum (CCA) center conductors that have lower DC resistance that CCS cables, but they tend to be more fragile due to the tendency of the aluminum center conductor to become brittle with flexure.

The device itself (splitter, tap) is designed primarily for 75 ohms and this means that its performance will be somewhat degraded in a system that is "completely" 50 ohms (e.g. 50 ohm cables with F-connector adapters) but these effects are largely as follows:

• The "through" loss may be slightly higher.  In the case of a 2-way splitter, the ideal loss will be 3dB - but even at the proper impedance, it will be slightly higher than this due to component losses, typically in the area of 3.5 dB.  Practically speaking, the main effect of using a 75 ohm splitter in a 50 ohm system was a slight change (only a few tenths of a dB) in the loss.

• Reduced isolation between ports.  The most obvious effect on splitters was that the isolation between ports (e.g. the "out" ports of a 2-way splitter) was reduced.  Compared to some specialized splitters, the isolation of inexpensive, consumer-grade "TV" splitters is lower overall.  As can be seen from the graphs, below, operating in a 75 ohm system resulted in better isolation - sometimes over 40dB at certain frequencies - but this assumes that all loads and sources are well-matched to 75 ohms, something that is not likely to be the case in a real-world installation.  Typically, isolation reduced to something in the 20dB area when operated in a 50 ohm system.  In many cases, this is "good enough".

• In splitters and taps, resistors are major components in determining their "native" operating impedance.  For example, a 75 ohm splitter or tap, depending on design, may have a 150 ohm or 37.5 ohm (2 times and one-half 75 ohms, respectively) resistor contained internally.  In theory, changing this resistor to a value appropriate for 50 ohms (typically 100 or 25 ohms) would optimize performance at 50 ohms - but doing this may or may not be worth the trouble. 

In short:

Unless your situation requires precision, the use of inexpensive, TV-type splitters and taps of the types described on this page will yield "reasonable" performance over the design frequency range - provided that the device is constructed as described by a reputable manufacturer.

The use of a (nominally) 75 ohm device in a 50 ohm system will require using connectors that are not normally used in 50 ohms systems (typically "F" connectors) which means that adapters of some sort will be needed - the expense, bulk and inconvenience of which must be considered in the overall design.

Finally, note that the above comments are for the general case:  Remember that your needs, requirements and results may vary and that you must do your own analysis and testing to verify that such components are appropriate in your specific case.

* * *

Plots of various devices:

Below are selected plots of devices representative of the types on-hand.  In general, devices with similar stated ratings performed in the same manner.  In all of these plots, the insertion loss is represented by the blue line while the complex impedance data is depicted on a Smith chart in the middle:  Numerical data at the frequencies indicated by markers is seen in the lower-left corner of the screen.  Again, remember that at higher frequencies, the nature of the 50 ohm test system, connecting cables and adapters will increasingly skew the results - particularly those depicted by the Smith chart.

The interpretation of a Smith chart will not be covered here, but there are many online resources that describe its use including this video in a series on this topic by W2AEW on his YouTube page.

A "satellite" splitter:

Figure 3:
The "through" loss of the HFS-2 splitter represented by the blue line
across the top.
Click on the image for a larger version.

This device - a "Tru Spec HFS-2" is representative of those intended for use on an (older) L-band system found in satellite receive systems, having on its label a "900-1500" MHz frequency range.  As noted above, the limitation of the measurement set-up made measurements above the 70cm amateur band (in the 440 MHz area) suspect - but the object here was to see if it was usable below that range.

At initial glance, the "through loss" of this device below 900 MHz (Figure 3) might seem to indicate that it worked below this frequency, but notice that at lower frequencies (below 50 MHz) indicates a loss less than 3dB indicating that it is not working as a proper 2-way splitter.  A look at the isolation plot (Figure 4) tells more of the story.

Figure 4:
Isolation between ports of this splitter.
Click on the image for a larger version.

As can be seen, at about 900 MHz and above, the apparent isolation between ports is reasonable but at 2 meters (146 MHz) it is only 3dB verifying the fact that at these lower frequencies, it less a proper splitter, but more equivalent to a device where the three ports are connected with a piece of wire.  The apparent isolation increase at low HF is more likely an artifact of its construction - the insertion loss being below 1 dB (in Figure 3) verifies this.

In short, these "Satellite only" splitters aren't really useful on TV and CATV frequencies or the amateur bands 70cm and below.

A "TV" splitter:

Figure 5:
The "through" loss of the Archer splitter.
Click on the image for a larger version.

