>From time to time, reflector interest and spirited discussions revolve around
>the 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
to each other and to the receiver tuned frequency as well.
2. Both signals must be transmitting at the same time.
3. The signal 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 at a syllabic rate 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 signal power in the coincidental
peaks having sufficient magnitudes to exceed the critical level would also
occur infrequently. Given the additional requirements that the two signals must
bear a certain frequency relationship and both must be on the air at the same
time, it is easily seen that the occurrence of these 3 relationships occurring
simultaneously is quite rare indeed. But when it does occur, the interfering
signal will be a series of relatively short bursts or spurts of noiselike
interference occurring in a random manner and difficult to identify accurately
unless one were using a three trace 'scope to view all 3 signals in a time and
peak-capture mode.
>From these considerations, whether the interference is objectionable is a moot
>point. Perhaps it is more of an annoyance than truly objectionable in those
>cases when it does occur.
Much more insidious and prevalent is wide band garbage generated by
transmitters and sent over the air. 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 generated out of
band signals as a widespread and common source of interference.
This type of interference is frequently blamed on less than ideal receiver
strong signal handling ability but that is not the cause. Many of us have
experienced this type of interference at multi-position field day sites wiping
out reception due to the noise and spurious signals transmitted by co-located
transmitter(s). Unfortunately, not much can be done to a receiver to alleviate
this condition!
It is my personal opinion 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, Al, w6wqc
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