The last challenge: reducing IMD

One of the last challenges to be conquered with the µBITx is the unacceptably high intermodulation distortion produced in the IF stages on transmit.

The IMD seems to be sourced in the Termination Insensitive Amplifiers used as bi-directional IF amps at both 45 and 12 MHz.  

Warren WA8TOD is experimenting with a prototype MMIC amplifier utilising the ERA-3SM+ (80 cents each on eBay) mounted on an RF prototyping board ($2.50 each from SV1AFN) that is the proper size and form factor to replace both transmit TIAs. The MMIC/board measure flat response from 300 Hz to well over 200 MHz, and will provide around 22dB of gain.

This test quantifies the IMD performance of the combo. The reference point on the SA is shifted +6 dB so that the readings reflect the power level that would be achieved with a single tone for convenience of interpretation.

Measurements were taken at Vcc of 3.3 volts and current draw was the recommended 35 ma.

The blue trace shows performance at 0 dBm out and is an acceptable -42 dBc (-36 dB minus 6 dB for a single tone carrier). Performance at – 10 dBm out, the level needed to drive the uBitx driver/PA chain, is an excellent -51 dBc (-45 dB minus 6 dB for a single tone carrier).

Yet to be determined is how to add the LM1117-3.3 regulator to hold the Vcc at the required level.

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

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.

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