I measured openloop transfer function of the phase tracking loop for the first characterization of phase tracker.
What is phase tracker:
See elog #6832.
For ALS, we use delay-line frequency discriminator, but it has trade-off between sensitivity and linear range. We solved this trade-off by tacking the phase of I/Q signals.
Figure below is the current diagram of the frequency discriminator using phase tracker.
OLTF of phase tracking loop:
Below. UGF at 1.2 kHz, phase margin 63 deg for both BEATX and BEATY. Phase delay can be clearly explained by 61 usec delay. This delay is 1 step in 16 KHz system.
Note that UGF depends on the amplitude of the RF input. I think this should be fixed by calculating the amplitude from I/Q signals.
BEAT(X|Y)_PHASE_GAIN were set to 300, and I put -3dBm 100 MHz RF signal to the beatbox during the measurement.
Other measurements needed:
- Linear range: By sweeping the RF input frequency and see sensitivity dependence.
- Bandwidth: By measuring transfer function from the modulation frequency of the RF input to phase tracker output.
- Maximum sensitivity: Sensitivity dependence on delay-line length (see PSL_Lab #825).
- Noise: Lock oscillator frequency with phase tracker and measure out-of-loop frequency noise with phase tracker.
- Sensitivity to amplitude fluctuation: Modulate RF input amplitude and measure the sensitivity.