[AMPS] IMD measurement scheme

[yo3ctk]

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	-----Original Message-----
	From:	George T. Daughters [SMTP:gdaught6@pobox2.stanford.edu]
	Sent:	None
	To:	yo3ctk@alltrom.ro
	Subject:	Re: [AMPS] IMD measurement scheme



	(snip)

	> Feed the PA with two signals of equal level with a known
spacing (say 
	> two kHz)  and tune the receiver to each of the two tones. 

	how do you recommend this be done?  a combiner/splitter?  a
resistive 
	mixer? (high power resistors would be necessary)  what's your
method?


	During my career as a R&D engineer for military equipment in the
eighties, I used the following setup for measuring IMD performance of
power amplifiers:

	A pair of HP signal generators (non-synthesized, for low phase
noise) feeding two power amplifiers (Marconi, 150W) connected to the
tested amplifier's input via a -3 dB hybrid combiner. This combiner must
have at least -40 dB crosstalk between its two inputs. We made it on a
Philips toroid, but I am sure that it can be replicated with Amidon
toroids as well. If you are really interested, I can dig into my notes
from that era to search for construction details. But I think it was
nothing unusual, just carefully made and designed for this power level.

	At the output of the amplifier under test, we used a -40 dB, 1
kW power attenuator (Rohde&Schwarz) and either a HP 141T spectrum
analyzer or a Rohde&Schwarz test receiver. I personally prefer the
spectrum analyzer due to its ease of use, but Tom has a valid point in
that a good test receiver has a superior dynamic range.

	Hope this helps.

	73 de Mike, YO3CTK



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