understanding error voltage link to pll locking status

[john23]

Hello , I have an intresting phenomena when I am trying to lock my PLL by the system shown below.
My error voltage which is IF of a mixer gets amplified and the amplified signal goes into YIG(VCO)
When the phase difference voltage is low and rippling as shown in photo 1, then I see the VCO going crazy back and forth changing the frequency as shown in photo 2.
However when I turn the phase shifter and the error voltage is getting higher then I get straight line error shape and a single tone.
I wonder about two things :
1.When I put big phase shift then IF signal voltage also is high.
High IF signal goes into my amplifier and its getting comppressed (final amount of current going into YIG)
So thats in turn creates a final amount of shift in the VCO so thats why i get a single tone.
But error signal after the feedback is a straight line.
What is the meaning of that?
2. My VCO needs to be connected as negative feedback.
Does it act like the feedback is negative in my system?
Thanks.
Video link:
https://technionmail-my.sharepoint.com/:v:/g/personal/yefimv_technion_ac_il/ES9y1VBxJVBAlNUUmKMyMT8BGzjR9Kd-cN36A6_6LWSdmg?e=duFpS5&nav=eyJyZWZlcnJhbEluZm8iOnsicmVmZXJyYWxBcHAiOiJTdHJlYW1XZWJBcHAiLCJyZWZlcnJhbFZpZXciOiJTaGFyZURpYWxvZy1MaW5rIiwicmVmZXJyYWxBcHBQbGF0Zm9ybSI6IldlYiIsInJlZmVycmFsTW9kZSI6InZpZXcifX0%3D

[David Hess]

Have you fiddled with the compensation of the servo amplifier?

What you describe seems like oscillation of the servo amplifier when operating in its linear range, which happens when the phase locked loop is close to locked.

[john23]

Hello David, My VCO is YIG so input voltage converts to current.
I have changed the amplifier so the voltage current slope will be smaller as shown below.
After that I see the phollowing photos ,Its moving around the point  with voltage and current varying.
How do you reccomend me to change the amplyfier to change the locking situation.
How can I see if  the locking situation is improving?

[john23]

UPDATE:

Hello David, I got a big problem.
my servo amplifier oscilates  in the lab i dont know why.
amplifier PCB file is attached.
How can i Investigate oscilations and see how can i get it out?

[David Hess]

The frequency compensation is shown in your schematic as the series capacitor and resistor connected between the servo amplifier output and servo amplifier input.  That network controls the gain roll-off of the servo amplifier so that gain of the entire loop is below 1 before the external phase delay reaches 90 degrees.  Lowering the gain within the feedback loop will definitely help, but is not ideal.

For simple testing, perturb the loop and monitor the output of the servo amplifier for settling time and lack of oscillation.  I usually attach a slow square wave current source (function generator output through a resistor and capacitor) at the input to the amplifier, but there are many ways.

Are you saying that the servo amplifier by itself is oscillating when not connected to anything?

[john23]

Hello , Regarding the controller for the YIG (VCO):
I have two coils , the first is large tune coil with drifting .
This tune coil I set at close as possible to the resonance frequency.
The second coil isFM coil:
1.5uH inductance and 1Ohm resistance FM coil.
sensitivity 400Khz/mA

We are tuning the error using the FM coil, so the large tune coil is out of the feedback correction picture.
Tune is not something I changethis coil getting all the time the same signal and its drifting.
I need somehow to change the signal in the FM coil so it will compensate for the drift coming from the tune coil.

I learned that maybe I should use PI controller .Is it correct?
Could you please give me general direction regarding such controller?