Colin Lamb wrote:
>And, what are the desired standards for 3rd order distortion anyway? This
>would perhaps be a good discussion topic. -30 db 3rd order distortion
>products do cause splatter. Is that the minimum level of distortion for a
>ham to transmit with. For years I listened to W7FY (now deceased) who ran a
>4CX1500 in grid driven service. His station was significantly cleaner than
>other signals and a real pleasure to tune off of.
You've hit it exactly with that last sentence. When you tune away from
the signal, you want it to be gone completely. This means that higher-
order IMD is more important in many ways than 3rd-order.
3rd-order IMD is always very close to the main signal, where some degree
of "bleed-over" of strong signals is very likely because of other (non-
IMD) problems such as filter passband overlap and phase noise in both
the transmitter and the receiver. For all those reasons, we mostly
accept that it isn't sensible to operate too close to a very strong
signal.
On the other hand, we do expect higher orders of IMD to drop very
rapidly as we tune away. Higher-order IMD is farther away from the main
signal, and can cover much more of the band - especially with a complex
speech signal containing many varying frequencies. In other words, high-
order IMD annoys far more band users than 3rd-order does.
Unfortunately there is no direct correlation between 3rd-order IMD and
higher-order IMD, so it's an over-simplification to concentrate on 3rd-
order only - it ignores a much wider problem!
73 from Ian G3SEK Editor, 'The VHF/UHF DX Book'
'In Practice' columnist for RadCom (RSGB)
http://www.ifwtech.demon.co.uk/g3sek
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