[Amps] Load Pulling

[rgroh at swbell.net]

Oh, boy, this is a difficult question. 

First off, with a 2 MHz (i.e. period = 500 ns) narrow (i.e. 20 ns wide for a duty cycle of 4%), you have very rich harmonic generator with (if my memory serves me right) the first null in the harmonic content at the 25th harmonic. 

So, to capture enough of the energy for a representative shot at the load impedance you should probably use a 'load' that will cover a frequency range from perhaps 2 MHz (your fundamental) out to maybe the 10th harmonic (assuming the energy content beyond that is reasonably low - check the Fourier series for the waveform for a better answer). 

To complicate matters quite a bit, the output impedance of the generator may not itself be flat over the frequency range of interest!  By this I mean the real and imaginary parts (the latter adjusted for sign and adjusting for frequency) may not be constant. But that gets complicated so lets assume that is not the case for now.

OK, so we are going to assume we need a 'load' that has constant  components from 2 MHz to better than 20 MHz.  Then we figure out a way to measure the energy transferred to the real part of our load. We might be able to get a reasonable answer simply by measuring the voltage across the real part with an oscilloscope particularly if we have an scope with the capability of to measure true RMS). Now we adjust the real and reactive parts of the 'load' to maximize the energy transferred. And, eureka, we can then measure the load, take it's complex conjugate, and we have an idea of the output impedance of the generator.

But lots of assumptions there. Should be interesting!  Lots of luck.

Oh, not said above, but the idea of using VSWR meters, antenna tuners, etc - totally out of the question due to the nature of your generator (i.e. pulse with a wide frequency content).

73
Bob, WA2CKY

----- Original Message ----
From: bumerang boom <bumerang.boom at yahoo.com>
To: amps at contesting.com
Sent: Wednesday, September 12, 2007 5:44:22 AM
Subject: [Amps] Load Pulling

Hello, one and all.
I need to characterize the output an unusual AMP. It
outputs a 20nS 70Square pulse every 500nS.
I would like to measure it's output complex impedance
but
I  am not sure if the usual equipment like an antenna
tuner and a SWR meter will indicate correct values.

Can any one help with ideas in the test bed
construction?
Any help will be greatly appreciated.

N2NNU


       
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