Intermodulation Interference

There are three basic categories of Intermodulation (IM) interference. They are receiver produced, transmitter produced, and "other" radiated IM. Transmitter produced IM is the result of one or more transmitters impressing a signal in the non-linear final output stage circuitry of another transmitter, usually via antenna coupling. The IM product frequency is then re-radiated from the transmitter's antenna. Receiver produced IM is the result of two or more transmitter signals mixing in a receiver RF amplifier or mixer stage when operating in a non-linear range.

"Other" radiated IM is the result of transmitter signals mixing in other non-linear junctions. These junctions are usually metallic, such as rusty bolts on a tower, dissimilar metallic junctions, or other non-linear metallic junctions in the area. IM products can also be caused by non-linearity in the transmission system such antenna, transmission line, or connectors.

Communication sites that are occupied by multiple transmitters, coupling between each transmitter and antenna system results in the signals of each transmitter entering the nonlinear final output (PA) circuitry of the other transmitters. When intermodulation (IM) products are created in the output circuitry and they fall within the pass band of the final amplifier, the IM products are re-radiated and may interfere with receivers at the same site or at other nearby sites. Additionally, these strong transmitter signals may directly enter a receiver and drive the RF amplifier into a nonlinear operation, or if not filtered effectively by the receiver input circuitry, these signals will mix in the nonlinear circuitry of the first mixer, creating IM products directly in the receiver.

The frequencies of IM products are derived from mathematical formulae. IM products are classified by their "order" (2nd, 3rd, 4th, ...Nth). Some of the more common forms of mixing are illustrated in the following examples. Note that The "A", "B", and "C" designations are the mixing frequencies. The numerical number assigned to the letter designation indicates the harmonic relationship of the frequency. Thus, 2A means the 2nd harmonic of frequency A.

Order Mixing Formulae First A=B, A=C, etc.

Second A ± B, A ± C, etc.

Third A + B - C, A ± 2B, 2A ± B, etc.

Fourth A ± 3B, 2A ± 2B, 3A ± B, etc.

Fifth A ± 4B, 2A ± 3B, 3A ± 2B, 4A ± B, etc.

Sixth A ± 3B ± 2C, 2A ± 2B ± 2C, 3A ± 2B ± C, etc.

Seventh A ± 6B, 2A ± 5B, 3A ± 4B, 4A ± 3B, 5A ± 2B, etc.

Eighth A ± 7B, 2A ± 6B, 3A ± 5B, 4A ± 4B, 5A ± 3B, 6A ± 2B, etc.

Ninth A ± 8B, 2A ± 7B, 3A ± 6B, 4A ± 5B, 5A ± 4B, 6A ± 3B, etc.

The above IM product formulae are just a few of the many possible combinations. When there are four frequencies involved at one time, the mixing possibilities increase tremendously. No t all of the mixing possibilities are significant in creating interference signals. Some fall out-of-band of the receiver and the higher order IM products are usually weaker.

Even if the IM product does not fall within the receiver pass band, the deviation(especially the higher orders) may swing across the pass band, causing some interference. The bandwidth of the IM product modulation sidebands is multiplied by a factor equal to the product order. For example, A FM transmitter with a deviation of ± 5 kHz, the third order products will have a deviation of ± 15 kHz.

Fifth order products will have a deviation of ± 25 kHz.

Transmitter Produced Intermodulation: Intermodulation in transmitters occurs when a signal from another transmitter is impressed on the nonlinear final output stage circuitry, usually via antenna coupling. The power level of the IM product is determined by the power level of the incoming extraneous signal from another transmitter and by a conversion loss factor. The conversion loss factor takes into account the mixing efficiency of the transmitter's final output stage. Conversion loss differs with transmitter design, adjustment, frequency separation of the source signals, and with the order of the IM product.

Conversion loss data from radio manufacturers is difficult to obtain, and some manufacturers will not release this "internal data". Therefore, ComSitePro uses fixed conversion loss values that are based solely on IM product order. These values are user changeable.

Receiver Produced Intermodulation: Within a receiver, when two or more strong off-channel signals enter and mix in the receiver and one of the IM product frequencies created coincides with the receiver operating frequency, potential interference results. This internal IM mixing process takes place in the receiver's RF amplifier when it operates in a nonlinear range and/or in the first mixer, which, of course, has been designed to operate as a nonlinear device.

Receivers have a similar conversion loss type factor and receiver performance is commonly described in terms of conversion loss with respect to the 2A - B type products. Here, conversion loss is the ratio of a specified level of A and B to the level of the resulting IM product, when the product is viewed as an equivalent on channel signal. Receiver conversion loss varies with input levels, AGC action, and product order.

In view of the absence of information from manufacturers on receiver conversion loss, ComSitePro uses the manufacturer's Intermodulation specification as partof the conversion loss factor. The IM specification is determined with adjacent channel frequencies. However, with today’s wideband receivers, the receiver IM specification is valid for determining IM product power levels in the receiver.

When three signal products are generated, theory indicates that the A + B – C product powers are 6 dB higher than the 2A - B type products. This increase in power level is added by ComSitePro to the IM product level when three or more transmitters are involved in the creation of a IM product.