Repairing a NanoVNA "V2" with nonsensical readings ("Blown up" input)

Figure 1:
The SAA-2N, a variant of the Version 2.  As noted in the text
I chose this version as it had N connectors and was suited
for my specific needs and has served well for several years.
Click on the image for a larger version.

The NanoVNA has become a ubiquitous device for the RF toolbox:  For less than US$200 you can get a device that is reasonably capable, accurate and covers a usefully-wide frequency range - from a few 10s of kHz into the GHz range, depending on the model.

The original NanoVNA - sometimes found for less than US$50 - works reasonably well up to about 250-ish MHz - the limit of the frequency range of its RF synthesizer (typically an Si5131).  Using harmonics for the higher ranges allows the unit to "work" higher than this, but with the dropping gain of the mixers and lower power of the higher-order harmonics diminish its usefulness and it pretty much "runs out of steam" by the time one gets to around 1 GHz.

The "Version 2" of the NanoVNA (which includes many variants, such as the "H2") improved on this using a different detector (often the Analog Devices AD8342 which is rated to work above 3 GHz) and two RF synthesizers - the original Si5351 and another device (usually the Analog Devices ADF4350 or similar) that takes over from the '5351 at the higher frequencies (usually above around 140 MHz) up through 3+ GHz.  The different architecture of the Version 2 necessitates greater complexity - which also implies increased vulnerability as we'll see.

Note:  This article will generically refer to the units thus-constructed as "V2" NanoVNAs - regardless of who actually makes/sells them.

Background

When I looked for an "upgrade" to the original NanoVNA I was looking for a device that would have durable connectors that would withstand hundreds of connect/disconnect cycles and be able to hang the unit from the interconnect cables themselves to measure gear "in situ".  This "need" was due to its expected use:  Being dragged around in the equipment box to repeater and radio sites, the houses of other amateurs, and being used on the workbench - and almost never above about 1.5 GHz - a frequency range for which I have other gear, anyway.  Having friends with other NanoVNA V2 variants, I've heard how physically fragile these devices are - mostly related to the supplied cables and the SMA connectors themselves, and how they may be physically attached to the circuit board.  All of this ruled out anything with SMA connectors.

For this reason I got the SAA-2N (metal case with "N" connectors) and have used it enough that the nickle plating is starting to wear off the threads of the N connectors, indicative of a number of connect/disconnect cycles (now in the hundreds) that would have trashed even the best-quality SMA connectors:  Had anyone else made a device as physically rugged as this, I'd have considered it - but as of the time of this writing, no-one else has done so!  (Why not?)

The problem

One issue that can befall users of all NanoVNA "V2" variants is that of static sensitivity - particularly on the S11 port (e.g. "Port 1" or "CH0").  While some users on the forums suggest that this is due to "inferior" components of some of the clones, this is simply false:  It's a result of the way that the unit is designed - and the components necessary to execute it which are intrinsically more vulnerable to ESD (ElectroStatic Discharge) than simpler versions.  

Additionally - and speaking for myself -  I'm much more likely to use this unit - with its "N" connectors - "out in the field" where it may be exposed to hazards (ESD) than someone who gently rests their NanoVNA on their workbench for its entire life.  At least for me, this increases the probability that a unit that is more likely to be used out in the field - away from the "safe" workbench - will be damaged in some way, electrically or mechanically.

Specifically, the problem is the Maxscend MXD8641 RF switch found in several places on the "Version 2" NanoVNA variants (e.g. those that work past 3 GHz).  This is a SP4T (4-way) RF switch used to route the signals that, among other things, is used to switch between the Si5351 oscillator and the ADF4350 (or similar) - but one of these devices may also be found right on the S11 port - and that's where the problem lies.  (This device is not on the original NanoVNA, which seems to be more rugged in this respect.)  As the list price of the MXD8641 is well under US$0.07 in any sort of production quantity, it's unlikely that so-called "cheap counterfeits" are being used.

While the data sheet for this device indicates that it has good ESD protection, it's clear from the NanoVNA forums - and my personal experience and that of friends who also have a V2 NanoVNA - that this protection is NOT necessarily adequate for in-the-field, casual use.  If your NanoVNA V2 never leaves your static-free workbench, you may never have this sort of problem - but if you drag it out in the field like I do, you may well have run across situations where the NanoVNA becomes damaged.

Figure 2:
If the RF switch chip is has a "dead" short to ground,
you may end up with chaos of lines like this after you do a
"short-open-load" recalibration.
Click on the image for a larger version.

