----- Original Message -----
From: "Jim Thomson" <jim.thom@telus.net>
To: <amps@contesting.com>
Sent: Sunday, November 10, 2013 9:40 AM
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.
** As well as early audio distortion meters before that; I believe WW2 saw a
lot of development in that area. I know National was building multichannel
HF in the 50's for the military.
There isnt too much that is actually new out there.
Carl
KM1H
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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