+- 90 phase shifter revisited

You are being excessively stringent. If the phase-locked loop is fast enough to track the planned-for phase excursions, it can track larger phase excursions than +/-pi radians (or +/-180 degrees if you aren't a physicist).

You need to have some mechanism to keep track of the more-than-one-cycle phase excursions, but that's what counters are for.

Floyd M. Gardners's book "Phaselock techniques" talks about this at length, and what sort of tracking errors various order of phase-locked loop can be expected to show.

So, "at low frequency offsets" screws with a synchrodyne situation?

Then, How to you make a low-noise phase shifter that can be played with a PFD? ...Jim Thompson

I posted one a day or two ago--the classical op amp phase shifter circuit, using a dual varactor with series and parallel inductors.

That won't quite get to pi radians per section, but it's pretty close, and the linearity is good enough to use inside a FB loop (+-4 degrees or so). Two sections will get nearly a whole cycle, which matches what a PFD can do.

For the lowest noise, you'd want to keep the impedance level down, which at 1 MHz would require parallelling a few BB201 dual varactors.

For really low noise PLLs, diode bridges are a lot better than PFDs, but to get a range of 2 pi, you have to have some scheme for figuring out which of the two nulls to aim for.

I did a 13-bit, 50 ks/s, 60 MHz successive approximation phase digitizer for my thesis project, using a digital version of that scheme--it used a

Cheers

Phil Hobbs

to track the planned-for phase excursions, it can track larger phase excursions than +/-pi radians (or +/-180 degrees if you aren't a physicist).

excursions, but that's what counters are for.

and what sort of tracking errors various order of phase-locked loop can be expected to show.

Gardner talks about cycle skipping and near-skipping (which he calls "clicks" and "anti-clicks" iirc) in a PLL at low signal to noise ratios.

I'm talking about a completely different situation--very high SNR, with a phase shifter with a limited range. There's only one way for that to lose lock, namely when the PFD runs into its sawtooth discontinuity at

Cheers

Phil Hobbs

I don't eat at fast food restaurants. I don't care for the rat juice seasonings >:-} ...Jim Thompson

They are amongst my most valued buddies >:-} ...Jim Thompson

If you read what it says below the logo's, in places where they have camera's, it clearly state's that all video taken is the property of FB and Twitter.

There are 2 restaurants that know of here with this very same logo and disclaimer on their door for you to read as you walk in. Most people don't bother, their stomach are more important. But if you enjoy being watched so be it.

Some people are so naive.

Jamie

If they want me, they can come and get me. I have nothing to hide and nothing they want. But the bastards should do it like real men instead of exploiting and misusing the social networks for devious task that go far beyond their duties.

If you enjoy being watched so be it, just remember this, professional crooks know how to use these systems.

Jamie

But once in a while you get a fresh rat.

Jamie

Yes, but only you post something, Maynard.

That is an interesting assertion. I happen to have read various T-1, T-3, and SONET standards and they do not do that at all. It is not like there aren't plenty of other things to do to keep it all running (many of which are described in the standards).

?-)

manipulate the

carrier.

you want).

oscillator.

I gonna be a bit blunt. This sounds like a homework assignment. Or possibly a training assignment for a very junior engineering assistant (undergraduate summer job). Worst of all the goal seem to suffer from requirements creep. The way you are coy about the task particulars indicates that you have little understanding of what has been done, how and when. Spend a few dozen hours patent searching for similar systems.

?-)

y"

tor

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an

hase > > excursions than +/-pi radians (or +/-180 degrees if you aren't a

gth, > > and what sort of tracking errors various order of phase-locked loo p can be > > expected to show.

And what I had in mind - and totally failed to communicate - was that it is trivially easy to keep track of larger phase excursions, provided that you have a second - quadrature - phase detector to keep track of the direction of the excursion when it crosses +/-pi.

Interferometers use this trick to measure distances which are multiple of t he wave-length of the light (or other waveform) involved, and the more mund ane Moire-fringe displacement measuring systems (which make Heidenhain load s of money) rely on the same idea.

You do need the second detector - and it doesn't have to be exactly in quad rature with the first, or anything like it - and you end up with a necessar ily digital system, because you add the "clicks" and subtract the "anti-cli cks" into some kind of digital register,

As it is a digital system, you have to worry about keeping the sampling rat e high enough to avoid aliasing, but that's a whole lot less stringent than avoiding more that +/-pi of phase excursion.

When you read the SONET standard, did you notice the part about pointer movements? Their role is to account for the frequency differences between network clocks. It is possible (but not usual) to pass data through a SONET network with a clock that is not locked to the network reference, by means of pointer movements.

Regards, Allan

Okay, that I would not like. If you happen to have a photo of the disclaimer I'd be interested.

Best regards, Spehro Pefhany

Sure, provided that what the OP is trying to do is measure absolute phase, which I don't think is the case--AFAICT he wants to do PM demodulation.

And of course that trick runs into major difficulties if the phase is jittering back and forth across the +-pi discontinuity, because you can easily lose count while the loop is misbehaving. Nulling methods are generally better for large excursions, because you never lose track of where you are.

The phase SAR approach that I mentioned earlier worked well because the phase was known to be stable during the measurement--it was a scanning heterodyne-interferometric microscope that was scanned in steps using an acousto-optic deflector. The parlour trick there was that there was a huge acoustic delay that showed up as this massive phase slope on the data. Fortunately it really was a pure time delay, so after unwrapping all it needed was a constant phase slope subtracted.

Cheers

Phil Hobbs

But does it really mean security camera "footage"? I find that hard to believe. Is it in fact really just customer video & pictures, that the customer decides to upload to facebook. "Look, I'm having a Big Mac". In other words, just the usual user content grab of these outfits, just the same as if you were sitting at home doing the same thing, but the shop does not want to be sued.

If he's trying to do PM demodulation, whatever he's trying to demodulate ha s been designed so that it can be demodulated. His local oscillator will be able to track the phase excursions because because the source oscillator i sn't going to generating some impossible - or even difficult - to demodulat e signal.

What would be the point?

synthetic aperture radar?

All very impressive, but just a tad irrelevant.

Based on his posts from some months ago, he appears to be trying to do it over the air, in the presence of atmospheric variations. The question is how large these are, and how fast.

Successive approximation register, see my earlier post.

You know, Bill, sometimes people talk about stuff because it's interesting, not because they're showing off. This is a *discussion group*, you see.

Cheers

Phil Hobbs