As it happens, a good-quality coaxial cable is capable of storing quite a bit of energy, stored as voltage.  With a typical 50 ohm coax having around 30pF per foot (100pF/meter) this, coupled with triboelectric effects - not to mention static on your person or on the antenna to which you might connect it - pose a hazard to sensitive electronics, and this is device is no exception.

Considering that the specifications for this part note that it should withstand 1kV from the "human body model" (100pF discharged through a 1.5k resistor) it's easy to see that discharging even a hundred or so volts from 3000pF of capacitance - represented by about 100 feet (28 meters) of coaxial cable - and not having any series resistance - could easily damage this part.  One can trivially develop hundreds of volts across a piece of coaxial cable just from handling - not to mention that which might be developed across an antenna from wind static!

When a NanoVNA V2 goes bad

On the NanoVNA forums I've seen people commenting on how they are suddenly getting nonsensical readings on their V2 (see Figure 2) or the apparent inability of the unit to maintain calibration - and this is indicative of possible damage to the MXD8641 found on the S11 port, likely caused by voltage discharge:  With a 0.047uF series coupling capacitor, it would not take much accidentally inject enough energy to damage that part!

My own NanoVNA V2 was, itself, damaged through discharge after having used it for several years without incident, apparently when connected to a fairly long length of coaxial cable - despite reasonable effort and care.  I have also encountered others with V2 NanoVNA variants that have also experienced similar issues and in all of the cases where I have been able to get the unit into my hands, it's been due to damage of the MXD8641.  One thing in common with these other failures is that these folks often - but not always - use their NanoVNA V2 places other than just on the workbench - to test antennas, cables, etc. out in the field:  I don't know about you, but I can detect a common thread here!

Comment:  

As can be seen in Figure 3, there is a 0.047uF monolithic ceramic coupling capacitor.  Ideally, a capacitor tester would be used to verify its integrity, as would an ohmmeter to assure that it's not shorted - even intermittently.

It is possible - particularly if the NanoVNA V2 has board-mounted SMA connectors - that if the things are flexed repeatedly (tightening of the connectors or other mechanical stress) that this capacitor and/or the solder joints in the signal path have broken or become intermittent.  It would be a good idea to use an ohmmeter to "buzz" out continuity from the end of the cable to the "connector" side of the coupling capacitor, and also between the other (circuit-side) lead of the capacitor and the trace.

Sometimes, reflowing both ends (allowing time in between for the two solder connections to cool) will restore connectivity if the solder joint has cracked - but note that the capacitor itself may be mechanically intermittent and thus require replacement, anyway.

Diagnosis:

Having corresponded with several others who have had the misfortune of damaging their V2 NanoVNAs, there are several ways that the damage will manifest itself that may appear to be totally different, but are actually the caused by the same problem.

• Sudden high return loss everywhere.  Sometimes the discharge will cause the '8641 to simply fail dead-shorted and the user will see a very high VSWR on everything and if one configures the unit for an S21 (e.g. "through") reading, the apparent insertion loss is extremely high - indicated by a very high noise floor (after attempted recalibration) as compared to what it was when working.  If one attempts to recalibrate (Open/Short/Load) a completely nonsensical display like that in Figure 2 is likely to result after doing so.
  

• Unit will not seem to "hold" calibration well.  This seems to be the more common failure:  The user will suddenly find that the unit is out of calibration for no obvious reason - and this is often worse at higher (VHF/UHF) frequencies than at HF (30 MHz or lower).  Using the Open/Short/Load, the unit will seem to calibrate, but the noise floor in "S21" measurements will likely be higher - and the calibration will seem to drift all over the place over time and with temperature.

This second failure mode is a bit more sinister in that the user may, at first, presume that the loss of calibration was a result of user error.  What has actually happened was that rather than fail shorted, the '8641 will be damaged, often putting between 30 and 80 ohms of resistance between the input port and ground  - a value that will vary significantly with temperature and over time.  This is insidious in that the unit may "seem" to be working - but it will likely give incorrect readings, even if it seems to calibrate properly.  This causes two problems:

• Calibration drift.  The NanoVNA's source impedance is nominally 50 ohms - but placing a varying resistance across this - possibly in the 30-80 ohm range - will lower this, causing calibration to seem to vary despite frequent recalibrations.  As noted above, if an "S21" measurement is done (e.g. a "through insertion loss"), the shunting of the source signal (on CH0/Port 1) will reduce its amplitude - possibly significantly - and you may notice that the noise floor is higher than expected as the signal levels have been reduced.
  

