Question about calculating values for a 2nd order PLLXO

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madisonears

Member
Joined 2007
#1
On the TLS.org site for calculating component values of 2nd order pllxo, the formula prescribes that C1 and C2 as well as R1 and R2 differ by a factor of 10. What is important about that factor? There are myriad combinations of values of those four components that will give a specific identical (or very nearly so) F3. Why is it defined that the two sections differ by a value of 10? What happens if C1 and C2 (and proportionately, R1 and R2) differ by a value of, say, 8?

Thanks to anyone who can shed light on this.

Peace,
Tom E
 
#2
10 is significantly greater than 1. It's an engineering rule of thumb. It's an order of magnitude. 1 only turns 10 into either 11 or 9, which are both significantly as good as 10.

In any case, why don't you get yourself a circuit simulator and take this to the next level by simulating a couple of stages together and making it work exactly?
 
#3
Um, okay, that doesn't really answer my question, but thanks for your input. I understand an order of magnitude, but why is that important in this circuit?

Please suggest a circuit simulator for someone with zero experience but a probing intellect, something relatively easy to learn, and I will gladly attempt to model this filter so it works "exactly" as I intend it. I am not evaluating anything complex, so a simple system probably would suffice. I see LTspice mentioned here all the time. Any others?

Peace,
Tom E
 
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