Thank you Bob :) I kind'a guessed that the regular
measuring instruments will not be of much help here.
And you are right about the output stages changing
impedance function of the harmonical content of the
pulse. However, just for starters, I am focusing on
the fundamental now. If I have that, I could use it
as a starting point for all harmonic analysis later.
A fast look on a spectrum analyzer with a 50OHM load
shows strong harmonics ( -10dBm under the fundamental)
up to 30MHz. The picture on the screen looks almost
rectangular and filled with harmonics. I do not know
where this unit was used as there are absolutely no
markings on it but I only can guess it is part of a
jammer or an over the horizon radar.
I was wondering if I could use it, as the mechanical
construction is quite exquisite :)
FFT and Wavelets show a rich harmonical content and
what is strange some mixing elements that I can
attribute only to pulse harmonic intermodulation in
the output stage.
I should probably sit down and painstakingly make its
schematic.
Thank you for your input
N2NNU
--- rgroh@swbell.net wrote:
> 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@yahoo.com>
> To: amps@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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