Two tone testing is indicative, and with the same tone frequencies and power
levels, can be a way of comparing transmitters. But as you say, varying the
parameters can give wildly variant (and not always predictable) results.
About 40+ years ago, I had a nominal 100 watt 2 MHz marine R/T in production.
Some would struggle to meet the specified 100 watts at 30dB below PEP on 3rd
order: another one would meet 200 watts at the same IMD.
But only a 3dB variation - a lot down to the driver, which used a number of
cheap transistors meant for field timebase driving in TV sets.
73
Peter G3RZP
========================================
Message Received: Nov 10 2013, 02:41 PM
From: "Jim Thomson" <jim.thom@telus.net>
To: amps@contesting.com
Cc:
Subject: [Amps] Measuring IMD
Date: Sun, 03 Nov 2013 22:25:53 +0100
From: peter chadwick <g8on@fsmail.net>
To: amps@contesting.com
Subject: Re: [Amps] Measuring IMD
### My problem with the two tone test is it is all steady state, including
all B+, bias, screen supplies etc. While the K3 xcvr has its best linearity
when run at aprx 30 watts output, as shown on a 2 tone test,
the fact is the higher order imd goes to hell..fast..when operating in SSB
mode. The steady state 2 tone test is flawed anyway. By altering the tone
spacing, you can make the imd levels change all over the map.
IE: find the sweet spot for tone spacing, that produces the lowest IMD3 +
IMD5.... then brag about how the linearity is wonderful. The old FCC imd
commercial spec wanted –30db for IMD3, and –40db for
IMD5 + IMD7......... then 40db + 10 x log of the power for
IMD9-11-13-15-17-19 etc. IE: with 1.5 kw, IMD9-11-13-15-17-19 should be 40db
+ 31.8db = -71.8db. For folks who run 10 kw, then its
40db +40db = -80db for IMD 9+. That FCC spec was based on db below one
tone of a 2 tone signal.
Jim VE7RF
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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