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Subject: [AMPS] IMD
From: (Michael J. Tubby B.Sc. (Hons) G8TIC)
Date: Tue, 18 Sep 2001 17:11:14 +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
> >> >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.
> >>
> >
> >But then there are transistors, transistors and transistors - for
> >as well
> >you probably know, there are the sorts of devices used in amateur radio
> >products
> >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".
> The output stage in the transverter is a Philips device specified for VHF
> use - I was told that its design origins came from military FM equipment
> 30-88MHz. Most devices are capable of this sort of performance if you are
> prepared to restrict the power to give the linearity, rather than the
> way round. Of course, you lose out on power (say 30% of cw rating) and
> efficiency. Maybe there's a rule of thumb here - for any given type of
> amplifier, cost x bandwidth = constant?
> Steve

Not sure about that relationship but I bet there is one similar to:

        cost = performance (eg. IMD) * power-output


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