M.L. wrote:
> I'm out of my element when it comes to practical PLL circuits.
> I'm considering using a PLL to create a 25MHz signal which should be
> phase locked to an input reference 20MHz signal. Do you know of some
> ICs that I could consider for this purpose?
> Most of the ones that I have seen quote significantly higher frequency
> values. Usually higher than 200 MHz.

My usual approach for this sort of thing is to use a VCXO module for the=20
oscillator, and control it using a small CPLD, or a portion of a larger FPG=
A
if there's one in the system anyway.

The CPLD contains the dividers and phase comparator, and drives the VCXO wi=
th
a PWM (or delta-sigma) digial signal through a simple RC low-pass filter.

One advantage of this approach is that it gives you a lot of flexibility wi=
th
regard to the VCXO behavior when out of lock, or when making the transition
into or out of lock -- sometimes I've had some very specific requirements i=
n
these areas.

-- Dave Tweed
--=20
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