RF POWER AMPLIFIER Test

Hello All

I have designed a power amplifier at L -band in three individual stages.

I fabricated it and test individual also.

but after cascading , i did not get the optimum power. so there is definately impedence mismatch issue.

hance, the only option is that, i have to measure the impedence (with VNA) at output (S22) of stage "1" and input impedence (S11) of stage "2", and then match them to each other using impedence transformer circuit in between them.

As its high power amplifier (CW mode ), and output power of the satge "1" is 45 dBm and to next stage (stage "2") is 250W (54 dBm) power amplifier device which to be match.

As the VNA is very sensitive device i am not able to measure the Impedence .
if i connect the Atenuater in between , VNA directly show the port impedence of attenuator (standart 50 Ω)

Kindly help me in this issue.

How i can measure the impedece of high power amplifier outport using VNA.

As the input impedance of the high power stage is likely to level dependent, it is unlikely that you would get an accurate value unless the drive level was about right. Use a directional coupler, measure the power at each tapping port, then by using a selection of mismatches, repeat the process at low power. Then you can get the impedance of the high power input, but not its sign. It could be 50 ohms anyway !
Frank

As chucky suggested, using directional coupler is the usual and effective way to measure S-para.
According to my experience the impedance of a PA is not wildly power dependent. The impedance doesn't vary much when the measured input power does. So that: bias stage 1 -> measure S11 and S22 and repeat with stage 2. By this way, you would not worry that output power will harm VNA because there is no signal source fed to the PA.
That I did and would suggest.

if any how the amplifier goes in oscillation then , at output port we may get the high osillated power which may damage to VNA.

you can use directional couples to measure . connect coupled port to VNA and other port to 50 Ohm or your device

i can see that you want to do/correct inter-stage matching. am i correct? if its true, you wont be able to connect to a VNA unless you have fabricated both stage/amplifiers separately.

measuring impedance using coupler is good solution for input and output stage of power amplifier but not inter-stage.

based on my experience, accurate EM simulation modeling will help in multistage amplifier design. if you measured your PA impedance based on loadpull (measured on your unique bias condition), you can rely on simulation tools like ADS to do impedance matching. the correlation between simulation and measurement wont be 100% but you will get a good starting point. with minor optimization you should be able to get results you wanted.

With RF amps working at lower frequencies with discrete Cs and Ls, the interstage tuning can be "Walked" into maximum power transfer. So after doing this if one of the tuning caps is at min or max, there is a likelyhood of a better point being reached if you had more/less capacitance or inductance.
I guess that you are using stripline, but I wonder if you added a temporary stub, you could determine if the line was properly tuned and the losses are due to some other effect.
Frank

If the circuit of the cascaded RF power amplifier is already done would be hard to interconnect couplers for interstage power measurements.
I think the best is to start doing tuning of the matching networks (input, interstage, output), starting from the input to the output.
Tune initially stage one and two for maximum PA gain, and at the end, tune the last stage for maximum PA output power.

kindly let me know the procedur to tune as VNA is very sensitive instrument.

Unless your design allows you to insert directional couplers in between the stages, you likely can't measure the reflection coefficients between stages with a VNA. In that case you're stuck with either trying to model the problem, or trial and error.

Did you consider temperature rise when designing the matching networks?

aks.rfms said:

kindly let me know the procedur to tune as VNA is very sensitive instrument.

Click to expand...

if you are looking for measurement to solve the problem, i believe this is the only way. you could have fabricated few PCBs for your 2 stage amplifier. use them as single stage amplifier instead. optimize stage 1 and connect a 50ohm RF cable at the output of stage 1. dont connect stage 2 device in there. on another board, connect stage 2 device and input and output matching but dont connect stage 1 device in there. at the input port of stage 2, connect a 50ohm RF cable. use VNA as many suggestion here to optimize individual stage's input and output matching to 50 ohm.

upon getting acceptable results, you can populate stage 1 and stage 2 in single PCB and test again.

no, i did not considerd the rise in temprature issue while designing.

kindly let me know how and where it will affect?

when temperature changes resistance will also change