> From: km1h@juno.com
To: <amps@contesting.com>
> Date: Tue, 22 Jul 97 21:17:19 +0000
> Perhaps someone would care to detail a proper test setup for evaluating
> IMD with professional equipment available to many hams. By that I mean
> test equipment from the 1960-80 era that is available at flea markets but
> is still adequate. Never mind the cheap hams that believe life ends at
> Radio Shack or the local Walmart or the other extreme being a modern HP
> equipped lab.
>
> 73...Carl KM1H
Hi Carl,
First let me say I agree with Ian. Screen regulation is important, as
well as adding a little negative feedback via an unbypassed cathode
resistor.
You can do IMD tests simply at home with a good selective receiver,
and attenuator pad, and TWO separate transmitters. As you know I
prefer a three tone test, since even a class C PA can look good in a
two tone test.
Couple the well shielded receiver to the dummy load via a pick up
probe in the load, and put the well shielded attenuator pad at
the receiver input.
Feed the PA with two signals of equal level with a known spacing (say
two kHz) and tune the receiver to each of the two tones. Add enough
pad, over 50 dB, to get the receiver down to some reference level
on the S meter.
Take the pad out and tune in the third order signal (it will be two
times one frequency minus the other one), add enough pad until the S
meter reads the same S reading as the carrier. The difference in
attenuation is the IMD below one tone of a two tone test, and that is
the commercial standard. The amateur radio standard is you add six dB
to the results, making everything look better.
A three tone test is normally accomplished by modulating the drive
levels with a third low frequency signal, but it is difficult to read
an S meter doing this.
Repeat the same procedure for higher IMD products.
By the way, this method, if you can shield the receiver well enough,
can be more accurate than most lab analyzers. Many receivers have
better gain compression specs and lower IM distortion than common
spectrum analyzers.
73, Tom W8JI
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