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Subject: [AMPS] IMD
From: (Michael J. Tubby B.Sc. (Hons) G8TIC)
Date: Tue, 18 Sep 2001 16:34:55 +0100
> >>
> >>Rich said:
> >>
> >>>Few AB1-grid driven. or AB2-GG amplifiers have as much distortion as
> >>>modern transceivers.
> >>
> >>As evidenced by the published test figures on, for example,  the TS2000.
> >>Interesting, since modern MOSFET PA's are supposedly very linear.
> >>
> >//  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
> You might find it interesting to investigate the specs of transistors used
> in UHF TV transmitters.

But then there are transistors, transistors and transistors - for example,
as well
you probably know, there are the sorts of devices used in amateur radio
designed by so-called professionals, like MRF247s and SD1477s for use at
144MHz, or worse SD1487 HF devices used at 50MHz/70MHz. There are
the "FM service" transistors that end up in amplifiers with some bias on and
called a "linear amplifier". At the opposite extreme there are VHF/UHF
used in video transmitters at full European TV/Video specification, however
these don't turn up in amateur service... price, availability... hen's

I think its fair to say that we are considering only devices likely to be
in amateur radio equipment rather than professional/broadcast service -
is in itself an interesting point beucase there is less difference when it
to valves (tubes) - there are broadcast stations using 8877's in amplifiers
albeit in FM service 87.5-108.0MHz and just up the road from me there
are 4CX1500Bs in a certain 500KW long-wave transmitter on 198KHz -
but then they are used as the screen stabilisers for the really big tubes!

> Your statement is far too simplistic a generalisation. There's situations
> where it's true, but there's many where it's just plain wrong.
> Steve

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