Intermod Settings
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

Clear Waves calculates the 3 types of intermodulation products that happen to be closest to our fundamental frequencies. These are:

• Two-Transmitter, 3rd Order Components (2Tx 3rds)
• Three-Transmitter, 3rd Order Components (3Tx 3rds)
• Two-Transmitter, 5th Order Components (5Tx 5ths)

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:

Stringency	2Tx 3rds	3Tx 3rds	2Tx 5ths
Strictest	YES	YES	YES
Strict	YES	YES
Moderate	YES		YES
Lenient	YES
Weak		YES	YES
Weaker		YES
Weakest			YES

What this means is that a frequency set created under ‘Strictest’ conditions is guaranteed to be free of intermodulation interference caused by 2Tx 3rd, 3Tx 3rd and 2Tx 5th intermodulation products. Similarly, a frequency set created under ‘Strict’ conditions is guaranteed to be free of intermodulation interference caused by 2Tx 3rd and 3Tx 3rd intermodulation products. And so on…

Ideally one would always want to use a frequency set created under ‘Strictest’ 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, for practical purposes, it may not be possible to always create frequency sets of ‘Strictest’ stringency when you have many transmitters 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. Furthermore, as the stringency is increased then the number of intermodulation-free frequencies that are available goes down. That is, the size of the frequency set is determined by (a) the frequency range, and (b) the intermod stringency or compatibility level.

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:

Stringency	Frequency Set Size
Strictest	16
Strict	17
Moderate	22
Lenient	32

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 equipment, 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. We suggest you leave the ‘Near Hit’ settings at their default values.




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). We suggest you leave the Step Size setting at its default value of 25 KHz.



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). If you prefer that Clear Waves ignore 3Tx 3rd products generated by 3 frequencies where at least one of them is more than 40 MHz distant from the others, then enable this setting by checking the CheckBox. By checking the CheckBox you are telling Clear Waves to ignore certain intermodulation products, thereby decreasing the stringency of the frequency set.


Likewise, if you prefer that Clear Waves ignore any product where at least one of the transmitter frequencies is more than 100 MHz distant from the other(s), then enable this setting by checking the CheckBox. Again, by checking the CheckBox you are telling Clear Waves to ignore certain intermodulation products, thereby decreasing the stringency of the frequency set.

By default Clear Waves does not ignore any intermod products when generating a compatible frequency set. However, if you find yourself in a situation where you need to slightly increase the size of a frequency set, then you might consider checking one or both of these CheckBoxes. Interference from intermodulation distortion may be less of a problem when transmitters are operating in bands that are many MHz apart from one another.



Signal Bandwidth
This parameter controls the minimal distance between adjacent frequencies in the resultant frequency set that is computed. From a practical standpoint the distance between adjacent channel assignments should be sufficient to ensure transmitters do not interfere with one another. The recommended, default value of 299 KHz is on the conservative side. It is important to note this value has a direct bearing on the final size of the frequency set. The larger the signal bandwidth, the less chance adjacent transmitters will interfere with one another — but the smaller will be the frequency set. The smaller the signal bandwidth, the greater the chance adjacent transmitters will interfere with one another — but the larger will be the frequency set. So, if you need more channels then experiment with decreasing the size of the signal bandwidth. On the other hand, if you do not require lots of channels then you can use the default value or even increase that if you wish.

Import Last Trace
Clear Waves allows you to generate a frequency set that takes into account interference from both RF interferers and intermodulation products. This is a very powerful feature — it allows you to make channel assignments using frequencies that are free of RF interference and intermodulation distortion.

The results displayed in the ‘White Spaces and Intermod-Compatible Frequencies’ view are sufficient for most wireless audio installations. The frequency set that is computed in real-time is free of RF interference and the most destructive types of intermodulation distortion. However, for more demanding scenarios, it is possible to take the analysis a step further.

When you select the Intermodulation Analysis tab, a new screen is displayed with additional settings, grid and chart. [NOTE: While scanning is in progress then this feature is disabled]. Initially the chart is blank and you’ll need to fill it with a spectrum trace. You can do this by pressing the Last Trace button, which will retrieve the last spectrum trace (assuming you had previously performed some scans).





After the RF spectrum data is loaded then its trace will be displayed — along with a horizontal, red line that indicates the White Space Threshold. The White Space Threshold slider controls the threshold signal strength (in dBm) below which is considered ‘White Space’ — that is, frequencies not subject to RF interference and which should be considered potential candidates for the frequency set.  As you adjust this control you’ll see the horizontal, red line move up and down accordingly.  Those frequencies whose signal strength amplitudes exceed the threshold are considered off-limits and are locked-out — that is, they will not be considered as candidates for a frequency set.

In addition to adjusting the White Space Threshold you should select a combination of intermod products you wish to test for from the Intermod Settings panel. The initial set of candidate frequencies from which intermod-compatible frequencies are drawn include multiples of the Step-Size that also fall within the “White Space” — i.e. below the White Space Threshold.

After the White Space Threshold and Intermod Settings have been accounted for, press the Start button to begin the computation.





Intermod Results
When the computation has completed the intermod-compatible frequencies that compose the frequency set are listed in the Results window. They are also displayed graphically on a spectrum trace chart, as shown above. Here you should see that the intermod-compatible frequencies (the vertical white lines) only occur within the ‘White Space’ regions.

Results Window
When the computation has completed the intermod-compatible frequencies that compose the frequency set are listed in the Results window.





The Results window displays 3 columns. Under the first column is listed all the frequencies that belong to this frequency set. These frequencies are guaranteed to be intermod-compatible based on the stringency applied during the intermodulation analysis. If channel bands are being used, then the second column indicates which channel band a particular frequency belongs to. The number in brackets is the total number of frequencies (in this frequency set) that belong to a particular channel band. And the third column displays descriptions for those frequencies that have been designated as “reserved” and which will always be included in an intermodulation analysis.