# Calculation of the loop filter impedances

#### Zyrlen

Joined Nov 4, 2019
1
Hello,
I have received a project where I am supposed to finish the work of another student. The project is about locking two clock frequencies to a reference clock to reduce the low-frequency noise of the two frequencies. Part of the job is to modify the loop filter, but I am unsure as to how the filter resistances have been calculated and was wondering if it is something obvious. Attached is a picture of the loop filter.
I hope someone can help me gain some understanding of how the calculation is done.

Thanks,
Zyrlen

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#### MrAl

Joined Jun 17, 2014
8,157
Hello there,

With a casual glance it looks like the Rfeedback goes down in proportion to the frequency while the Rseries goes down as the square of the frequency. I can only base this on the two examples given however.

So if you have 28k series and 41k feedback for 20Hz, then for 40Hz you would have:
Rseries=28k/(40/20)^2= 28k/4=7k
Rfeedback=41k/(40/20)=41k/2=20.5k

You would then divide Rseries by 3 to get the three series resistors.

Again that is based solely on the two examples they give. I can look up more info on loop filters but at these low frequencies they usually dont use inductors just RC filters and op amp(s).