[Amps] Measuring IMD

[Dave Trainor]

So here is a newby question,  Let's say I have a HP8924C SA and it has a max of 60W input, and I want to check IMD on a 1.5KW amplifier. Do I do this with a T connector in the line to the dummy load?  Or do I need to simply pick off a sample of the RF with a mica cap from the dummy load? I have one of the old "Cantenna" dummy loads I use and could add something to pick off a sample. What I don't want to do is somehow burn out my analyzer by making a silly mistake.  I usually use it for repair/alignment work on VHF and have never tried to measure IMD on an HF amp but since I am in the process of collecting the parts I need for a tetrode amp, measuring the IMD to be sure it's clean is now something I'd like to do, and I am not quite sure how to safely measure that amount of RF, or if picking off a sample would not be a valid testing method.

73 - Dave N8ZFM 


-----Original Message-----
From: Amps [mailto:amps-bounces at contesting.com] On Behalf Of peter chadwick
Sent: Sunday, November 03, 2013 4:26 PM
To: amps at contesting.com
Subject: Re: [Amps] Measuring IMD

The reference given to Pappenfus gives the equations for determining the level of 3rd and 5th order products from the curvature of the line.

Noise testing is good, provided you are sure about the peak to average ratio: to that end, noise with a notch in the middle of the AF passband leading to measurement of the IMD power in that notch is good. The technique was used over 50 years ago in measuring FDM telephone systems, and was applied to multichannel HF SSB tx's.

The question  at the end of the day is 'How much use is the information about more accurate levels of IMD measurement except as an academic exercise?' More to the point is the IMD power in a SSB bandwidth at various offsets - as SM5BSZ has measured. Plus the fact that the vast majority of amateurs neither understand or care about how bad their tx is to the last few dB!

73

Peter G3RZP
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