I tested several splitters that were intended for general VHF/UHF/FM use - one of these being an "Archer" (Radio Shack) two-way splitter being typical of that type.  The implied frequency range is from at least 54 MHz to 700 MHz - the extent of the cable TV, FM broadcast, and off-air VHF and UHF TV frequencies at the time it was made.

Figure 5 shows the measured "through" loss in a 50 ohm system.  Compared to a plot done at 75 ohms (not shown, using resistive matching) the insertion loss barely changes across the frequency range.  In both 75 and 50 ohm systems, at least at 2 meters, down to 20 meters (14 MHz) seems to be "ok" - but the "dip" in the 3-4 MHz area - and the fact that the attenuation below it drops below 3dB - indicates that it's not likely acting like a splitter at these lower frequencies.

Figure 6:
Port to port isolation at 75 ohms for this splitter.
Click on the image for a larger version.

Figure 6 shows the port-to-port isolation at 75 ohms and we note that in the "low" and "high" VHF band (U.S. channels 2-13 - which more or less includes the 6, 2 and U.S. 222 MHz amateur bands, that the isolation is quite decent - well above 20 dB.

From this plot we can see that the "dip" in the 3-4 MHz area seen on Figure 5 is quite telling as the port-to-port isolation is pretty much gone below this frequency

Figure 7:
Port to port isolation in a 50 ohm system for this splitter.
Click on the image for a larger version.

The plot of Figure 7 shows what happens if the splitter is operated in a 50 ohm system.  The main effect is that the port-to-port isolation is reduced - being on the order of 15 dB or so from the 20 meter band through the 2 meter band (14 MHz - 144 MHz).

From this we can conclude that this splitter is quite usable from the middle of the HF spectrum through at least 2 meters - and is probably usable through 70cm.


A "TV/CATV/Satellite" splitter - preferred for HF use:

Figure 8:
Holland HFS-2P through loss in a 50 ohm system.
Click on the image for a larger version.

I have on hand several splitters that have on their label a frequency range that starts at (typically) 5 MHz with a high end of between 600 MHz and 2450 MHz.  The reason for this extended "low end" is likely due to their being designed for use in systems that have "Cable Internet" where the return (upstream) signal from the user's modem to the cable system are likely to be in the 5-50 MHz (or, possibly, a bit higher) range.  The plots included are those of a Holland Electronics HFS-2P which is a 2-way splitter/combiner that has a stated range of 5-2050 MHz and the results of this device are typical of that type.)

Figure 8 shows the "through" loss in a 50 ohm system showing a reasonable insertion loss (4 dB or below) from below 40 meters (about 5 MHz) through at least 70cm (440 MHz) - but again, the limitations of the measurement set-up make readings higher than this a bit suspect.

Figure 9:
Port-to-port isolation at 50 ohms.
Click on the image for a larger version.

Again knowing that the "isolation" measurement is the way to get the "true" story, port-to-port isolation in a 50 ohm system is depicted in Figure 9.

This verifies - to the extent that the test set-up can - the 5-2050 MHz range showing that the port-to-port isolation from 5 MHz to 1 Ghz is well over 15dB.  A port-to-port isolation measurement at 75 ohms (not shown) is slightly better (by a few dB) over the same range.

The combination of Figure 8 and Figure 9 show that this device may be usable down to the 160 meter band (1.8 MHz) provided that a slight amount of extra insertion loss (about 1dB) and lower isolation (approximately 12dB) can be tolerated.    (The Holland HFS-2D has characteristics similar to the HFS-2P down to 1.8 MHz.)

Figure 10:
The through loss, the other 7 ports being terminated with 75 ohm F-type
connectors.  The insertion loss is reasonable - between 10.5 and
11.5 dB over the range of 1.8 to 450 MHz.
Click on the image for a larger version.

An 8-way splitter:

The final splitter to be tested was the Holland Electronics GHS-8 8-way splitter-combiner.  Often, splitters with an even number of outputs greater than two contain multiple two-way splitters which means that this 8-way splitter might contain seven such devices - but I didn't break it open to check.

Figure 11:
The port-to-port isolation between two adjacent ports with the "in/out"
port and unused ports terminated with 75 ohm "F" loads.  The apparent
isolation is on the order of 35dB from 1.8 through 450 MHz - but this would
likely drop to something closer to 20dB.
Click on the image for a larger version.

