### The PRFIntermod Calculator.

The intermod product calculator programs that one finds today generally just do brute-force number crunching. They generate huge imposing looking lists of possible intermod products for a technician to investigate. Though this much is helpful, the technician gets little other assistance from the program. He or She is left only with a pencil to eliminate possibilities from the list. And the technician needs to provide the pencil!

The results are clouded with repetitions of the same calculation, both within and across orders. Clouded with results that trouble shooting has already eliminated as possible. And not presented in a way which helps the technician identify the most likely offending transmitters.

PRFIntermod is designed to provide that extra assistance. Repeated results are eliminated, Phantom results that repeat across orders are eliminated, the results are filterable to remove the impossible, or to force inclusion of the proven. And everything is summarized in a way that allows the technician to determine which transmitters are the most likely offenders.

With the ability to save an intermod study to disk, PRFIntermod becomes a field service tool that provides the technician a lot of help with the practical trouble shooting process. Complementing his or her expertise and helping to narrow the field of possibilities.

#### Test out YOUR intermod calculator

Here is a rather extreme test. Fire up your favorite intermod calculator, enter a single transmitter frequency, and a hit frequency identical to the transmit frequency. Now ask for odd order results. Do you see any hits? Maybe ones like:

ftx + ftx - ftx = fhit

Now think about this for a monent. is this a real hit? Or is it a phantom of the fundamental? A number, plus itself, minus itself, is just the original number.

While your intermod calculator is running try another test. Enter three transmit frequencies of 100, 200, and 300 MHz. And enter a hit frequency of 600MHz. Now look at the third order results. Do you see:

100 + 200 + 300 = 600
200 + 100 + 300 = 600
300 + 200 + 100 = 600
etc

One popular intermod calculator reports SEVEN third order hits. And in the fifth order FIFTY-SIX hits are reported. A little inspection and thought shows that the majority of these are repeats or phantoms.

Here is what PRFIntermod reports for the same intermod study:

In the third order:
300 + 200 + 100 = ±600
200 + 200 + 200 = ±600

and in the fifth order:
200 + 100 + 100 +100 + 100 = ±600
300 + 300 + 200 - 100 - 100 = ±600
300 + 300 + 300 - 200 - 100 = ±600

In order to provide such a clean result set PRFIntermod enforces some restrictions as it calculates hits:

• Any frequency on the left is always greater-than-or-equal-to any frequency to its right.
This means that the sequence 300 + 200 + 100 is calculated, but repeats of this sequence like 300 + 100 + 200 are rejected as repeats.
• Never will the same frequency appear with both plus and minus signs in the same sequence.
This means that the sequence 300 + 200 + 100 - 100 + 100 will be identified as a phantom of a third order hit and will not be calculated.

Applying these restrictions on the calculation means that the sign on the result of the calculation is lost. It could wind up being positive or negative. Thus the ± in the calculation. The sign is of little value though. The important point is that PRFIntermod has identified this particular group of frequencies and harmonics as a mathmatical hit.

Though designed as a trouble-shooting tool, PRFIntermod can help with frequency selection as well. The adjustable display filtering allows the system planner to drop in a new transmitter in with a group of local existing transmitters and see the new intermod products that appear by simply forcing the inclusion of this new frequency in the displayed results.

Add to this the ability to generate customized reports, to save studies to files, and a future ability to search frequency databases for local transmitters, should make PRFIntermod a very useful tool.