Replacing relays with Axicom brand relays

 

Jim Sheldon W0EB has now replaced all stock relays in one of his µBITx transceivers with Axicom D2n 12v relays.

All five relays have been replaced with Axicom D2n 12v parts.  Photo courtesy Jim W0EB.

This improved filtering by a wide margin on all bands except 40m.   The results:

80 Meters 3.500 MHz
7.000 (2nd harmonic) – 65.7 dBC
14.000 (3rd harmonic) -76.4 dBC

40 Meters 7.000 MHz
14.000 (2nd) -58.0 dBC
21.000 (3rd) – 47.2 dBC

30 meters 10.000 MHz
20.000 (2nd) -76.9 dBC
30.000 (3rd) -75.8 dBC

20 meters 14.000 MHz
28.000 (2nd) -65.2 dBC
42.000 (3rd) -64.8 dBC

Jim didn’t test 17, 12 and 10 meters, but he suspects their harmonic suppression is in the same boat as the rest.

Closeup of the four Axicom relays in the output stage. Photo courtesy Jim W0EB.
Reference

Graph showing results

The graph above provided by Gary AG5TX shows the resultant effects of using Axicom relays (using Jim’s data points).

Gary says:

“Your shared data maybe useful as you have shown the same board and relay with different measured results. For that particular board and the measurement data you gathered, I would say the results ARE encouraging from this viewpoint:
You have 4.5 dBc more margin to FCC spec on your worst case data point (40m 3rd harmonic) with the supplied data set.”

Reference

 

Sourcing Axicom relays

These relays can be obtained from a number of sources.  If you live in the US they can be found at Arrow, Digikey and Mouser for less than US$3 each.  They are also available from Aliexpress for under US$2 each in a set of 10 and from Component HK in Hong Kong for under US$1 each.

More details on using Axicom relays for RF switching

For those interested in the benefits of Axicom relays see this webpage for a switched Bandpass filter bank:

http://www.i1wqrlinkradio.com/antype/ch97/chiave50.htm

There are wealth of tips on this page for those building switched filter banks.

Conclusion

It appears that the Axicom relays are the answer, and Ashhar Farhan has already committed to replacing these relays in production µBITx, presumably when the current µBITx stocks have been depleted.  Hopefully Raj’s mod for removing spurs will also feature in a v4 board upgrade shortly.

Raj’s second spurs mod … simpler again

Raj VU2ZAP has found good reduction of spurs with the following mods:

1. Replace L5 with an SMD inductor 680nH or 681nH.  Mount at right angles to old toroid

2.  Replace L7 as above.  Note that C211 is bypassed with this mod.

Don’t be distracted by the missing C216 and C210.  These parts were never populated.

An additional 45MHz filter is not required and, therefore, you should get almost the same power out as stock boards.  If you ALSO use the second 45MHz filter the spurs get further reduced, but with some loss of output power.  Raj used Murata inductors 1210 size. 1206 size will do.

With Mikes W0MNE relay fix and this mod, Raj has the uBitxv3 and v4 boards now operating within normal emission standards.

Raj thinks that a combination of high Q, small size and possibly internal shielding contribute to the reason why.   Another hypothesis is that the original inductors caused ringing.  Raj has also replaced L1-4 with 330nH SMD and that helped a little on one test board.    His interest was to fix the  problem of spurs with the simplest solution and without major surgery.

Reference

W0MNE simply replaces the existing relays to fix the LPF blow by!

Mike W0MNE has come up with a fix for the harmonics problem with the µBITX that is simpler than other fixes …

He has two Ver 3 and one Ver 4 uBiTX rigs. He simply replaced K3, KT1, KT2, and KT3 relays with Axicom relays.

This has knocked down the 3rd, 5th, and 7th harmonic levels from -35dB to below -50dB on all 3 rigs.   No board modifications are required, and no changes to the LPF circuitry is required.   This may not be reproducible on all rigs,  but with 3 rigs showing the same results it may be something constructors should consider.

Mike purchased the relays from Mouser – part # 655-V23105A5403A201. Cost was US$2.89 per relay plus shipping.

Raj VU2ZAP found a picture of the “innards” of the Axicom relay, and suspects the improvement in isolation is the result of the contacts being closer to the ground plane (as the relay coil is on top in this relay, whereas the standard relays supplied with the main board have the relay coil underneath the contacts).

