The choice to use of the TX Offset value in TAP means that intermod products can be viewed in two different categories.

• Principal products result from combinations considering only the actual center (carrier) transmitter frequencies that you specified when you set up the study.  In real-world operation, these intermod products are the most likely to result in actual intermod problems at a site.  When the program prints these products, it is usually obvious that the computed intermod product is within the protected receiver offset range. 
• Instantaneous products are those products that result if and when the transmitter modulation generates sideband frequencies that combine to produce an intermod product within the offset range of a receiver frequency.  When you enter a TX Offset value, the offset for each transmitter is included in the calculation of the potential intermod products.  Even though the center frequencies you entered when you set up the study do not produce an offending product, the inclusion of the offsets may create a such a product.  The center frequencies are printed to enable you to easily identify which transmitters are involved in the product.

As an example, look at the following product from an intermod study:

153.85500 0.02500 MHz from 153.83000(Fire 5 RX)

-1*(154.25000)-1*(154.84500)+1*(462.95000)

This information printed on the output from the intermod study indicates that an intermod product of 153.855MHz (shown on the first line) is generated and is 25kHz (.025MHz) from a protected receiver frequency of 153.83, identified as the "Fire 5" receiver.

The second line of the output result shows the three center transmitter frequencies (such as 154.25) and the coefficients (such as  -1) that will generate this predicted intermod product.

The protected receiver band is 153.83MHz, +/- 15kHz, or a range of 153.815 to 153.845MHz.

If only the TX center frequencies (without deviation) are included in the intermod study, the computed principal intermod product is:

-1*(154.25000)-1*(154.84500)+1*(462.95000) ==> 153.855

This product is outside of the protected receiver range and can be shown graphically as:

When "TX Offset" values are included in the Intermod Setup to incorporate the effects of transmitter deviation, the actual instantaneous intermod products cover a range of values.  In this example, using a transmitter deviation of +/-5kHz, the range would be:

-1*(154.25500)-1*(154.85000)+1*(462.94500) ==> 153.840MHz

-1*(154.24500)-1*(154.84000)+1*(462.95500) ==> 153.870MHz

This range of potential instantaneous intermod products overlaps the protected receiver range.  Therefore an intermod product is printed.

The potential intermod interference for the protected receiver can be show graphically as:

The deviation value will vary continuously for the modulated signal.  You can see that there could be essentially an unlimited number of instantaneous products generated for this one example.  That's why only the center frequencies are printed to show which ones contribute to the intermod product without trying to print every possible combination.

The program looks at the range of the TX center frequencies +/- the offset for each one, combined with the other TX frequencies +/- their offset. Then the program combines the possible ranges for each combination (all the +/- permutations) to see if any possible combination produces a range of intermod products that would overlap the RX guard band.

If such overlap is found, the program prints the simplified output consisting of the center frequencies and the harmonic coefficients that result in the potential intermod problem.  These instantaneous intermod products (with printed values beyond the specified RX offset) should be considered in the evaluation of the intermod results.  However, they have a lower probability than products generated exclusively by the transmitted carrier frequencies with no modulation considered.

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