2008/10/17 Mike Monett <[EMAIL PROTECTED]>: > Those are all variations of an XOR. This is perhaps the worst > possible phase detector to use for precision applications. It has > very high ripple at lock, which generates unwanted sidebands in the > PLL oscillator output. > > It requires signals 90 degrees out of phase, so you have to add > extra logic to lock to signals that are in phase. This adds jitter. > > The output is not referenced to ground, but to some floating level. > This means very small offsets and drift will change the lock point. > > It is also unsuitable for measuring phase angle. The slope of the > transfer curve for the basic XOR reverses at +/- 90 degrees. So you > can't use it to measure phase angle. > > The AD9901 hase all the above problems but it rails at +/- 180 > degrees instead. It cannot be used to measure phase angle either. > > The standard phase/frequency detector (PFD) is better for precision > PLL's since the output signal is zero at lock. This means minimum > ripple to generate sidebands. It can be reset to a known state so it > can be started in phase with an incoming signal. This greatly > reduces the lock time. > > It wraps at 360 degrees, so it is also better suited for measuring > phase angle. > > These are only some of the issues in analyzing a phase detector for > a precision pll. Here is a quickie LTspice analysis showing the > phase detector transfer curves for the three detectors. I posted it > so people would not get their mailbox clogged with attachments. > > http://silversol.net63.net/spice/phasedet/phasedet.htm > > [...] > > > The quickest and easiest is to use the AD9901 phase detector as it > > only has a few ns of non linearity at the end of the range. For > > higher resolution just add a sigma delta ADC. > > The AD9901 is a very bad design. Probably the poorest I have ever > seen. It is unsuitable for measuring phase angle. Adding a high > resolution ADC will do nothing to help. > > I notice there is very little in the way of analysis on your site. > This means it is impossible to distinguish one circuit from another. > > What you really need to do is go through and put everything in > LTspice so people can look at the circuit behavior in detail. > > Then you need to do a tolerance and sensitivity analysis to show how > stable the circuits are over long periods. > > Then you need to do a noise analysis to show why some circuits are > better than others. > > This would make your information much more valuable and interesting. > > One of my big problems right now is trying to understand why the XOR > works so well in one of the GPSDO's in Tom's Allan variance pages. I > can't find it at the moment, but I need to take a very close look at > this design and see what is going on.
One of the greatest problems in any pll circuit is instability caused by time delays in the feedback low pass circuit. In a locked system where the feedback voltage approaches the rails, stability improves due to the lower error signal. Getting the time constant of a simple low pass feedback circuit right is not easy especially in high precision phase locked systems. Ideally the filter should have a very low frequency cutoff but have very low time delay and the two are usually at odds with each other in simple designs. 73 Steve -- Steve Rooke - ZL3TUV & G8KVD Omnium finis imminet _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.