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[AMPS] Distortion Measurement Distortion

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Subject: [AMPS] Distortion Measurement Distortion
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Wed, 19 Sep 2001 17:12:48 +0100
2 wrote:
>If such a measurement is made to determine how much splatter one is 
>generating on the air, on adjacent frequencies with human voice 
>modulation, why not measure the relative signal strengths the same way?   
>Is the high-tech laboratory approach better, or is much adoo about 
For you and me, it's what happens on the air with speech that really
matters. For labs and reviewers, they need some way to make consistent
tests that are reproducible from rig to rig. However, these two sets of
requirements are now converging. 

It's now possible to call up a digitized speech signal which is always
the same, and to measure spectrum occupancy with the analyzer in 'peak
hold' mode. If you set up each rig according to the manufacturer's
instructions, and continue to loop the test signal until the spectrum
ceases to 'grow' at any point, the display then shows the highest levels
that the sideband signals have ever reached, at any moment. Compared to
a two-tone test, the result look horrifying - usually a straight-sided
'pyramid' of solid IMD!

If you wish, you can then set pass/fail standards in the same way as any
other swept measurement, by drawing limit lines on the display.

Anyone who has an old HP analyzer with a 141T storage display can get a
pretty good approximation of what you'd see with modern digital
equipment. Set the display to max persistence, crank the resolution and
sweep speed right down, zero in on your SSB signal, and talk away. Just
watch your IMD grow... 

73 from Ian G3SEK          Editor, 'The VHF/UHF DX Book'
                          'In Practice' columnist for RadCom (RSGB)

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