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Phase Margin Question for Phase Locked Loops

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.

Reply to
Tim Wescott
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You mean the magnitude of [GH] through unity. GH=Kpd*Kfilter*Kvco/(s*N)

Slick

Reply to
radio913

I most certainly do. See Barkhausen in my original post.

An inverter between the

Well, we have two integrators in this case, the op-amp with capacitive feedback and the VCO.

Slick

Reply to
radio913

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.

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

Reply to
radio913

Yes. I forgot the N. I was thinking of a loop I did with N=1. I actually prefer to use G(s) for open-loop gain and H(s) for closed loop gain; so, personally, I would write it as:

G(s) = kPD * A*(s+B)/s * kVCO/s * 1/N

You say in another post that you want to improve the lock time. I recently designed a loop that had to lock very quickly. I found it useful to do Bode plots and step response using SCILAB.

Reply to
Andrew Holme

Most of the literature uses G(s) for the forward gain and H(s) for the feedback path.

But, to each their own.

What sort of phase margin did you shoot for? And what type of filter did you use?

Slick

Reply to
radio913

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