• Misleading results.  What's worse is that even though the unit will seem to calibrate correctly using the Open/Short/Load, the fact that it is no longer sourcing or loading 50 ohms on the Ch0 port can cause misleading readings if you are trying to sweep a filter, antenna, cable or other device that is expecting a source impedance in the area of 50 ohms.  This may be proven by using a known-good 50 ohm cable of significant length (10s of feet/meters long) and terminating it with a known good 50 ohm load:  Ideally, it should remain "flat"(low VSWR/return loss) but if the unit itself no longer sources/loads 50 ohms, this test will reveal something other than a flat response.
  

  

  Figure 3:
  The screwdriver tip is touching the "circuit" side of the
  0.047 uF coupling cap.  It is between this point and ground
  that one would test with a digital Ohmmeter to determine if
  the RF switch chip has been damaged.
  Click on the image for a larger version.
  

A quick check with an Ohmmeter will reveal the problem and figure 3 shows where a measurement may be made, on the "circuit" side of the blocking capacitor on the CH0/Port1 terminal. This photo shows the SAA-2N, but your model may have a slightly different layout, but it should be pretty easy to locate this capacitor and do a similar test on other variants.

If all is well, a typical digital Ohmmeter will likely read around 10k or higher with the unit powered down - but if you get anything lower - specially if it is in the hundreds of Ohms or lower (it is likely to be in the 10s of ohms if you have noticed a problem) - the MXD8641 is likely blown.  (Do NOT use the "diode test" function of your Ohmmeter.  Be sure to check your Ohmmeter in both directions to rule out a reading a protection diode on the chip.)

As mentioned earlier, the MXD8641 is a very inexpensive part - but it's difficult to find from U.S. suppliers - and for this reason I sourced a strip of the MXD8641 on EvilBay with each part costing well under US$1.00, including shipping.  Replacing this part, however, is another matter as we'll see shortly.

A promising equivalent part would seem to be the Skyworks 13414-485LF (available from DigiKey) which is also a SP4T switch - in the same package and with the same pin-out - but despite what look like identical specifications, parameters and truth tables in the data sheet, the units that I got from DigiKey did not work in the 'V2 for reasons still unknown.

Figure 4:
Highlighted section shows the installed TVS - the 30 volt
0603-package part mentioned in the text.  It is laying on its
side, which was the easiest orientation for mounting.
Click on the image for a larger version.

Input protection

If one makes it standard practice to always place a resistive attenuator (say, 6-10 dB) on CH0/Port1 of their VNA - and does calibrations with it in place - this may reduce the probability of damage, but doing such may complicate certain types of measurements due to the added loss and reduction of signal levels.

Unfortunately, none of the V2 variants that I've seen have included overt protection in addition to that intrinsic to the MXD8641 - and this would likely be in the form of a low-capacitance TVS diode.  As it happens, there are a number of TVS diodes that have very low capacitance (0.5pF or less) that are specifically intended for protection of GHz-range devices (HDMI, USB3 and RF/antenna) which make the suitable candidates for use with a NanoVNA.

The device that I chose was the Inpaq EGA10603V12B0DG - a 12/30 volt, 0.2pF part in a 0603 SMD package (DigiKey P/N:  3526-EGA10603V12B0DGCT-ND).  There are other devices with low capacitance with even lower voltages (which would have been preferred) but these were available only in 0402 (or smaller!) packages, making their handling very difficult:  As it is, a 0603 package is about 1/4th of the size of a grain of rice!

With a fine tip and small-diameter solder, it's possible to add the TVS diode.  Figure 4 shows how this might be done on the SAA-2N:  Adjacent to the blocking capacitor, the green coating is scraped off the board to bare the ground plane and the TVS diode is installed.  I find it a bit easier to set the TVS on its side, but your mileage may vary.

You will note that the TVS is placed on the "circuit" side of the blocking capacitor rather than on the antenna terminals.  The reason for this is that if there is a voltage across the input (and thus across the capacitor) that is then shorted, the energy of the 0.047uF capacitor (plus any "ringing" from inductance/resonance) will be dumped into the circuitry.  By placing the TVS on the "circuit" side, it will be able to dissipate at least some of the energy dumped by the capacitor, placing an absolute limit on the voltage peak applied to the '8641.

How much effect does this TVS have on the readings?  I've added one to a perfectly-functioning Version 2 NanoVNA and then re-checked the calibration:  Below several hundred MHz, there was no detectable effect of the device's small amount of capacitance - and even well into the 2+ GHz range, the effect was very minor.

