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Re: Best loop filter for the CS812, Wildmonkeysects?

Posted by wildmonkeysects on March 13, 2001 at 15:23:39:
Ya hoida me?
Try a 470 or 500 ohm resistor in series with 0.22 microfarads; and up the loop to 3rd order by adding a 3300 picofarad cap from the loop filt pin directly to ground. This pin is *extremely* sensitive to noise pickup, keep lead lengths short. This attempts to keep adequate loop phase margin, maintaining settling time and avoiding peaking. Unfortunately there is no single best number here, and elsewhere for that matter.
Notice the vague square law correspondence here, the resistor is about half of the data book, and the cap is about 4x. Coincidence? Synchronicity? Just math...
Within [or "below"] loop bandwidth the source phase noise dominates, and above it the local vco phase noise dominates. Hence the pll "tracks" the incoming frequency and artifacts with a settling time approx inverse to the loop bandwidth, so any peaking is to be avoided.
If one has the time to twizzle, a better solution [but not the one true way (tm)] is to place the master clock right at the DACs [fed by it's own regulated power supply] registering/reclocking the DACs and "slaving" the transport to that master clock, guaranteeing synchronicity [of the clocking type, not the cosmic type] and eliminating dependence on the pll/vco.
But, I admit, it is *fun* to meet the challenge and twizzle in an attempt to extract a clean enough clock.
Share and enjoy...