I checked electronic noise, of the UPDH box while tuning its gain, to understand why the beat signal gets higher when the gain on UPDH increases.
I still don't understand ACAV loop's behavior, but it is not the current limiting noise source, either RCAV loop or PLL is.
Right now, the beat signal at high frequency is limited by an unknown source(s). But by changing the gain of the UPDH which is used
for locking the beam to ACAV, the beat signal gets higher, before the oscillation in the loop occur.
I would expect the loop to become unstable first, so the beat gets worse, but this is not the case. So I measured
error signal, Vout to VCO, TF of the UPDH, noise of the UPDH, and varied the gain.
This UPDH has knob to adjust its gain. The dial reads from 0 to 10.
I check that the beat signal does not change when the gain of the UPDH is changed from 0 to 1.5, then it starts to get higher, broadband at high frequency, 200Hz and above.
So I varied the gain and recording the following(I made sure that the loop is still stable, no oscillation in ACAV_RCTRANSPD)
1)Above, beat measurement
2)Above, error signal, during closed loop
3) actuator signal to VCO, during closed loop
4)noise of the UPDH box, 50ohm terminated input,
5) TF between input and output(UPDH TF)
6) so from, 4 and 5 I can compute the input referred noise of the box. The result is unexpected. The input referred noise is getting lower
as the gain increases. With the error signal slope of 0.11 MHz/V the input referred noise is still lower than the beat (~ 10^-1 Hz/rtHz.)
So the input referred noise of the UPDH that changes with gain might not be the limiting noise.
However, when I project the noise from error point to beat noise with the slope of the error signal being 0.11 MHz/V.
The noise when the gain is 1 and 1.5 are lower than the beat. However, when the gain is 3.2 and 4 where the beat is getting higher,
the noise projection match the beat signal.
This means that
1) the noise projection factor I get this time is correct,
2) the limiting noise at gain 1.5 (usual setup) is not from ACAV loop.
3) UPDH will be the limiting noise source very soon. Since at gain 1.5, it is only a factor of 2 lower than the current beat.
So the next thing is to check if the limiting noise source comes from RCAV loop or from PLL loop.
* note: the phase shift dial on UPDH box is probably broken. I could not change the error signal with it.
I'll have to think over about what causes this.