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Application Note #2

Understanding Intermod

Copyright (C) 1994-2005 by Signal Intelligence All Rights Reserved.

Introduction

The term intermod is often used to describe any type of undesirable signal in a radio. This is incorrect. There are a number of ways that a radio can pick up unwanted signals, intermodulation distortion is just one of them. Some of the others are image response, synthesizer sidebands, blocking or desense, and adjacent channel rejection. Each of these maladies has it's own symptoms and cures. The purpose of this note is to explain what intermod is, what it isn't, how it differs from other problems and what can be done about it.

Image Rejection

Most modern high quality receivers use triple conversion designs. This is done to distribute gain to different frequencies and improve image rejection. In a typical triple conversion design, the front end mixer up converts signals from the on-air frequency to the first intermediate frequency (or I.F.), typically 1 GHz or more. Because mixers produce sum and difference outputs, both the desired frequency and another numerically related frequency will be converted to the I.F. Older designs which used a low I.F. at 21.4 or 10.7 MHz suffered greatly from image problems.

For example, consider a scanner with a 21.4 MHz I.F. tuned to 161.4 MHz. The first local oscillator is producing an output of 140.0 MHz. Signals at 161.4 AND 118.6 will be converted by the mixer to the 21.4 MHz I.F. The front end should provide some rejection at 118.6, but strong signals will make it through. Since 118.6 is a relatively low powered aviation band frequency, this would usually not be much of a problem. But if you were tuned to 150.5 MHz, the image would fall on 107.7 MHz, a high powered FM broadcast station sure to cause an unwanted response in half the major markets in the U.S. Image response is easy to identify. Intermediate frequencies are usually listed on the specifications page of radio user manuals. Take the I.F. frequency, double it and write that number down. If you think you might be getting an image response, subtract the doubled I.F. frequency (add if you have an up conversion receiver) from (to) the displayed frequency. If what you hear is reasonable for the calculated image, you probably have one.

Getting rid of image responses is usually difficult without redesigning the radio, but special cases can be dealt with. If you live near a high powered station, consider a trap filter. Several types of filters are available from radio dealers and catalogs. Adjacent Channel Rejection: This type of interference is usually not much of a problem at VHF/UHF, but can be fixed in some cases. Ultimately, after all the amplification, mixing and filtering is done, the "close in" performance of any radio is only as good as the narrowest I.F. filter. The job of the I.F. filter is to allow signals at the center of the passband to be received and to reject all others. This is a tall order, and most filters roll off with a steep but not perfect slope. Thus a strong signal "next door" can make trouble for weak on-channel signals.

Most receivers use standard I.F. frequencies, to take advantage of existing filter parts. These frequencies are most often 455 KHz, 10.7 MHz or 21.4 MHz. Many people replace stock filters in their radios with more expensive (steeper side skirts) models. This is most often done with H.F. receivers, but can be done in scanners as well. Some dealers and catalog houses do this kind of work.

Synthesizer Sidebands

This rare problem is often mistaken for an adjacent channel problem, but is caused by a the frequency synthesizer in the radio generating more than just the desired local oscillator frequency. A poorly designed or adjusted synthesizer can produce a comb-like output which can cause the reception of multiple channels at the same time! The spacing between the combs is the synthesizer reference frequency. Determining this frequency is difficult without a schematic diagram and detailed information on the radio, but a good guess is the channel spacing in the band in question. For example, a UHF receiver would need a 12.5 KHz reference. Adjustment of a synthesizer requires expensive test equipment. This problem can be easily identified by the same signal being heard at fixed intervals above and below the actual frequency. The strength of the signals will decrease as they get farther away from the original. Fixing this problem requires a trip to the factory or someone who is a real expert in synthesizers.

Blocking/Desense

Desense is caused when a powerful signal drives the front end amplifier out of its linear region. As long as none of the signals entering a radio saturate the front end, things are OK. When one signal rises above a certain point, weaker signals begin to be attenuated, this is known as desensing. When this gets out of hand, complete blocking occurs. All radios will suffer from desense given a strong enough local signal. The signal causing the desense is usually not heard in the radio. The only remedy is to eliminate the interfering signal.

Intermod

Properly termed third order intermodulation distortion, intermod occurs when two or more strong signals appear in a radio front end. Neither signal needs to be strong enough to desense the radio, and neither signal needs to be on the desired channel frequency. Interference occurs when signals interact with each other in the first mixer, creating intermodulation products which fall within the bandwidth of the receiving frequency. Good band pass filtering in the front end will eliminate most problems, but all receivers are susceptible to some degree. Intermod is such a difficult problem to deal with because it is the result of normal operation of the first mixer in the receiver.

A mixer blends different signals together to create new ones on frequencies that are the sum and difference of the originals. The goal is to mix the on-channel signal with the local oscillator in the radio to produce an output at the intermediate frequency. Unfortunately, other signals will enter the mixer creating potentially undesirable effects. For example, if there are ten strong signals present at the mixer (not unlikely), there will be one hundred eighty strong first order products created. If the mixer were perfectly balanced and matched to the surrounding circuitry, these first order products wouldn't be too bothersome since they're usually far away from the I.F. A perfect match doesn't exist, and many of these products will be reflected back into the mixer where they will be mixed again, combining with all the other signals already there to produce a new set. It's these new, third order signals that cause most of the problems. If you crunch through the numbers, you'll find that many of them land close to or in the I.F. bandwidth.

The good news is that the strength of these third order responses falls off rapidly after each recombination.

The bad news is that it's likely that one of them will fall exactly where you don't want it.

There is hope, though. Since these third order signals are the product of signals that came in through the antenna connector, attenuation has a tripling effect. If you insert a 10 DB attenuator in the antenna line, the third order products are reduced by 30 DB. Fifth order products are attenuated by 50 DB, and so on. The best way to prevent intermod is to use a well designed receiver. Quality receivers use "strong" mixers which are designed to minimize third order products. In addition, good band pass or track tuning in the front end reduces out of band responses. A variable or step attenuator is ideal for getting rid of intermod. Start with a small setting, say 3 DB, and increase attenuation until the interference is gone.