ClearWaves -- RF Spectrum Analyzer & Frequency Coordination Software

Intermod Settings

Intermodulation (IM) or intermodulation distortion (IMD) is the amplitude modulation of signals containing two or more different frequencies, caused by non-linear behaviour of the signal processing (physical equipment) being used. Since practically all audio equipment includes non-linear components, then some degree of IMD is unavoidable.  Intermodulation specifically creates non-harmonic tones due to undesired mixing of near frequencies, and these spurious emissions create minor to severe RF interference to other nearby signals.

The good news is the physical basis for IMD is well understood and the mathematical formulas used to describe it are relatively simple -- so, computational methods can be applied to generate frequency sets that are free of intermodulation interference and which mitigate its effect.

When performing calculations for intermodulation analysis, there are several settings that affect the size and stringency of the resulting frequency set. These include the following:

• Intermod Stringency / Compatibility Level
• Step Size
• Ignore Certain Intermod Products

When working with wireless audio/video equipment one is concerned with the 3 types of intermodulation products that are closest to the fundamental frequencies. They are:

When creating an intermodulation-compatible frequency set, any combination of these 3 tests can be applied in order to obtain the desired level of reliability. In fact, these three tests can be combined in different ways in order to generate frequency sets with 7 different levels of stringency:

What this means is that a frequency set created under 'Strict' conditions has a high probability of being free of intermodulation interference caused by 2Tx 3rd, 3Tx 3rd and 2Tx 5th intermodulation products. Similarly, a frequency set created under 'Moderate' conditions has a high probability of being free of intermodulation interference caused by 2Tx 3rd and 2Tx 5th intermodulation products. And so on...

Ideally one would always want to use a frequency set created under 'Strict' conditions because the frequencies within the set could be assigned to different transmitters and the level of reliability would be very high (since interference caused by intermodulation distortion would be eliminated for all intents and purposes). However, just as in real-life, everything comes with a trade-off -- and in this case the stricter the conditions then the smaller the size of the resultant frequency set. As it turns out, it may not be possible to create frequency sets of 'Strict' stringency when you have many transmitters that you need to assign channels to. Assuming that transmitter channels fall on 25 KHz boundaries, then for a particular frequency range (e.g. 470 MHz - 500 MHz) there are a limited number of intermodulation-free frequencies that can be computed. Again, this is because as the stringency is increased then the number of intermodulation-free frequencies that are available goes down.

For example -- let's take the frequency range of 470 MHz - 500 MHz and assume that transmitter channels fall on 25 KHz boundaries. The following results are approximations:

As you can see, the number of transmitter channels you require will dictate the level of intermod stringency that can be applied in order to generate a frequency set of sufficient size to accommodate your needs. In this example, if we required 26 channel assignments then we would create a frequency set of "Lenient" stringency.

As a side note, when it comes to configuring audio gear, at the very least your frequency set should be free of interference from 2Tx 3rd-order intermod products -- i.e. be of "Lenient" stringency or higher -- since these are the most destructive.

Near Hit Settings:
Related to ‘Stringency’ is the concept of ‘Near Hits’. This means that in order to qualify as a compatible frequency and be a member of a frequency set a candidate frequency must not match an intermod product nor be within a specified distance of an intermod product. A ‘Near Hit’ setting specifies the minimum distance a candidate frequency must be from an intermod product in order to qualify as a compatible frequency. If a candidate frequency is too close to an intermod product then it is disqualified. For example, by default, intermod-compatible frequencies must be at least 99 KHz from 2Tx 3rd-order products, 49 KHz from 3Tx 3rd-order products, and 89 KHz from 2Tx 5th-order products. As the ‘Near Hit’ distance is increased then the frequency set becomes more stringent, but that comes at a cost — fewer candidate frequencies will qualify as members of the frequency set and, hence, the frequency set will be smaller. 

Step Size
Most audio transmitters are designed to operate on channel frequencies that fall on 25 KHz boundaries — i.e. they are “tunable” in 25 KHz steps. Given a frequency range (e.g. 470 MHz to 500 MHz) and a step-size of 25 KHz, then only frequencies within that range and which also are multiples of 25 KHz are potential candidates to be included in the frequency set that Clear Waves generates (provided they also meet the stringency requirements described above).

Ignore Certain Intermod Products
Intermodulation products are calculated from either two transmitter frequencies (e.g. 2Tx 3rds, 2Tx 5ths) or three transmitter frequencies (3Tx 3rds). For example, if you were to ignore the 3Tx 3rd order products generated by 3 frequencies (where at least one of them is more than 40 MHz distant from the others) then this would decrease the stringency of the resultant frequency set.

Signal Bandwidth

This setting controls the minimal distance between adjacent frequencies in the resultant frequency set -- by default, this is set to 299 KHz. From a practical standpoint, the distance between adjacent channel assignments should be sufficient to ensure transmitters do not interfere with one another. This setting has a direct bearing on the final size of the frequency set -- i.e. the larger the signal bandwidth, the less chance adjacent transmitters will interfere with one another — but the size of the resultant frequency set will be smaller. And the contrary is true, as well -- the smaller the signal bandwidth then the greater the chance adjacent transmitters will interfere with one another — but the larger will be the frequency set.