strategy for designing a locking component in PLL

Thread Starter

yef smith

Joined Aug 2, 2020
755
Hello, i am trying to implement the circuit shown bellow.its a PLL device.the mixer produses DC voltage signal which is error between the reference oscilator and the main oscilator as described in the attached article.idially the error voltage needs to be zero and the reference frequency needs to match the main oscilator frequency.
basickly the heart of it is the SERVO amplifier.the servo amplifier takes the error voltage and converts it into chaging the reference oscilation of frequency.
looking at the diagram bellow is there a way to predict wether servo amplifyer could achieve the desired locking between the refrence and main oscilators?
Thanks.
1702933145234.png
 

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MisterBill2

Joined Jan 23, 2018
18,517
This is the identical circuit to a recent post, and neither had a "reference oscillator. The effort then was to lock the YIG oscillator to the resonant frequency of the cavity. The plan appears to be based on the cavity having a much more stable frequency. So why not use the cavity itself as the frequency determining element of an oscillator, and then phase lock the YIG oscillator to that? Itwould be less complicated and require fewer adjustments.
 

Tesla23

Joined May 10, 2009
542
This can be analyzed the same as a basic phase locked loop. If I take a PLL using a mixer:

1703804209787.png

the reference oscillator and mixer (in the red border) take the VCO signal at A and returns a signal at B which is the phase difference between the VCO signal and the reference signal.

You should be able to recognize your circuit as replacing the reference / mixer as:

1703804567096.png

where the VCO signal at A enters, and what returns at B is a signal representing the deviation of the frequency of the VCO from the centre frequency of the resonator. You should design the 'servo amplifier' as you would design a loop filter in a PLL and ensure you have adequate bandwidth to achieve the desired phase noise suppression, and sufficient gain/phase margin for stability. I imagine you will have to account for the (probably) low pass response of the YIG control. You will need to think carefully as to how many integrators there are in the loop as superficially it looks like the error signal depends on the frequency and not the phase. There is plenty of literature on PLL design available.

As this is a school project, you need to make some progress.
 

Thread Starter

yef smith

Joined Aug 2, 2020
755
Hello ,In the diagram below they did do carrier supression with A1 and phi1 but then they amplified the whole signal.
What is the purpose of first nulling the carrier and them to amplify it?
it seems to me like the mixer will compare the YIG signal to amplified white noise and i dont see any logic in that.

1703855627335.png
 

MisterBill2

Joined Jan 23, 2018
18,517
It appears to me that the goal is to phase lock the YIG ooscillator device with the very stable resonant frequency of the cavity. Thus the motivation is to collect the resonance signal from the cavity and use that as the PLL reference signal. I question the validity of that concept, which is why I suggested using the cavity exclusively as the frequency determining element in an oscillator. But the published article in the link appears to claim that the intended scheme will work. I am not quite so convinced of that.
 

Tesla23

Joined May 10, 2009
542
Hello ,In the diagram below they did do carrier supression with A1 and phi1 but then they amplified the whole signal.
What is the purpose of first nulling the carrier and them to amplify it?
it seems to me like the mixer will compare the YIG signal to amplified white noise and i dont see any logic in that.
I answered this back in June when you were last working on the problem:
https://electronics.stackexchange.com/a/669009/276605
 

MisterBill2

Joined Jan 23, 2018
18,517
None have challenged or agreed with my understanding of the intention., and the original referenced technical paper left me wondering. And then, none have commented on my interpretation stated in post #5.
Repeating ones goal in some effort is entirely acceptable in this forum, in fact it is often suggested. Very few have mind-reading abilities such that they can clearly discern what some TS is actually seeking.
 

Tesla23

Joined May 10, 2009
542
None have challenged or agreed with my understanding of the intention., and the original referenced technical paper left me wondering. And then, none have commented on my interpretation stated in post #5.
Repeating ones goal in some effort is entirely acceptable in this forum, in fact it is often suggested. Very few have mind-reading abilities such that they can clearly discern what some TS is actually seeking.
I think that there is nothing fundamentally flawed in the approach. The referenced paper does show that the phase of the reflection coefficient from the cavity does vary rapidly with frequency around resonance (not unexpected), and that can be used in a control loop with a VCO to stabilise the VCO frequency. This is novel, I haven't seen it before, a method of disciplining an external oscillator with a high-Q cavity. As to how practical or useful it is, I have no idea.

You could use the cavity as the resonator in an oscillator, but that is not the issue here. I haven't stuidied this in detail (it seems unnecessarily complicated fo the things I do), but an obvious question is does this method load the resonator less, i.e. can you get a higher loaded Q in this setup than when you use the resonator in an oscillator. If so, then on at least one level, this approach may offer some advantage.
 

MisterBill2

Joined Jan 23, 2018
18,517
There is always a trade-off. Higher oscillator circuit gain with looser coupling improves the long-term stability but tends toward greater phase noise, depending on the circuit design and component noise. So it seems that the choice must depend on the application, and probably the project budget.
 

Thread Starter

yef smith

Joined Aug 2, 2020
755
Hello Tesla23,yes i agree that with this method i can null the leakage of the circulator.
However as you can see below in the article they dont only want to kill the leakage from the circulator but also the return signal from the cavity oscillator.

so if we killed all the feedback signal how are we going to compare the YIG with the resonator?
we are just comparing a YIG to a NOISE in the mixer.

" One way to do it is to replace the cavity with a matched load so that (ideally) there is no reflected wave, then adjust the amplitude and phase of the cancellation circuitry to null out the leakage through the circulator."

1703912295911.png
 

MisterBill2

Joined Jan 23, 2018
18,517
In post #10 I see a reference to a "cavity oscillator", which is rather confusing, given that the circuit drawing shows a cavity with a claimed very high "Q", of 59,000. I have not seen any reference to it as an oscillator able to produce any signal of it's own.
 
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