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Old 7th Dec 2017, 12:35 am   #8
Join Date: Sep 2010
Location: Cheltenham, Gloucestershire, UK.
Posts: 1,344
Default Re: 'Kalmus' wideband RFPA: info.?

One thing to bear in mind is that you need to be careful what test gear you use to measure the RF power level from some of these amplifiers if you run them close to full power. This is because the test gear could give false readings in the presence of relatively high harmonic distortion produced in the amplifier.

You can typically expect to see three or four stages within these amplifiers. If this is a budget 50W amplifier it may use a fairly simple design for the final amplifier stage. It could be as simple as a push pull output stage. It could be a BJT design but it could also be a VDMOS design.

Push pull amplifiers like this tend to produce a lot of odd order harmonic distortion when driven up towards the onset of compression. This can cause a lot of uncertainty in the test gear if you are using something like a classic (ham) power meter that uses a simple/linear diode detector. You can get the same issue if you use a simple analogue scope to measure Vpk voltage. The harmonics can cause confusing results because of the influence of the harmonics on the peak (or peak to peak) voltage of the overall RF waveform.

A typical push pull amplifier just running into compression can easily have a third harmonic at -15 to -20dBc. Depending on the phase of the harmonic relative to the fundamental this can cause a lot of uncertainty in a typical ham power meter. Easily adding +/- 1dB of measurement uncertainty because of the limitations of this type of power meter. So this can make the amplifier frequency response look worse than it really is.
Regards, Jeremy G0HZU
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