>From time to time, newsgroup interest and spirited discussions revolve around
>third order intermodulation distortion performance of specific receivers. I
>would like to offer a few words on this subject with a view towards clarifying
>what we are talking about.
Three conditions must occur simultaneously in order for third order
intermodulation distortion products to be heard. As a minimum these are:
1. The presence of two out of band signals in a precise frequency relationship
not only to each other but to the received frequency as well.
2. Both signals must be transmitting at the same time.
3. The signals peak strengths must exceed a specific critical amplitude
threshold or level.
In the case of SSB signals, requirement 3 deserves additional discussion. Voice
power varies unpredictably over time and consists of peaks and valleys
occurring in a non-periodic manner. Thus two voices speaking simultaneously
would have peaks occurring and coinciding at exactly the same time only very
occasionally. Furthermore, the combined power in coincidental peaks having a
sufficient magnitude to exceed the critical level would also occur very
infrequently. The interfering signal would be a series of relatively short and
random bursts or spurts of noise and difficult to identify accurately unless
one were using a three trace 'scope to view all signals at once in a time and
peak-capture mode.
If any of the three conditions occurring as described above is missing, no
interference from that source will be heard.
>From these considerations, it is seen that interfering signals occur in a very
>sporadic and very infrequent manner. Whether it would be objectionable is a
>moot point. Perhaps it would be more of an annoyance than truly objectionable.
Much more insidious and prevalent is wide band garbage generated by
transmitters and sent over the air. All too often this type of interference is
taken as less than ideal receiver performance but it is not. A wide band
spectrum analyzer can identify this type of interference immediately.
We hear a lot about the spurious free dynamic range of receivers and their
strong signal performance and that is all well and good. But we do not hear too
much, nor do we seem to be as acutely aware, about transmitter emitted out of
band signals as a widespread source of interference.
It is a common thing for multi-op field day activity to be wiped out not as
much by poor receiver dynamic range performance as by out of band noise and
garbage emitted by co-located transmitter(s).
My personal opinion is that while third order intermodulation distortion rating
is an extremely valuable receiver design specification, receiver performance in
a practical environment is better categorized by strong signal desensitization
and oscillator phase noise. It is these two specifications that better describe
and determine and enhance the weak signal performance of a receiver.
Fortunately for us, it is the American manufacturers that are the leaders in
providing equipment that excel in these two areas.
73, Frank
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