When the indicated measurement points are used and if there is not enough isolation between the transmitters in a separate transmitter configuration, or if the combined signal transmitter is not linear enough, you will see the unwanted signals products on the SA display. In reality, there is always some undesired energy there but the real question is whether that undesired energy is below the power limits identified by the emission mask. Because of modulation of the analog signal, the unwanted signals are further spread out in frequency.

Figure 8. Combined IBOC and modulated analog signal with undesired energy. Click to enlarge.

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A real spectrum with analog modulation and undesired signals is shown in Figure 8. When this spectrum is observed, it can be due to one of two sources: Either the SA is being overdriven and causing the distortion or the distortion is really there and the transmitter is at fault. Here again, it is important to know how much total energy is being fed to the SA because you cannot determine that just by looking at the amplitude on the display.

Unless your measurement is near the SA noise floor, you can determine whether the analyzer is being overloaded by adding 5 or 10dB of additional attenuation. If the signal amplitudes relative to each other do not change when the attenuation is changed then the problem is with the transmitter.

Assuming this is not a problem, the steps to determine compliance with the emission mask are straightforward.

Connect the transmitter(s) to a dummy load and accurately measure analog power and digital power. If it is a combined transmitter, you will have to use the spectrum analyzer to ensure the digital power is correct. (Remember to set the analog power reference on the SA with no modulation on it.)

• Ensure analog to digital power ratio is correct and apply analog modulation • Optimize transmitter linearity if required and recheck power ratio. • Define the measurement points and bandwidths for the NRSC mask • Record data at each point using 1kHz RBW and SA channel power mode. • Apply any correction factors for bandwidth necessary. • Compare emission measurements to the NRSC emission mask.

- continued on page 5