C4FM vs CQPSK: Modulation Techniques Compared

This page describes the difference between C4FM and CQPSK modulation techniques.

C4FM

The full form of C4FM is Continuous 4 level FM. It is used in project 25, also known as P-25. Project 25 is a family of standards used for digital radio communications.

There are four symbols with designations +3, +1, -1, -3. Each of the symbols carries two bits of data information. The symbol rate in C4FM modulation is 4800 symbols/sec, which gives a data rate of about 9600 bps.

In C4FM modulation, the transition between any two symbols will involve a change in the frequency, and hence the resulting signal envelope will have a constant amplitude.

Figure 1 depicts the C4FM eye diagram after the filtering stage. As shown, the “eye” is open for about 40% of the time. Hence, the signal envelope is much cleaner compared to LSM modulation. The occupied bandwidth of C4FM is about 8KHz.

CQPSK

The full form of CQPSK is Continuous Quadrature Phase Shift Keying. Refer to the basics of QPSK modulation for more information.

CQPSK modulation offers a constant envelope, which allows the use of class-C amplifiers in the saturation region. It can also allow the use of a linear amplifier to be employed with very minimal backoff.

CQPSK is a similar modulation technique to C4FM, but generates a cleaner signal compared to C4FM modulation. CQPSK is a more linear modulation technique in comparison to C4FM. The occupied bandwidth of CQPSK is about 6KHz.

Figure 2 depicts the eye diagram of 4 symbols of CQPSK modulation after filtering. The eye patterns are discernible, and 5 possible phase transitions are clearly visible.