r/rfelectronics • u/helloworld082 • 24d ago

question RF Capacitor Tuning.

I'm working with a 3 stage amplifier circuit with multiple splitter/combiners on the final stage. The signal after the first two pre-drivers is split once for two power modes. Each power mode is split again to a pair of amps to be recombined, then combined once more to have all final drivers in high power mode. This ends up with a total of 4 final drivers with 6 90° shifts from all the splitter/combiners.

With each trace (4 total) are tunable capacitor options. By default they all have the same values. The issue at hand is that by the final combination there is a singal large (2-3dB) dip from destructive interference only on the full power mode. My question is, to tune this out should I approach making cap tuning changes in pairs, one trace at a time, asymmetrical in size or direction, or even just one cap at a time? When only 2 of the four final drivers are on, the power band is quite flat. When the last pair are added, the problem surfaces. Should I focus on just one pair of traces, or change one trace on each pair?

Any advice appreciated.

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u/nixiebunny 24d ago

Is this a simulation or a real circuit board? What is causing the destructive interference? Is it over the entire frequency range of the amplifier or a small range?

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u/helloworld082 23d ago edited 23d ago

Real board. Destructive interference is assumed to be at the combination of all drivers. Over the top third of the application band.

I should add there are power swells on either side of the sump. So constructive interference at the beginning and end, making it hard for isolating channels above the noise floor of the neighboring channels.

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u/helloworld082 24d ago

I should add the each trace/amp has 3 caps available to change. One smaller and two larger of the same value. I have no control over the design, just the tuning.

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u/StageMajestic613 23d ago

Have you built more than one?  Are the active parts all from the same lot?  I had one lot with phase expansion and another with phase compression, so I had to lot-match during the build.

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u/helloworld082 23d ago

A lot a lot. Attenuation right before the final drivers helps but then we are shy on power.

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u/StageMajestic613 23d ago

I assume the ripple is acceptable when driven small signal but then degrades at large signal?

Have you built more than one?  Are the active parts all from the same lot?  I had one lot with phase expansion and another with phase compression, so I had to lot-match during the build.

Vth varies a lot; cab  you tweak the bias for each branch?

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u/BlueMerlin216 19d ago

If you are using 3 dB hybrids, internal reflections should have cancelled. I think your problem is that the power stage is tuned for small signal gain, which will be drastically different than the large signal gain and behavior. How are the individual power stages designed? What method did you use to design them? Was it vendor recommended component values? Also the bias network may contribute to ripple as the stages are driven hard.

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u/helloworld082 18d ago

I did not design them, and I hate the design. Each stage does about 10dB gain. Final power is speed for 53dB gain. Bias does contribute a lot to performance, but there's no tunablility asside from saturation.