Before the fix on 40m …

And after the fix on his Ver 4 rig …

Reference

IMD using an alternative MMIC amplifier

Warren WA8TOD has now tested the MMIC based wideband amplifier board from SV1AFN https://www.sv1afn.com/wideband-mini-amplifier.html in two different IF stages in the TX chain.

First Test

This is a replacement for the 45 MHz transmit amp comprised of Q20 – Q22 on the µBITx main board.    Warren removed C20 and C22 and used two short lengths of miniature coax to take the signal off-board to a 7 dB attenuator and then on to the MMIC amplifier, and then back onto the board.

The resulting MMIC amplifier gain was +16 dB to match that of the BI-DI amp on the stock µBITx.

Test 1

The yellow trace is before the amplifier board was inserted and the purple trace after. 3rd order IMD was reduced by nearly 10 dB over the stock µBITx by using the MMIC amp.

Second test

Warren restored the Q20 – Q22 amplifier and moved the MMIC amp to the Q40 – 42 amp with similar results. Here he found that he required the full 23 dB of gain provided by the MMIC to achieve the same level of main signal.

Test 2

Yellow and purple traces are as before with the new measurement indicated on the blue trace. Results are almost identical indicating replacing these two amps together would provide 19 – 20 dB of IMD improvement which would make the transmitter completely viable and, in fact, better than some commercial radios in terms of IMD.

In both cases the indicated power out from the two tone test was a little over 4 watts.

Self-oscillation test

Warren also tested this MMIC board for its susceptibility to oscillation.  He connected a 60 dB attenuator between the input and output while feeding a signal into the input.  He then gradually reduced the attenuation one dB at a time until oscillation was visible on a 1.5 GHz spectrum.

The amplifier broke into oscillation very reliably when the attenuation was stepped below -14 dB. Higher than that and it was completely stable.

In the course of these two tests the board was hanging unshielded about 3 inches from the PA heat sinks and the output was a little over 4 watts. In both cases he saw no indication of oscillation.

Reference

Details of the anti-spurs mod (added 45MHz filter)

The spurs can be conquered by a relatively straight forward modification.  The circuit diagram for the mod is shown above and the photo below:

The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit.   Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.

Warren’s implementation of the filter uses a single transformer on the output of the crystal filter.     A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer.  He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output.  No doubt a through-hole component could also be deployed.

The filter input is attached directly to the other R27 pad.  Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.

In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.

Other implementations of the anti-spur filter

A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs.   uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.

Glenn  VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board).  Photos of their board designs follow:

Reference

About the IMD and where it is being generated

Warren WA8TOD previously raised concerns about the level of intermodulation being generated in the µBITx TX signal path.   IMD may not be as pressing an issue as harmonics and spurs, but the µBITx seems to generate more IMD than it should.  This can cause audio distortion and splatter (including out of band transmission if you are operating near a band edge).   IMD only occurs in SSB modes, not on CW.  If you are CW operator you only need to address spurs and harmonics for which there are now solutions.

General expectation about intermodulation

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products.

Warren’s observed IMD

Warren’s µBITx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. His board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

Where is the IMD being introduced?

In order to better understand where the IMD is being introduced in the radio Warren started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points  (on the v4 main board) up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of the measurement configuration.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total… too much and probably indicative of low injection levels as has been stated in the past. On the other hand, it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be increased by RV1 adjustment to significantly higher levels.

Cause of the IMD

The IMD problem is rooted first in non-linearities in both bi-directional (bi-di) amps and then in both mixers.  The focus is likely to be initially on addressing concerns about linearity in the bi-di amps (on TX only of course).

Possible improvements to the bi-di amps

Glenn VK3PE notes that the original article by Wes Hayward and Bob Kopski on bidirectional amplifiers shows a slightly different biasing arrangement and feedback in the first stage compared to that used in uBITX. As shown, gain is 15dB and flat to 100MHz within 1dB.

A more conventional resistive voltage divider is used and the feedback is AC only. Two extra parts are used. The article doesn’t mention IMD though specifically, as a performance target or measure it.

Glenn has plotted the  gain difference between uBITX and Hayward versions of the bi-di amplifier after building a prototype.

