Process for characterising an RF PA's linearity

Thread Starter

jgodvesmar

Joined Jun 15, 2021
1
Hi all,

I am looking into using linear RF power amplifiers within a marine VHF radio application. I have limited experience with linear RF PAs, and was wondering;

What processes/tests/benchmarks can I use to characterise the linearity of an RF PA?

Ideally, something I can use to make apples-to-apples comparisons between different RF PA options, as well as determining how the linearity changes over my input power range. I do not have access to a network vector analyser, but I do have signal generators and spectrum analysers available for testing.

Thanks in advance.
 

Papabravo

Joined Feb 24, 2006
17,316
For an RF power amplifier the only linear relationship is:

RF Power Out is a linear function of RF Power In​

Unless you have a way of measuring RF power a you are going to have difficulty evaluating the linearity of any potential candidates.

Some caveats:
  1. Maybe your signal generator is calibrated to drive a 50Ω and you trust that if you set it to a given power level that it will deliver that power level.
  2. A standard signal generator will produce a sinewave, square wave, and so forth, but the marine transmitter might be FM, not AM, so a single signal frequency might not be a good test.
  3. The antenna is a critical element. If the match is worse than 2:1 a significant amount of power will be reflected back to the source, which can be bad for the source.
In short you need a way to measure things you can't measure at the moment.

In the amateur radio community we have channels that are close to the marine bands and we use both narrow band FM as well as SSB signals. We spend our money on the best antennas and coax feedline that we can afford before we waste a nickel on an amplifier. I like 7/8" hardline for low loss to the antenna.

 
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