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Subject: [AMPS] IMD
From: Ian White, G3SEK" < (Ian White, G3SEK)
Date: Tue, 18 Sep 2001 18:00:17 +0100
Steve Thompson wrote:
>>//  FETs are more linear than bipolars, however, both need RF-NFB to
>>reach the limit of human ability to detect distortion -- i.e. 1 part in
>>10,000, or c. -40db below peak amplitude.
>So when the spectrum analyser shows -50odd 3rd order IMD out of my 2m 
>transverter at 15W pep from a bipolar PA with no rf nfb, it's telling 
>lies? You might find it interesting to investigate the specs of 
>transistors used in UHF TV transmitters.
>Your statement is far too simplistic a generalisation. There's 
>situations where it's true, but there's many where it's just plain 
There's a confusion here between in-channel IMD and off-channel IMD.

In-channel IMD is heard as distortion of the audio, and -40dB distortion
is excellent quality for a band-limited speech signal.

Off-channel IMD is totally  different, because levels as low as -80dB or
even less can cause trouble to other weak signals.

Anyway, it's not reasonable to talk simply of "IMD" or how "IMD gets
better at lower power levels" (or doesn't). There are many different
orders of IMD, and it's too easy to get fixated on how only the 3rd
order behaves. In many ways, 3rd order IMD matters least of all on the
air, because it's very close-in. The higher orders are what determine
the "width" of your signal, especially if they fail to dwindle away into
the noise as someone tunes away from you.

On any kind of real-life amplifier, the different orders of IMD will go
both up and *down* almost independently of each other at various levels
of drive. Vary the two-tone drive level and watch them dance up and down
on the spectrum analyser. Even with a simple two-tone test it's too
complicated to generalize; with speech it's even more difficult to pin
down exactly what you're measuring.

OK, you can say that so if you drive the amp hard enough the "overall"
level of IMD increases... but you still don't really know what you mean
by "overall".

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

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