There is a difference in input levels before output clipping occurs. The biasing arrangement also gives different Iq.  Gain is reduced about  4dB in the Hayward version over the uBITX. It accords closely with test results from Hayward’s paper of 15.5dB.  Glenn got 16dB of gain at 30MHz.

Haywards paper gives some values for varying the gain to other values also, so there is more experimentation to be done.

Henning DK5LV notes that the designer of the Bi-Di amp states his version is designed for 15 dB gain, which is why he has a series feedback system (680 ohms + cap) from collector to base, and the biasing is done with two extra resistors.

Ashar VU2ESE in designing the µBITx uses only the biasing and feedback with the two resistors.

Henning suggests that the result is that Ashar’s amp has about 20 dB gain, but the frequency response is worse due to the limited fT of the 2N3904 and input reflection (S11).   Due to the higher gain, IMD must be worse for the same input level. If the textbook curve applies (3 dB for every 1 dB of input power change) the IMD will be 15 dB worse.

Reference #1
Reference #2

External LPF filter board from K5BCQ under test

Kees K5BCQ has received his first set of boards. These boards will fit in the space currently occupied by the on-board LPFs.   The piece jutting out and marked “WOOF” is the space for the TX/RX relay (was K3).  The filter parts need to be transplanted from the main board onto small PCBS that are designed to plug in to the board as illustrated above (they are the same size as QRP-Labs filters).

Kees has already made some modifications for better wireability as follows:

1)   Eliminate R3 on the uBITX board and instead add it to the LPF Relay Board.  This frees up space on the µBITX main board and allows more efficient RF wiring for receive. Since the “old R3″ also switches M1 to M2, this requires modification of the board to short M1 and M2 and insertion of the audio pop mod on a v3 board (a v4 board already has the anti-pop mod included).

2) Replace 3 sets of dual 2N3904 Transistors in the PA driver with 3 sets of dual 2N2222A Transistors. Allison had suggested changing the emitter resistors from 22 ohms to 10 ohms (for dual 2N2222A’s).

3) Add a three 1N4148 diode-OR from TxA,TxB, and TxC driver inputs to a TxD driver to pick that relay, which removes the 10m/12m/(15m) LPF when inserting the proper (A/B/C) LPF during Tx. Yes, that means 4 relays are active during TxA,TxB, or TxC transmit. All four of the LPFs use the existing inductor/capacitor components off the uBITX board. They are just moved over to a small 1.5″ x 0.5″ bare LPF board (that footprint is also the one QRP labs has for their LPF/BPF filters, but they don’t sell blank boards.

4) uBITX Antenna wired to either one of two SMA connector pads.

5) T11 will be connected to the RF input on the LPF Relay Board with about 1″ of coax.

6) The smaller 12V 10 pin DPDT bifurcated contact relays can be found on ebay for about $1 each. They fit nicely.

7) Leave all the traces, just make a cut and remove 1/16” of the trace where needed.

[Photos of installation still to come]

Reference

Removing M1/M2 audio switching from Relay

In considering adding a new LPF module to replace teh original on board LPF matrix, it is helpful to remove the audio switching from K3.   This reduces RF signal routing on the uBITX board.  Many have found the signal routing via K3 causes RF to get into the audio so this represents good practice.

It is suggested that Audio M1 and M2 are tied together at R70, and does not  involve Tx/Rx switching them with K3. 

Power is Tx/Rx switched to Q70 on v4 boards, and the audio pop modification does the same thing on the v3 board.

The procedure is:

  1. Remove R70
  2. Bridge the front pad to the adjacent track
  3. Cut the adjacent track and short the relay sides of both tracks to ground.
Reference

Testing removal of spurs with additional 45MHz filter

The photo above shows an additional 45MHz filter (15khz passband) inserted in place of R27 (you can’t see the centre wire on the filter, which is attached to the ground end of R13).

Early indications are that this removes all of the offending spurs.   This will make it  a uBITx.net ESSENTIAL MOD.  The mod has, however, yet to be tested by uBITX.net.

Warren WA8TOD has completed spectrum analysis plots for each band, and these have been reproduced below.  The plots show removal of all unwanted spurs.

Conditions for the test:

  •  eBay filter in place of R27. No other changes.
  • Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

Yet to be verified:

  • 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance.  It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.
  • Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance.  Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Reference