[TenTec] rec intermod

Alfred Lorona w6wqc at dslextreme.com
Sat Nov 29 23:32:29 EST 2003


>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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