As I rarely use my NanoVNA above about 1.5 GHz, this effect was acceptable - and the added protection against damage (that is, finding your NanoVNA non-functional at a radio site - possibly disrupting the planned activities - perhaps resulting in a wasted trip unless you have a spare VNA or equivalent!) is worth it. The 0.2pF capacitance of this device is probably less than stray capacitances in the circuit/layout, anyway.

Figure 5:
Location of the failed chip (U551) - the first active component
in the signal path on CH0/Port 1 of the VNA - and the one
most likely to be damaged.  The SAA-2N is shown above.
Click on the image for a larger version.

Note:  I haven't put similar protection on CH1/Port 2 as it's less-commonly used - and it's typically used in conjunction with a device under test that is less likely to produce ESD.  If you are measuring the loss of coaxial cable, however, it would be a good idea to dissipate any stored charge on it (shorting the center to the shield - or even touching across the two with your fingers) prior to connecting it to either terminal of the NanoVNA.

Replacing the MXD8641

If your 'VNA has already been damaged, it may be too late unless you have the equipment to do small surface-mount work.  For such work, it's not so much skill that is needed - but the right gear (hot-air rework, ceramic tweezers, etc.) and good magnification is required:  None of this is particularly expensive as a suitable hot-air rework station can be had new for well under US$100.

Using the hot air rework wand with a small-diameter blower tip (Figure 6) the chip is carefully heated and using ceramic tweezers (which are fairly heat-insulative and non-conductive) the old, blown-up chip is lifted off - noting the orientation of the small dot on the original chip - which, on this board, corresponds with the "arrow" symbol seen at about the 7 o'clock position on U551 in Figure 5.  This image shows the location of this chip on the SAA-2N, but it will be located near the connector itself on other models - you may need to trace the signal path for other models.

Figure 6:
Heating the failed chip with a hot-air rework tool.
Be sure to note the orientation of the tiny dot on the chip
before you remove it!
Click on the image for a larger version.

Flooding the area with a drop of "no clean" solder flux from a pen, the replacement chip is dropped onto the spot where the original was - taking care to orient its dot properly - and the area is re-heated with the rework tool.  Gently nudging it back and forth, when the solder melts it will "stick" in place as surface tension does its work - but it may be necessary to make sure that it's centered on the pad - or even push down on it slightly.  If it seems to not be centering itself, let it cool a bit and apply a bit more flux and re-heat it:  The trick here is to let the flux and surface tension do the work!  It is unlikely that more solder will need to be added as the new chip was pre-tinned at the factory and there will likely be plenty still on the pads.

Letting the unit cool - and verifying that its dot is oriented in the same manner as the original part - the unit may be reassembled and tested:  I've had extremely good luck with this method and have never had a failure - and having replaced '8641's in several Version 2 NanoVNAs, I've been 100% successful - but then, I've been doing surface-mount work for quite some time.

Figure 7:
The repaired NanoVNA, recalibrated.  The flatness and level
of the noise floor (<=-70dBm) indicate that it's well matched
and that the losses are low between 100kHz and 1.5 GHz.
Click on the image for a larger version.

Conclusion

Without careful examination of your Version 2 NanoVNA it may not be possible to determine if it has the added protection of the TVS diode - although this may be noted in its documentation.  If it does not - and you regularly take it out in the field rather than having it sit comfortably on your workbench - it's worth considering adding it.

Note:  As much as I would like to help, I'm not going to get into the business of repairing/modifying NanoVNAs in the manner described, so please don't ask - sorry.  If you have a "blowed up" NanoVNA and don't have the gear/skill to do SMD rework, find someone who can.  At the very least, you now know why this issue might happen and this may prevent you from damaging other devices in the future.

 * * * * * * *

This page stolen from ka7oei.blogspot.com 

[END]

Ali Express "SDR TX/RX Switch" - A design NOT well thought out...

Figure 1: 
Front panel of the SDR TX/RX antenna switch showing the
the 3.5mm audio connectors and the red/green RX/TX LED -
which have been swapped to be in their proper location.
Click on the image for a larger version.

        IMPORTANT:        

If you have one of these devices, DO NOT connect it to your transceiver and second receiver (SDR) UNTIL you have read and understood the issues described here. 

Failure to understand how this device works may result in you blowing up your SDR when you transmit! 

If you wish to use it ONLY as an audio muting switch when you transmit (e.g. when using a remote receiver) see the section labeled "Audio Muting" near the bottom of the page.

