Demodulators 4 Demodulators of digital modulation types

GMSK demodulator

The simplest type of digital modulation is GMSK. When working with this type of modulation, you can use the frequency detector circuit, which is well proven in communication systems with frequency modulation. The timing diagram of the GMSK signal at the output of the frequency detector is shown in Figure 1.

Figure 1. The timing diagram of the voltage at the output of the frequency detector

This voltage is shown subject to no interference. The first thing that comes to mind is that in order to restore the original (transmitted) signal, it is necessary to strengthen and limit this signal. This scheme is part of the majority of the circuits of the second intermediate frequency paths, such as К174ХА26 or MC3361. As a result, we get a sequence of zeros and ones that should be written to a sequential register.

But at what points in time to record? Where can I get a clock synchronization signal? The GMSK signal also contains this signal. The fact is that when a signal transitions from a zero state to a single state and, conversely, a component appears in the signal spectrum with a frequency equal to the repetition rate of binary symbols. Unfortunately, the phase of this component when a signal transitions from a zero state to a single state is opposite to its phase when a signal passes from a single state to a zero state. As a result, the average value of the spectral component of the signal with the symbol frequency is zero.

To highlight this component, the signal is used to square the frequency detector output. As a result of this operation, a signal component with a doubled frequency of transmitting binary symbols appears in the spectrum of the signal at the output of the squaring circuit. An example of the spectrum of such a signal is shown in Figure 2.

Figure 2. GMSK signal spectrum at the squaring device output

Now, a double symbol frequency signal can easily be extracted using a narrowband bandpass filter or phase locked loop.