Figure 10 shows the typical "through" insertion loss with the seven unused ports being terminated with 75 ohm "F" type terminators:  I don't have enough F-male to BNC-female adapters on-hand to terminate the 7 ports at 50 ohms - but if one were going to use one of these devices, it's probably more convenient to use F-type terminators on the unused ports, anyway.  The typical "through" loss is measured to be about 10.5-11.5 dB - slightly higher than the predicted "ideal" 9dB insertion loss, but typical for these devices.

The port-to-port isolation was also measured and the use of 75 ohm terminations on the other ports and the "common" in/out port likely improved this:  The isolation would likely be significantly worse if all ports were at 50 ohms, for the same reason as the other splitters tested.

Based on these readings, this device is useful down to 1.8 MHz and up through 2 meters - and probably 70cm.

Figure 12:
Coupling coefficient at 50 ohms for this tap
Click on the image for a larger version.

A TV-type signal "tap":

Likely unfamiliar to many, a signal "tap" is a very useful device in multi-drop TV installations found in hotels, hospitals and other larger buildings.  Unlike a splitter - which usually divides a signal equally to its output ports - a "tap" will siphon only a certain amount of signal off the cable and leave the majority of it intact - which is very useful for systems such as those in a hotel or hospital to distribute and split a signal hundreds of times to serve all of the devices.

In some ways it can be considered to be similar to a part of an SWR bridge where only a small amount of signal is sampled - and in only one direction - allowing the majority of the original signal to pass with minimal loss.  Several taps - all from Holland Electronics - were tested as they were what was on-hand and the "DCG-6SB" is represented in the plots. 

Figure 13:
The "reverse isolation" loss of the tap (e.g. turned "backwards") with a
50 ohm termination.
The reverse isolation is described as being the absolute amount of isolation
(e.g. that in the chart above) minus the coupling coefficient which means
that the actual forward coupling loss - which means that using Figure
12, we know that the actual reverse isolation is about 7 dB lower than
indicated by the graph above.
Click on the image for a larger version.

Figure 12 shows the "coupled" energy in a 50 ohm system:  Compared to the coupling in 75 ohm system (now shown) the insertion loss was slightly higher (about 1dB) but the frequency loss/flatness was about the same, being pretty consistent from about 1.8 MHz through 1 GHz.

Figure 13 shows the reverse isolation of the tap:  Rather than 6dB of coupling from the main line for signals going the "other way", the absolute is closer to 20dB - about 13dB lower.  (The actual reverse isolation is the absolute isolation minus the forward loss).  In a 75 ohm system (not shown) the reverse isolation was quite a bit better (closer to 30dB over the 5 MHz-1GHz range) - but this result is completely expected:  The reverse isolation is akin to measuring VSWR, and operating a 75 ohm device at 50 ohms implies a VSWR of 1.5:1 - a "return loss" of 14dB - very close to the values depicted in Figure 13 over much of the frequency range when the "forward" loss is taken into account.

On a tap there is yet another measurement to be taken - the loss between the in and out port.  Because we are measuring a 6dB tap - a device which siphons off about 25% of the signal - we would expect at least that amount (theoretically 1.25dB for 6dB) to be lost as it is coupled to the "tap" port. Figure 14 we can see that the measured loss is slightly higher than this between 1.8 and 200 MHz- a bit over 2dB.  Some of this "extra" loss is due to the intrinsic losses of the device, but a smaller amount is a result of the use of a 75 ohm device on a 50 ohm system.

Figure 14:
Through loss of the 6dB tap in a 50 ohm system.
Click on the image for a larger version.

This device - which is rated down to 5 MHz - may be useful through at 160 meters (1.8 MHz) - but the insertion loss goes up rather quickly at lower frequencies.

This device is NOT suitable for passing DC (e.g. for amplifiers, control signals) as it has a DC short across it - but that is not true of all taps.  For example, the Holland Electronics "HDCS" series does allow low frequency RF down to DC to flow through it - but like the DCG-6SB, its coupling coefficient deteriorates quickly below about 1.8 MHz.

* * *

General conclusions:

If you are going to use TV-type splitters for HF, make sure that you get devices that are explicitly rated down to 5 MHz.  Based on the (limited!) sample of devices that were tested, these devices can be expected to work into the 160 meter amateur band (down to 1.8 MHz).  While these devices may be usable thoughout the entire AM broadcast band (down to 540 kHz) expect performance to drop quickly in terms of added "through" attenuation and worse port-to-port isolation.

A "TV" type device - one that may indicate a start frequency of 5 MHz, or just any device that is claimed to work at TV (VHF/UHF) and FM broadcast frequencies will likely work from 6 meters through 70cm (50 MHz - 450 MHz).