I've you've been following this blog you'll note that I've used SDRs (Software Defined Radios) quite a bit - particularly for reception.  Transmitting in the vicinity of any receiver - or trying to use an outboard receiver in conjunction with a transmitter on the same antenna - is a bit problematic for several reasons:

• If the transmitter and receiver are in close proximity and on very nearby frequencies (e.g. on the same band) then it is (nearly) inevitable that the receiver WILL be overloaded when the transmitter is active.
  
• Unless the frequencies (transmit and receive) are very well separated AND both the receiver and transmitter have adequate filtering, the receiver will be overloaded by the transmitter.
• It is possible that even if you have a separate receive antenna, it may intercept enough energy from the transmitter to damage/destroy the receiver.  If the two are on the same band, this is more likely - but even if the receiver is being operated on a very different frequency range than the nearby transmitter and there is insufficient filtering at the receiver the receiver could sustain damage.
• It is often the case that one might have a single antenna on which there are two receivers (e.g. the receiver built into the transceiver and an outboard SDR receiver).  In this case one clearly must protect (e.g. disconnect) the outboard receiver when transmitting.

It's worth noting that most SDR receivers do NOT have particularly strong filtering in them:  Unlike an amateur transceiver - which may have separate filtering for each amateur band (or groups of bands) - this is rarely the case for wide frequency-range software-defined radios:  RTL-SDRs, SDRPlay, Funcube and others have either no band-specific filtering or rather broad (e.g. covering about an octave or even wider) filtering in them.

What this unit does

Some modern radios actually have external receive ports on them to allow you to "share" the RF while protecting the external receiver.  If your radio doesn't have that, there are/were several devices to allow this that may be found on the market (e.g. the MFJ-1708) but my attention was brought to an inexpensive unit (pictured above) that has appeared on the various seller web sites (Amazon, EvilBay, Ali Express, etc.) so I obtained one via a U.S. seller.

The description of this device is typical of those found on at the stores of Chinese sellers, curiously being both under and over-descriptive at the same time:  "160MHz 100W Portable SDR Transceivers Aluminum Alloy Box Device Radio Switch Antenna Sharer Practical Signal Equipment Accessory"

By the description, with this device it should be possible to connect your transceiver and SDR (receiver) to the same antenna, perhaps receiving using both (e.g. the addition of a waterfall to an older radio) without fear of damaging the SDR or the transceiver.

This device also has another feature:  To re-route audio when transmitting - which is probably the most usable feature of this device as it comes out of the box as we'll see.

As we'll see, this device doesn't quite work as you might think that it should.

"Documentation?  What documentation?!" 

This unit arrived in a package with (surprise!) no documentation at all - which was somewhat disappointing:  Sometimes one gets a (badly!) translated half-sheet of paper that hurts one's brain to parse - or even a URL to a page with... something... but not the case here.

From a practical standpoint, it's somewhat "self documenting" in the sense that if you ordered this device in the first place, you already had an idea as to what it was supposed to do, so it's possible to figure things out.  Referring to Figures 1 and 2 (the front and back panels) we have:

Front panel (Figure 1):

• LED on the left-hand side.  This LED is illuminated when the unit is in "Receive" mode - that is, the "SDR" rear-panel RF connector is connected to the "ANT" rear-panel connector.  (The PC board shows this as a green LED, but on mine the red and green were interchanged during assembly:  I swapped them back.)
• 3.5mm jack labeled "SDR".  This is a stereo (2-channel) audio jack and, during receive, both channels are connected to the "Audio Out" connector.  It is disconnected during transmit.
  
• 3.5mm jack labeled "AUDIO OUT".  This is a stereo (2-channel) audio jack that is intended to be connected to speakers.
• 3.5mm jack labeled "TRX".  This is a stereo (2-channel) audio jack that is intended to be connected to the transceiver during transmit.  Is is disconnected during receive.
• LED on the right-hand side.  This LED is illuminated when the unit is in "Transmit" mode - that is, the "TRX" RF rear-panel connector is connected to the "ANT" rear-panel connector.  (As noted, this should have been a red LED according to the marking on the PC board but mine was populated with a green LED, which I swapped.)
  

Figure 2:
Back panel of the SDR antenna switch.  SO-239 connectors
are for the radio (transceiver) and antenna with the SMA for
the SDR.  The 3.5mm PTT and power connectors are visible.
Click on the image for a larger version.

Rear panel (Figure 2):

• RADIO connector.  This is an SO-239 (female) UHF connector to which the transmitter/transceiver is to be connected.
• ANTENNA connector.  This is an SO-239 (female) UHF connector to which the antenna is to be connected.
•  PTT ("Push To Talk") connector.  This is a 3.5mm connector in which the center pin (tip), when grounded, will switch the unit from"Receive" to "Transmit" mode.  It's likely that a 3 or 4 wire 3.5mm cable was provided in which case only the tip (PTT) and sleeve (outer-most ring - ground) are needed.
  
