FM demodulation PLL analysis memo by Kenji Rikitake

7-JUN-2021

* By comparing rather unstable stereo PLL pilot signal from Inter FM 89.7MHz
  Tokyo, nesting two biquad filters showed less stable result than the
  original single biquad filter implementation
* The periodic (period: ~21000 blocks) Inter FM signal stereo PLL signal
  loss (level < 0.01) were continuously and steadily observed while received
  in Setagaya City, Tokyo
* The PLL filter is rolled back by the original single biquad filter
  implementation

15-JAN-2021

* Still unable to find out how the PLL system constants were defined
* Replacing the first 2nd-order LPF with the following
  2nd-order Butterworth filter looked OK
  scipy.signal.butter(2, 30, 'lowpass', False, 'sos', 384000)
* Adding another 2nd-order LPF might or might NOT decrease the
  lock frequency error
* After the lock frequency measurement, the lock delay time can be
  shortened to 0.5 second while maintaining the stability
  (previously 0.667 second) when the lock bandwidth is 30Hz

[End of memorandum]