Again, for general signal splitting and combining, these 75 ohm devices, used at 50 ohms, are quite usable for non-critical applications - provided that they be used at low power levels (a few 10s of milliwatts at most) and where one need not have precise 50 ohm matching and high port-to-port isolation.  Remember that most 50 ohm devices (receivers, amplifiers, filters) have only "approximately" 50 ohm source/load impedances - and filters in particular will, out of their design frequency range (outside the band-pass, on a notch frequency, above the low-poss cut-off, below the high-pass cut-off) will likely have anything but a 50 ohm characteristic impedance, so even a "proper" 50 ohm splitter/tap device would not necessarily yield any better performance in those situations.

For information about the design and use of splitters/combiners in general, a good reference is Mini-Circuits AN10-006, "Understanding Power Splitters" - link.

* * *

Far more data was gathered than was presented here, but that depicted above is representative of the devices that were on hand.

* * *

This page stolen from ka7oei.blogspot.com

[End]

Examining the Glencom VC510 UHF to L-Band Upconverter

This is a curious little device - of which several have fallen into my hands.

Often available on EvilBay for fairly cheap, these are in some nice, die-cast Hammond (tm) aluminum boxes approximately 7.25"L x 4.5625"W x 2.125"H (185mm x 118mm x 54mm) in size with two good-quality "N" type connectors connected with short lengths of UT-141 PTFE cable and an board-mounted "F" connector.

The question that seems to be asked by others who run across these devices on the GoogleWeb is "What are these for?"

Well, I can answer that.

Figure 1:
The case of the VC510 "Upconverter"
Click on the image for a larger version.

From the early 90's and into the mid 2000's Hughes Network Systems had a VSAT (Very Small Aperture Terminal Satellite) product referred to as "ISBN" - and an early version of this was called the "Type 2" with much of the hardware being made by NEC in Japan.  Connecting the rooftop satellite transceiver - typically operating in the U.S. market on the Ku band - to the indoor data interface unit was a single coaxial cable that carried not only the power, but all of the myriad control signals used for transmitting - but also the entire 500 MHz of the satellite passband.

Now, you would think that, like everything else satellite that the receive signal would occupy the "L-Band" range of 950-1450 MHz, being down-converted from 11700-12200 - but you would be wrong.  For various and sundry technical reasons, the receive signals were conveyed on the cable from 1000 to 500 MHz - "upside-down" owing to a "high-side" local oscillator within the rooftop unit itself, making it incompatible with L-band gear.

Except that NEC/Hughes had thought of that:  They'd handily included a simple converter within the unit that, using a 1950 MHz oscillator, converted that "upside-down" signal to the proper 950-1450 MHz L-band signal again.

Except that it didn't really work all that well.

Figure 2:
The circuit board of the VC510.  There is also a version that has a
surface-mount 74LS parts instead of the DIP parts shown that is
(pretty much) electrically identical in all other ways.
Click on the image for a larger version.

You see, at about this time, digital signals - data, voice and video - were starting to appear on the satellite bands and this built in L-band converter - while adequate for wideband analog video signals was too unstable and inaccurate for digital signals so the device pictured above was devised to fit the bill, doing what the built-in converter should have done correctly in the first place!

Dissecting the VC510:

Essentially, the VC510 does the same thing as the converter in the original NEC unit should have done:  Mix the 1000-500 MHz signals with a 1950 MHz local oscillator to yield a stable, clean 950-1450 MHz L-band signal - but how did they do it?

To answer this question, I decided "reverse-engineer" the board and came up with the diagram, below.

Figure 3:
A reconstructed circuit diagram of the VC510.
The component designations are arbitrary and are not marked on the board anywhere but with a a circuit this straightforward, it should be pretty easy to work out what's what!
Click on the image for a larger version.

How it works - The frequency converter portion:

The signal from the rooftop unit is coupled via the "line sampler" - a stripline directional coupler etched onto the circuit board that also extracts a bit of the DC power from the coaxial cable as well:  This directional coupler has a negligible effect on the signals passing through it.

From this directional coupler is an elliptical-type low-pass filter that removes signals above approximately 1000 MHz (there may have been a signal at around 1350 MHz - I don't know this for certain) and is amplified by U1 by about 12dB which is then applied to U2, an RMS-11X doubly-balanced mixer which causes a loss of approximately 7 dB.  Mixed with the 1950 MHz signal from the local oscillator the output is passed through an attenuator and then another low-pass filter with a cut-off frequency of approximately 1800-2000 MHz and then amplified by U3 by for another 12dB gain which is the L-Band output.

