CPM (Digital Demod)

Use the Format dropdown menu to select the CPM (FM) format in order to demodulate Continuous Phase Modulation signals.

CPM Continuous Phase Modulation signals send data using the change of the carrier phase. The amplitude of the carrier stays constant.

CPM signals are demodulated using the FSK Frequency Shift Keying: A form of modulation using multiple carrier frequencies to carry the digital information. The most common is the two frequency FSK system using the two frequencies to carry the binary ones and zeros. demodulator instead of decoding the change in phase from symbol to symbol. What this means is that IQ traces do not contain an imaginary component and the values represent the frequency of the signal with reference to the carrier.

Below is an example CPM constellation diagram. The center point is the center frequency and the points to the left and right of the center represent frequency shifts above and below the center frequency, respectively.

The marker shows that the frequency shift at symbol 3 is 2.0054 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second).. The phase shown on the marker applies to IQ traces in general, but is not applicable for CPM (FM Frequency Modulation) IQ traces.

CPM signals can be demodulated this way because the rate of change of phase is the same as the frequency shift.

The change in phase can be ± pi or ± 3pi times the H parameter for the current symbol. Since there are 4 possible phase changes for each symbol, 2 bits of data are transmitted each symbol.

There are two H parameters, H1 and H2. The H parameter used to calculate the phase transition to the next symbol changes between H1 and H2 each symbol.

Filtering for CPM occurs on the phase component of the signal, similar to MSK Minimum Shift Keying and FSK. Therefore, the combination of filter definition and the values of H1 and H2 will determine the number of distinct FM states for a particular CPM signal. For example, HCPM as defined by APCO-25 Phase 2 generates a CPM signal with 7 unique FM states.