Eric Scace K3NA eric at K3NA.ORG
Wed Mar 19 12:08:10 EST 2003

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-----Original Message-----
From: Jim Ussailis [mailto:ussailis at equinox.shaysnet.com]
Sent: 2003 March 19 Wednesday 11:59
To: eric at k3na.org

Once upon at time...

I was involved with a large military airframe, where everybody wanted to
talk. They were adding 14 military UHF band trabsceivers to the beast.
These units were freq hopping.

An intermod nightmare, to say the least.

At a meeting at the airframe mfg, in Seattle (who shall remain nameless),
I was told that 3rd order, two-tone IMD was the worst condition. To which
I replied, "Balderdash!" I had to translate that too.

Then I spent the next month calculating three and four tone IMD. I found
what you might expect. There is more energy in three and four tone IMD,
because there are more signals...Duh!

I also found that much of this IMD falls in-band, for second order
signals. Of course second order IMD is more powerful than thrid order
because it involves second order harmonics, rather than third
order harmonics...Again, Duh!

I also predicted a particular fourth order product that would be 6 dB
more than an adjacent third order two-tone product. I set up an
experiment, and foumd the predicted product at 5 dB more than the third
order product. Close enough.

Several papers were accecpted for presentation at Mil-Comm, but I left
the company I was working for before I could finish them.

So, what do I think of using a noise test? Probably very interesting. If
enough noise is used it might well simulate contest conditions. And, it
is certianly a lot easier to instrument than a four-tone test.

I don't know about notching out the channel of interest tho, I think I
would fill the channel of interest and many adjacent channels with noise,
then add a signal into the channel. Adjusting the level of noise and
signal independantly might prove interesting.

Filling adjacent channels would simulate signals getting into the

You might also consider two signals in the band of interest, in addition
to the noise. This could simulate the conditions I heard on this past DX
contest. Noise represents the "mush" of stations calling a rare one,
while the two signals represents the realitivity strong signals that one
is trying to listen thru. I had that experience while trying to work a
multiplier. The rabble wouldn't shut up while I tried to get the guy's
call correct.

At the time I wondered how much of the "mush" was real, and how much IMD.

It is a measurement that needs to be developed. I would suspect that the
math behind it would follow something like McKay's two-tone IMD stuff.
(Electronics, 2 Feb, 1967, I think).

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Your email has been forwarded to me by a friend, from some reflector. Would