• SDR connector.  This is an SMA connector to which the SDR (or other auxiliary receiver) is to be connected.
• 13.8 VDC connector.  This is a 2.1x5.5mm coaxial power connector (center positive) through which DC power is supplied.  This voltage is not critical and could be anywhere from 11.5 through 15 volts.

Also in the box my unit came with an SMA-SMA jumper, SMA-BNC adapter to adapt the SDR connector to BNC, three "audio" cables with 3-conductor 3.5mm connectors on each end, and a 12 volt switching supply (with a European "pin" plug) and a universal plug adapter:  The 12 volt switching supply seems to be the cheapest, meanest possible unit with no brand name and should NOT be trusted or used - but at least its DC cord is useful!  (In other words, do not use this power supply - particularly as it is unfiltered from an RF standpoint and it would be a really bad idea to use it on an RF receive device of any type.) 

How it actually works

As you would expect, the antenna is to be connected to the "ANTENNA" port.  When in receive mode, the "SDR" connector is also connected to the "ANTENNA" port - but the "RADIO" port is not!

What this means is that as shipped from the factory, if you connect your transceiver, antenna and SDR to the unit, when it's in receive mode, you will get no receive signals on your transceiver.  This is by design, apparently.

It is expected that the PTT connection on the back should be grounded when the transceiver is in transmit mode - and when this happens, the RADIO and ANTENNA ports will be connected to each other.  There is also an RF sensing circuit that is supposed to detect when the transmitter is producing RF, but this has its own issues as will be discussed later.

There is a jumper...

If you take the unit apart (via the four screws on the back panel) you'll see a jumper (J5 - see the schematic of Figure 3 and the photo of the board in Figure 4) and some awkwardly-worded text indicating that if you remove the jumper that you'll have "dual receive" - which means that the Radio's receiver and the SDR will be connected to the antenna at the same time.

This is technically true - but there are a number of "gotchas" here

First, let's take a look at a reverse-engineered schematic of the unit, below:

Figure 3:
Reverse-engineered schematic diagram of the unit.
The parts designators are those shown on the silkscreen of the circuit board.
Click on the image for a larger version.

Circuit description:

DC power

The DC input power via J4 is protected with F1, a 500mA self-resetting fuse and D10, a diode for reverse polarity protection while L2, a 220uH inductor, isolates the connection at RF.  Capacitors C6 bypasses RF while C7, a 220uF electrolytic, provides smoothing/filtering - likely enough to even allow an AC power source (from a 12-ish volt transformer) to be used. 

It's worth noting that there are two "grounds" on this device:  The "antenna ground" of the rear panel and RF connectors and the "shack ground" of the DC power and audio which are isolated by L1, a 220uH inductor.  On paper, this isn't a bad idea - but on this unit there's a flaw:

The back panel being used to mount the connectors and it - and the entire case - is at "RF" ground - which would be fine as that would be the same "ground" as your radio.  The problem is that the circuit board's ground planes are not set back from the edges of the board meaning that it's possible that the green insulating coating could scrape off the board and contact the case it the mounting slot, connecting the two "grounds" together - perhaps intermittently.  (Practically speaking, most people would not be likely to ever have a problem.)

Oops.

Keying

J6 is the PTT input, activated by grounding the tip of the 3.5mm connection with the outermost sleeve being the "shack" (not antenna) ground.  Diode D6 blocks positive voltage and when the PTT is keyed, the gate of Q3, an N-channel MOSFET, goes low, de-energizing all of the relays.  When the PTT line is "un-grounded" capacitor C3 and R9 charge, preventing Q3 from re-activating the relays instantly, providing about 100msec or so of delay through the charging by R3.

Comments about the keying via the PTT port

This relay keying scheme assumes that there is either NO voltage, or that there is a POSITIVE voltage on the keying line from the radio when in receive that is greater than a few volts.  There are several caveats to this:

When powered up, the relays are energized whenever it's in "Receive" mode (PTT ungrounded or no transmit RF)  What this means is that if power is removed, it's as if it's in "transmit" mode.

• This connects the ANT to the RADIO port and the SDR port is grounded when in TX mode or powered down.
  
• Additionally, the front-panel AUDIO jack gets connected to the TRX jack. when in TX mode or powered down.
  

