GFSK

A simple GFSK demodulator circuit inspired by this IEEE article: A GFSK demodulator for low-IF Bluetooth receiver

Summary:

1. The input GFSK signal (1 Mb/s , modulation index of 0.32) is using 2MHz intermediate frequency (IF). Since this is only for simulation purpose, there is no gaussian filtering on the repeating bits of 10101010 that are generated from the Vfm voltage source.
2. out_zd is just a pulse train from the zero-crossing detector (ZD) circuit that operates on the IQ (in-phase (I) and quadrature (Q) inputs) differential I+, I-, Q+, Q- signals.
3. As for shape-keeping (SK) circuit, it consists of a clever mechanism of using window comparator circuit to set lower and upper voltage thresholds that helps the SR latch to work by using the mosfet M1 together with the RC charging circuit (R9, C6). Check the simulation waveform at the bottom to understand further. Hint: The filtered output levels V(out_lp) won't reach +1V/0V. The only way for the filtered output to reach +1V is if the pulses from the SK one-shot approach 100% duty-cycle, and the only way for it to reach 0V is if the pulses approach 0% duty cycle.
4. The demodulated signal V(out_lp) output variation for bit 1 and 0 is at 125mV. So, a simple voltage comparator U6 is added to generate baseband digital bits for downstream DSP processing work.
5. The use of 4th-order Bessel active low-pass filter circuit is due to the fact that the demodulating signal is a pulse, so a linear phase might be preferred.
6. The circuit now does not need .IC statements to solve pseudo transient analysis issue. However, initial_reset_pulse signal is needed at the input of both A2 and A14

TODO:

1. Investigate the need/purpose of the td parameters of SR latch

2. Debug why the demodulator circuit does not work when passive low-pass circuit is used instead of active low-pass circuit.

3. Study the GFSK demodulator using digital circuit and 5GHz SERDES IO alone. See A unified GFSK,p/4-shifted DQPSK, and 8-DPSK baseband controller for enhanced data rate Bluetooth SoC and Digital Down-Converter articles.

4. Study the possibility of demodulating bluetooth enhanced data rate (EDR) : pi/4-DQPSK for 2 Mb/s , while 8-DPSK is used for 3 Mb/s air data transmissions

Credit: A big thank you to @Andy and @Vlad from LTspice forum