The local oscillator:

Q1, an AT-41511 transistor along with varactor diode D1 forms a VCO, the output of which is coupled via an attenuator pad to U4, a MMIC that amplifies the signal by 10 dB - some of which is siphoned off and applied to U6, an MB506 divide-by-256 prescaler that takes the 1950 MHz signal down to 7.6171875 MHz (when the PLL is locked) - while the remainder goes to U5 to be amplified again and applied to U2, the RMS-11X mixer.

The main reference oscillator is based around a 7.6171875 MHz (approximately!) crystal, using a 74LS00 NAND gate and fed to a pair of 74LS74 D-type flip-flops wired as a "charge-pump":  If the frequency is too high, a bit of charge is subtracted from C28 and added to C29 and vice-versa if the frequency is too low. U9, a TL071 op amp which is used as a loop filter/integrator and does the phase/frequency control, locking the VCO to the frequency reference provided by the crystal.

In all, there's nothing about the above circuitry that is particularly fancy or requires exotic components - just the application of fairly inexpensive, standard components using designs that had been around since the late 60's or early 70's - except, perhaps, for U6, the prescaler.

Notes:

• U1 and U2 are very similar to the MSA-2086 (but a different package) and good from DC to at least 2.5 GHz and typically have 10-12 dB gain over this range and a 6-7 dB noise figure with a 1dB compression power output of around +4dBm  The typical bias current is 25 mA with 5.0 volts at the output terminal.   This device is generally equivalent to the Mini-Circuits MAR-2.

• The MSA-1105 used for U4 and U5 is good from below 50 MHz to 1300 MHz at the -3dB points with a typical gain of 10-12 dB and usable to over 2 GHz with a gain reduction to around 6dB.  Up to 1.3 GHz the 1dB compression power output is typically +18dBm dropping to around +15 dBm at 2 GHz with the noise figure below 1 GHz typically being below 4 dB and rising to around 5.5 dB at 2 GHz.  The typical bias current is 60 mA with 5.5 volts at the output terminal.  This device is generally equivalent to the Mini-Circuits MAV-11.

Testing on the workbench:

Surprisingly, the unit produced a fairly good CW "note" - almost suitable for CW/SSB operation - something that could have probably been cleaned up had a better crystal reference oscillator used.  With no modification at all, the VCO's lock range turned out to be approximately 1600-2150 MHz by varying the frequency fed to the crystal oscillator from 6.25-8.398 MHz - but it could probably extended by modification of the cutting/bridging some traces in the VCO section.

As it is, the "gate-type" crystal oscillator based on the '7400 is not accurate/thermally stable enough for SSB/CW operation - or even narrowband FM operation - so if this sort of operation is anticipated, a different, more thermally-stable oscillator is likely required!

So, what's it good for?

It's hard to say, but some of the ATV folks seem to have found use of these devices as 23cm and/or 13cm ATV converters and in theory it could be used to convert 2 gig WiFi to other frequency ranges or even be the front end of a simple spectrum analyzer for the low GHz range.

Because the RMS-11X mixer is rated for as low as 5 MHz on all ports, up to 1000 MHz on the IF port (to which the F-connector sends the signal) and to 1900 MHz on the LO and RF ports, it should be perfectly usable to at least 2500 MHz - perhaps higher, especially if preceded with a low-noise amplifier.


A few comments about modification:

- L1/L2/L3 are circuit board inductor traces that can be sliced.  If the accompanying capacitors are removed, the low-pass response of this filter is eliminated and useful response is extended well past 2 GHz.

- L4 is a circuit board inductor and its low-pass response is also eliminated if its accompanying capacitors are removed.

- If the L1-L4 filtering is removed, additional (narrowband) filtering for the frequencies of interest should be added to the input and output to prevent/minimize spurious responses.

- As noted on the schematic, there are also some traces that could be sliced/jumpered in the VCO section.  It is likely that modification of these could change the VCO tuning range from that noted above.

Please note that the rating of the prescaler, MMIC amplifiers and the mixer would limit the upper end of the useful range of the VCO to something in the 2.2-2.4 GHz range at most, but it should be possible - in theory - to take the VCO down to well below 1 GHz with the addition of a physically larger inductor.  If this is done, one might want to rewire the prescaler as well to give a different divisor ratio (e.g. divide-by-128 or even divide-by-64) using the information on the diagram.


Now you know!

[End]

This page stolen from ka7oei.blogspot.com