[Amps] Measuring IMD

[Carl]

----- Original Message ----- 
From: "Jim Thomson" <jim.thom at telus.net>
To: <amps at 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 at fsmail.net>
> To: amps at 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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