Understanding P25 Modulation Fidelity

The objective of this application note is to clarify how P25 C4FM modulation works, and to explain the critical components of the modulation and how the Aeroflex 3920 provides the best means to accurately analyze P25 signals.

With the proliferation of the P25 standard, it is important to understand the meaning of special measurements found in P25. In the P25 standard (TIA/EIA-102) there is a measurement called modulation fidelity. Modulation fidelity is defined as; "the degree of closeness to which the modulation follows the desired ideal theoretical modulation". This is a very important measurement for P25 C4FM modulation as it is an indication of the quality of the signal being transmitted by the radio. Before we can understand the modulation fidelity measurement, we will need to look at the type of modulation that this measurement analyzes and how test equipment must be able to accurately measure this type of modulation.

Most people familiar with the Aeroflex 2975 and the 3920 product lines will assume that modulation fidelity is one measurement pertaining to the P25 C4FM modulation. In fact, the term "Modulation Fidelity" is really a combination of a number of parameters, including frequency error, symbol deviation, and RMS deviation error. Aeroflex utilizes the term "Modulation Fidelity" to be consistent with TIA/EIA-102-CAAB-C, Section 3.2.16 where Table 3-18 is labeled "Modulation Fidelity" limits.