C4FM vs CQPSK | Difference between C4FM and CQPSK modulation

This page on C4FM vs CQPSK describes difference between C4FM and CQPSK modulation techniques.


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

C4FM Modulation

There are four symbols with designation +3,+1,-1,-3. Each of the symbols carry two bits of data information. Symbol rate in C4FM modulation is 4800 syms/sec which gives data rate of about 9600 bps.

In C4FM modulation, transition between any two symbols will involve change in the frequency and hence resulting signal envelope will have constant amplitude. The figure-1 depicts the C4FM eye diagram after filtering stage. As shown "eye" is open for about 40% of the time. Hence signal envelope is much cleaner compare to LSM modulation.

Occupied Bandwidth of C4FM is about 8KHz.


The full form of CQPSK is Continuous Quadrature Phase Shift Keying. Refer basics of QPSK modulation.

CQPSK modulation

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

CQPSK is similar modulation as C4FM , but generates cleaner signal compare to C4FM modulation. CQPSK is more linear modulation technique in comparison to C4FM. Occupied Bandwidth of CQPSK is about 6KHz.

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

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