[Amps] Measuring IMD

Mon Nov 4 10:11:17 EST 2013

Dave,

You definitely want to reduce the power input to the spectrum analyzer down to something in the 0 dBm to +10 dBm level. I am not familiar with the HP 8924() family but I have had, in my prior life as a radio design engineer (I am now retired), extensive experience with spectrum analyzers, IMD measurement, etc. 

The required power attenuation can be obtained in a number of ways - use a 'probe' (e.g. like Bird wattmeter's do), use small capacitor for decoupling, use a power attenuator, etc.

The probe, capacitor, etc techniques are frequency dependent (e.g. the output of the sampler decreases as frequency does down) but that won't affect the accuracy of your IMD measurements  because your are measuring IMD over a very small frequency range (i.e. a couple of kHz).  

If you use a good power attenuator (e.g. Bird) then you don't have to worry (much) about frequency variations at all.  As a side note, you can pick up high power 50 ohm attenuators (on eBay, etc) for not a lot of money - I have a 40 dB, 250W Bird attenuator I got for $60 or so - a 40 dB attenuator is a darn decent load all by itself and the 250W rating is a continuous duty rating - the peak rating is determined by the voltage breakdown of the connectors and load which is quite a bit higher than 250W (figured that out once but have forgotten the details).

If you designed the system for (example) 2,000 watts and want a maximum input signal to the SA of 0 dBm then you need 63 dB of attenuation (i.e. 2,000 watts = +63 dBm = +33 dBw). 

As a side note, I would worry a bit about the 'linearity' of the Heathkit load when being pounded as you are talking about doing.

Much more to say but I won't burden the forum with it at this time.  If you have any other questions, you can contact me directly and I'll be glad to give you some more info.  You can also call via landline if you want (my address is in the Callbook)

73
Bob Groh, WA2CKY

________________________________
 From: Dave Trainor <dave at n8zfm.com>
To: "amps at contesting.com" <amps at contesting.com> 
Sent: Sunday, November 3, 2013 8:51 PM
Subject: Re: [Amps] Measuring IMD

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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