The keying line from the radio must go to GROUND when keyed.  If the keying line is shared with an amplifier, that amplifier CANNOT put a negative voltage on the keying line as that will hold the SDR switch in "Transmit" mode at best, damage the SDR switch in worst case - and in either case it would hold the amplifier in a "keyed" state.

If there IS a positive voltage on the keying line when "unkeyed" it must be at least 5 volts just to assure that Q3 will turn on reliably when the radio is un-keyed - and this lower voltage may affect the duration of the "un-key" delay.  If the voltage is less than 10 volts, the full delay caused by C3 and R3 may not occur.  The 100msec or so of "un-key"delay afforded by R3 and C3 is insufficient to prevent the relays from "chattering" during SSB and CW transmissions if RF sensing is used!

• If you are a CW operator, the unit will not switch back to receive mode as quickly as your radio might.  If you are a CW operator that prefers QSK (full break-in) you probably don't want to have this unit inline.
  

There is also an RF-sensing keying circuit:  Transmit RF is tapped from the RADIO RF line (from the transmitter) by C1, a 47pF capacitor and rectified by D1 and D2 and used to turn on Q1 - grounding it in the same way that grounding the PTT line does - which in turn keys the transmitter.  There is a fatal flaw in the design of this device exacerbated by RF sensing which I will discuss shortly.

Audio switching

Figure 4:
The circuit board, showing J5 in the center.
The power supply filtering is in the upper-right with the audio
relay (K3) on the left.
Click on the image for a larger version.

Relay K3 may be used to switch audio based on keying.  Let's assume that you are using a separate SDR with a computer to receive audio:  By connecting the computer speakers to the "Audio Out" connector and the computer audio output to the front-panel 3.5mm "SDR" jack the SDR audio will be muted when transmitting at which any audio from the radio connected to the front-panel 3.5mm "TRX" jack will be passed through to the speakers.

Practically speaking, you would probably never use the "TRX" jack to mute your radio's audio, but a more likely scenario is that if you are using an online WebSDR (see the WebSDR.org web site for a list) to listen on the air, you could use this to mute your speakers when you transmit to prevent your own transmitted audio from coming back with a delay and causing an echo.

RF Switching

This is where it gets a bit scary.  First, consider the configuration - from the factory - with jumper "J5" in place.

Remembering that when powered up, the relays are energized, you can see that when in "Receive" mode, the ANT port is connected directly to the SDR port - but you'll also note that the RADIO port is not connected to anything (e.g. floating).  When you transmit, the relays de-energize, connecting the RADIO port to the antenna and grounding the SDR port.  This means two things:

• There is no receive RF at the transceiver.  Most people that I know don't use their transceiver's receiver instead of the SDR, but use them at the same time - perhaps turning down the volume on the one not being used or, more likely, using their transceiver for audio and the SDR to display a waterfall.  Not having antenna RF to be able to receive anything on the transceiver is likely not what you really want to do.
  

• When using the RF sensing, the transmitter is connected to an open circuit before the relay switches.  This has several implications:
• If you don't have the PTT line wired to your radio, there will be a split second when RF first appears that the radio will see an infinite VSWR before the relays de-energize and the contacts close, connecting the radio to the antenna.  This repeated burst of infinite SWR can progressively damage a transmitter's finals, despite the SWR protection circuitry within the radio.
• When relay K1 does de-energize and connect to the antenna, it will be "hot switching" the relay contacts, which is to say that they will be carrying RF power at the instant that the antenna is connected.  This tends to burn contacts and shorten the life of the relay.
• The RF sensing circuit doesn't have adequate "hang time" to ride through word pauses and CW elements meaning that it will likely "chatter", repeatedly causing the hazards noted above.

As can be seen from the picture of the circuit board in Figure 4 there's a jumper, J5, on the board with the following somewhat confusing text:

J5 Usage
Open = Dual receive wheh [sic] RX ("wheh" was probably intended to be "when")
Short = Normal Operation

By removing J5, relay K1 is never energized meaning that it is always connected to the antenna:  This helps to mitigate the problem that - when RF sensing is used - that the transmitter is connected to "nothing" as it would be the case with J5 installed - but this also means that in receive mode, the transceiver and the SDR are connected in parallel. 

Simply paralleling two (nominally) 50 ohm devices (the transceiver operating in receive mode and the SDR) isn't a great idea - but it will generally work "OK", particularly if the SDR and the transceiver are tuned to the same frequency range.  When the transceiver is OFF - or on a band  other than that to which the SDR is tuned - it may cause its filters to "suck out" RF and a loss of signal/sensitivity on the SDR.   

