PA phase shift changing with amplitude

I also have problems with this T/R switch, but when it contains quarter wave section not shown in this circuit, it may function.

Click to expand...

Yes, hopefully. The other thing missing in the schematic is a load impedance. This also matters in output filter behaviour (if it's also the case in your real setup), because the filter is designed with 50 ohm load.

If you lowpass filter that distorted waveform....you will have a phase shift, as I indicated. Think of the Fourier analysis of the waveforms that make up the distorted one, and you will see that the fundamental has shifted.

FvM said:

Unfortunately, all relevant components, except for the diodes are missing in your schematic. Seeing the paper, I get an idea, how the T/R circuit could be connected. To understand possible phase shift issues, we should see the real circuit used in the test. Presently, it looks like a shorted filter output with no antenna feed at all.

Click to expand...

As I said, the matched coil (antenna if you want to call it that) connects to the cathode of D1. At my operating point, the lumped element pi network there approximates a quarter wave line. It works fine as a narrowband TR switch, but thought I'd include it in the schematic anyways.

WimRFP said:

#Regarding output capacitance and matching.
I would first breakdown the problem (so studying each (amplifier) stage separately).

Click to expand...

In the next version I build I'm planning on having the two stages separated but on the same board, so I'll match both their inputs and outputs to 50 individually. Should make things more straightforward.

Adding inductance to ground (as close as possible to the drain) will compensate this current, you may use L4 for that (make sure you use a good bypass capacitor(s) for C10 as it will carry reasonable RF current).

Click to expand...

Yeah I'll probably use the chokes for that purpose just to save space.

I would not recommend full conjugate match. Though it gives maximum gain, it may lead to slightly voltage-saturated operation.

Click to expand...

Could you elaborate a bit more on this please?

You are correct that the output capacitance reduces with increasing DC drain voltage, but for the AC case it is different. The negative half of RF drain voltage experiences an increasing capacitance though the positive half experiences a reducing capacitance. The increase is larger then the decrease, so effectively, the drain capacitance increases somewhat.

Click to expand...

Yes that makes sense, noted.

You may transform your 50 Ohms load to a lower value (as seen by the drain). This reduces the effect of the capacitances. This requires more RF current and it may become difficult to maintain class A operation.

Click to expand...

You mean something like using a 1:4 impedance transformer so my 50ohm load looks like 12.5ohm (or whatever)? Yeah I'd have to increase Id, but then I could probably lower my drain voltage as well.

Do you have good familiarity with a simulation package (that allows non-linear simulation)? If so, you may change an existing transistor model (including the non-linear capacitances). It takes time, but in simulation you can turn on and off certain behavior to see the effect on phase versus output power. I learned a lot by doing this for a 100 MHz linear PA with a spice simulator.

Click to expand...

The only simulation software I'm competent with is LTspice, which doesn't have nonlinear models for any RF FETs, and I've never been able to find a nonlinear spice model for any of the FETs I use. I tried ADS for a bit but was pretty much overwhelmed by it (though they at least had a model for the MRF136 which I could use, and possibly there are more libraries out there).

Class D (or other switching scheme).
I reread you original posting again. You mentioned you only need about 6 dB change in power. In that case a switching class amplifier based on some medium power upper UHF ldmos can be an option. You only have to halve the supply voltage to get a 6 dB step. Note that such amplifiers are very load sensitive. Some inconvenient load and yo have to place a new mosfet.

Click to expand...

I need 6db difference between each pulse within one pulse sequence, but I will need a decent overall range of output power. So I might need to change between 33dB and 39dB, or 28dB and 36dB. Sorry for the confusion. Overall I'd be happy with a working dynamic range of 20dB.

I also have problems with this T/R switch, but when it contains quarter wave section not shown in this circuit, it may function.

Click to expand...

FvM said:

Yes, hopefully. The other thing missing in the schematic is a load impedance. This also matters in output filter behaviour (if it's also the case in your real setup), because the filter is designed with 50 ohm load.

Click to expand...

See reply above.

Regarding modelling and simulation.
When you visit Polyfet Home Page try to locate a device the looks like yours. The provide Pspice model files for virtually all of their devices. The files for eagle are text files, so you copy them into your favorite simulator (and modify parameters / syntax).

Start with some simulation to get the datasheet specs (DC transfer curve, capacitance versus voltage, etc). If there are significant differences, you need to modify some parameters (can be time consuming). Last year I used a Polyfet SK702 device model for a feasibility problem.

Regarding impedance transformation, you can do that with a L network (series L, parallel C to load) or CLC Pi network, so you aren't fixed to standard impedance steps as known for transmission line transformers.