Hi Peter,
I'm not talking about perfectly designed receivers, I was conveying
how my FT-1000 actually measures.
The worse thing in my FT1000 was the MOSFET in the noise
blanker left hanging on the IF strip even when the noise blanker
was off.
When the blanker was off, that FET remained biased heavily into
conduction, causing all kinds of IMD and cross mod.
After repairing that design error, the next worse problem was the
RF amp with four push-pull parallel FET's.
From: Peter Chadwick <Peter_Chadwick@mitel.com>
To: "'amps'" <amps@contesting.com>
Subject: RE: [AMPS] Re: Ferrite Rod for 6M Amp
Date sent: Wed, 2 Jun 1999 09:14:57 +0100
>
>
> Jon says:
>
> >THe FET's will be over-driven before the
> >mixer is. That's why you can turn the preamp-off and get a
> tremendous
> >improvement in IP3 performance.
>
> In any properly designed receiver, the IMD performance of the RF amp will
> be a relatively small contribution. The mixer, and occasionally, high
> frequency crystal filters are the major causes of IMD. Switching off the
> RF amp reduces signal levels into the mixer, and thus mixer IMD. Getting
> the performance in an RF amp is relatively easy, especially if you use
> noiseless feedback and a good linear transistor running well in Class A
> with plenty of current. (For the effects of current on IMD, see my paper
> in the WESCON 1981 session on High Performance Mixers). Crystal filters
> are a problem close in, especially at higher frequencies. This is because
> the electric field stress is higher for a given input power in a thinner
> crystal, and the stress reaches the point where the crystal no longer
> follows Hooke's law. At one well known company, filters were selected for
> IMD, and the worse ones went into transmitters. Alsocrystal filters are
> not symmetrical in this respect, and turning a filter round can improve
> matters.
>
> >So while harmonic distortion doesn't affect your signal it might piss off
> > someone trying to work a DX station on 10M that is on your harmonic!
>
> I can hear some of my 'locals' second harmonics on 10 from 15 miles away,
> at about S7. Trap beams don't help, of course, as they offer no antenna
> selectivity.
The "harmonic distortion" of ferrites and mixers all pale to the point
of vanishing when compared to the harmonic distortion of a tube
operated in class AB near cutoff.
Let's get back in the real world of amateur transceivers and
equipment we actually use!
Mixers are often the least of our worries, and so is harmonic
distortion.
73, Tom W8JI
w8ji@contesting.com
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