(Note:  A "properly-designed" device that shared the antenna for receive would likely include a built-in 2-way splitter which can reduce such problems, and it will also contain circuitry to protect the second receiver should excessive RF manage to find its way into it.)

The bad part here is that if you transmit - and, for some reason relay K2 doesn't de-energize instantly, as would be the case with RF sensing only - you will transmit directly into your SDR, likely destroying its front end.

Oops, again.

What this means is:

• If you remove J5, DO NOT operate the unit UNLESS you are using the PTT cable - which is to say DO NOT rely on RF sensing alone as transmit power will briefly enter the SDR's front end before the relay can switch.  The SDR is likely to be damaged due to the lack of RF power protection on that port.
  

• If your PTT cable accidentally becomes disconnected - or external keying is turned off in your radio's menu - you will transmit into the SDR and destroy its front end due to the inability of the RF sensing to act instantly and due to the lack of protection to the SDR.

The reason for this as as mentioned above:  Not only are the transceiver and SDR connected together without any protection circuitry, but also the RF sense needs to detect transmit power before it will activate - and by the time that it does, a brief burst of full transmit power may have found its way into your SDR.

A hardware bug

There is also a more subtle bug that I uncovered.  While testing the unit on the bench, I disconnected J5 - but was confused when the ANT and RADIO ports were not connected.  What was happening was that when I removed J5 - while the unit was powered up and in receive mode - enough current was flowing through LED D8 and resistor R4 to hold relay K1 closed.

Simply removing the power temporarily caused K1 to release - and there wasn't enough current to close it again, but if you are messing with the configuration, this "bug" could bite you, too!  I suppose that it's also possible that jarring the unit could cause the armature of K1 to hold in place - but I didn't try this.

The "fix" for this - if you want to bother with it - is to change resistor R4 to a 10k resistor:  This also tones down the TX LED's brightness a bit, too.

Overall comments

I get the sense that whoever designed this thing may have been copying the general idea from other, similar devices - but not really understanding what was being done, and why.  For example, the separate "grounds" implies an understanding that having them separated would be a good idea - but whoever laid out the circuit board made the "rookie mistake" of making it possible for the two "grounds" to be connected, anyway if the green coating on the board and the black anodization of the case wear through.

The description of this unit implies that it's useful up to 160 MHz.  I suspect that this is probably "true-ish", but the VSWR starts to climb when one gets to and above 6 meters (50 MHz) meaning that it's likely most useful at low power at these higher frequencies.

The cardinal - and unforgivable - sin has to do with the fact that if you want to use both your transceiver's receiver and your SDR simultaneously, you WILL want to remove J5 - but if you do - and you don't absolutely have the PTT connection working properly, you WILL blow up your SDR!

Fixing this problem is possible with the addition of some simple protection circuitry to allow the SDR to survive brief bursts of transmit power - perhaps the topic of a later post. 

AS IT IS, I WOULD NOT USE IT FOR ITS INTENDED PURPOSE, AS AN SDR ANTENNA SWITCH - at least not without significant modification.

"Audio Muting"

What it IS useful for, out of the box

What it IS useful for is an audio switch to mute your computer audio when you transmit - as you might do when using a WebSDR or other remote receiver.  For this, I would:

• Remove internal jumper J5
  
• Connect the PTT to your radio's PTT
• Connect your computer's speakers to the "AUDIO OUT" jack
• Connect your computer audio output to the "SDR" jack

If you are hell-bent on not using the PTT cable, the RF sense may be useful, but you would also need to:

• Be sure that you have removed internal jumper J5
• Connect the transceiver to the "RADIO" port
• Connect the antenna to the "ANTENNA" port 
• DO NOT connect anything to the SDR port

As noted before, the "hang time" imposed by the time constant of C3 and R3 may not be enough and the relay may "chatter" - in which case R3 could be replaced with a higher-value resistor of, say, 330k - or with a 1 Meg potentiometer in series with a 47k resistor to allow "hang time" adjustment.  (Adjustment of R3 is preferable to increasing the value of C3 much as the latter could also slow the activation time when RF is detected.)

Final comments

Other than to switch audio as described above, I wouldn't use this device as it is shipped for any other purpose without appropriate modification.  This makes this device a possible  "starting point" for another project (e.g. there are already some relays and a metal box!) - one to provide proper RF protection for the SDR and make the RF sensing more useful when using modes that have variable power levels (e.g. CW, SSB) which can cause the current design to "chatter".

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This page stolen from ka7oei.blogspot.com

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