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For those who haven't yet been initiated into the world of inter-modulation interference, this will serve as a quick introduction, or it will simply serve as a basic review for the “old salts” who have been around for a while.

Intermodulation interference has always been a problem for those who work in the land mobile radio industry. In channelized communications systems, odd-order intermod products (resulting from in-band signals) will fall back into the same band.

Intermod can be predicted and calculated mathematically. There are many software applications ranging from commercial software to shareware and even freeware, that can be used to study the intermod phenomenon through mathematical analysis.

The requirements for the formation of an intermod signal are a non-linear mixing point and at least two signals (of different frequencies) applied to the non-linear mixer. The non-linear mixer can be the Class C RF amplifier output stage of a transmitter, an overloaded receiver input stage, a rusty point of a tower or a bad RF connector. In short, almost anything can become a non-linear mixer.

When dealing with large signal levels the mixer doesn't have to be highly efficient to produce an intermod signal of sufficient intensity to cause serious interference to an on-site receiver.

Figure 1 is a simple diagram of a plate-modulated AM transmitter. When a 1kHz sine-wave audio tone is applied to the input of the modulator, the signal is mixed with the RF carrier in the modulated stage to produce a sum and difference frequency of Fc 2 Fa and Fc 1 Fa. If the carrier frequency is 1MHz (1,000kHz), then the difference frequency will be 999kHz and the sum frequency will be 1,001kHz.

Thus, the output from the transmitter will be the carrier frequency of 1,000kHz and the sum and difference frequencies (sidebands) would be spaced 1kHz above and below the carrier frequency. (See Figure 2.) These sidebands contain the intelligence of the signal and are the result of an intermodulation process.

Intermodulation occurs in a superheterodyne receiver where the local oscillator and RF signal are mixed to form an intermediate frequency. So, intermodulation isn't always a bad thing. Without intermodulation, our communications equipment wouldn't work. Yet, when undesired intermodulation occurs, it can cause our communications equipment to not work.

Odd-order intermod

Let's look at a channelized land mobile radio band where the channel spacing is 30kHz. (See Figure 3.) Here, seven frequencies are listed from a mobile radio band. Let's plug a couple of these frequencies into the third-order model: 2A - B.

If we let A = 159.390MHz and B = 159.360MHz, then the resulting intermod frequency will be 159.420MHz. So, it falls right back into the band and will cause interference to a receiver on this frequency and near this site. This is called third-order intermod because the sum of the coefficients of the mixed frequencies is equal to 3.

Let's try a fifth-order product in the form of 3A 2 2B where A is 159.330MHz and B is 159.300MHz. Then, 3(159.330) 2 2(159.300) 5 159.390. Again, the odd-order intermod falls back into the same band where it was generated. The odd-order intermod products are the most troublesome in land mobile radio — especially those of the third order.

Even-order intermod

Even-order intermod caused by in-band frequencies will fall out of band. Referring again to Figure 3, let's plug a couple of these frequencies into the even-order model A 2 B — a second-order intermod signal. In this example, let A 5159.450 and B 5 159.300. The resulting intermod signal is 150kHz and falls far out of band. This results in no interference to any of the in-band channels.

The rule

The rule that says odd-order intermod products fall in-band while even-order intermod products fall out of band is true as long as the frequencies forming the intermod are in-band frequencies. If the frequencies are from different bands, this statement no longer holds true.

Next month we will look at practical examples of intermod and how they can be resolved by taking advantage of the “leverage” factor. Until next time — stay tuned!

Contributing editor Kinley, MRT's technical consultant and a certified electronics technician, is regional communications manager, South Carolina Forestry Commission, Spartanburg, SC. He is the author of Standard Radio Communications Manual, with Instrumentation and Testing Techniques, which is available for direct purchase. Write to 204 Tanglewylde Drive, Spartanburg, SC 29301. His email address is