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To: <>
Subject: [AMPS] IMD
From: (Steve Thompson)
Date: Tue, 18 Sep 2001 17:06:23 +0100

-----Original Message-----
From: Michael J. Tubby B.Sc. (Hons) G8TIC <>
To: Steve Thompson <>; 2 <>; Peter Chadwick
<>; @AMPS <>; 'John T. M.
Lyles' <>
To: <>
Date: 18 September 2001 16:40
Subject: Re: [AMPS] IMD

>> >>
>> >>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
>> >>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
>> 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
>called a "linear amplifier".
The output stage in the transverter is a Philips device specified for VHF FM
use - I was told that its design origins came from military FM equipment for
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 other
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?


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