Phase Margin Question for Phase Locked Loops

Jerry Avins wrote:


I most certainly do. See Barkhausen in my original post.
An inverter between the

The
of
Well, we have two integrators in this case, the op-amp with capacitive feedback and the VCO.
Slick
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snipped-for-privacy@aol.com wrote:

Then why do you need any help?
If the circuit provides negative feedback at DC and has insufficient phase margin at some frequency, then slapping in an inverter will give you positive feedback, with some _very_ peculiar results. If you're using an XOR or other mixer for the phase detector then it'll invert automatically, and you won't be changing the gain/phase relationship of the loop.

No, a few more than that: Passive, single ended filters. Active filters using op-amps, inverting, non-inverting, and double to single-ended. Closed-loop control systems using several different types of microprocessors, DSPs and DSPs roped to FPGAs.
I haven't used pneumatics, mechanical amplifiers, magnetic amplifiers or dynamotors yet, but you never know.

From what population of PLLs are you drawing this "most"? If you're working with the current crop of PLL-based synthesizers-on-a-chip then using an inverting filter means you have to reverse the sense of the VCO command/frequency relationship, or you need to flip a bit in the phase detector someplace. If you're using an XOR phase detector then the point is moot, of course.

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Tim Wescott
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Tim Wescott wrote:

We would like to improve the current design, if we could. But it certainly already works. We would like to omit some fast acquisition 1N4148 back-to-back diodes that we have in parallel to the output resistor, as they supposedly distort our audio modulation at low freq. However, the lock-up time is slower without them, so we'd like to adjust the poles and zeros for faster lock without them, without losing too much phase margin/stability.

then
VCO
phase
Well, that was my orginal question, do all phase detectors, whether XOR or phase/freq, have the 180 degree phase shift that you need. Apparently, yes.
Slick
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snipped-for-privacy@aol.com wrote:
[snip]

They detect the phase difference. If the inputs are a(t) and b(t) then the phase difference is:
a(t) - b(t)
The -ve sign in front of b(t) is where the 180 degrees comes from.
a(t) = reference b(t) = vco
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snipped-for-privacy@aol.com wrote:

For some reason your postings show your messages as going to sci.engr.control, and our replies go there, but your postings don't.
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Tim Wescott
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I read in sci.electronics.design that Tim Wescott
com>) about 'Phase Margin Question for Phase Locked Loops', on Mon, 3 Jan 2005:

Maybe his ISP doesn't carry that newsgroup.
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Regards, John Woodgate, OOO - Own Opinions Only.
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