PLL problem - SIG to COMP In phase between changes

[carl]

Phase lock loop problem

Hi, every one,

I designed one PLL with 4046, the VCO is set at 8MHz with a exter divider of 1024, and the filter is a simple lead-lag filter.

Now the problem is the frequecy is locked well, but the phase between the SIG and COMP In changes when I change the input freuqency.

I suppose I need to change the filter to an active filter to make a type 2 system, so that there is no steady phase error. Is this right?

thanks in advance

 

[keith1200rs]

Phase lock loop problem

I seem to recall there are two phase comparators in the 4046 - one doesn't maintain the phase, the other will maintain zero degrees phase. Try the other one to the one you are currently using.

Keith.

 

[carl]

Re: Phase lock loop problem

I seem to recall there are two phase comparators in the 4046 - one doesn't maintain the phase, the other will maintain zero degrees phase. Try the other one to the one you are currently using.

Keith.

yes, there are three Phase Detector in 4046, and I used PC 1. The phase difference between the input signal and reference signal is 90 deg at the center frequency. But if I change the input frequency, the reference freqeucy change accordingly, only the phase is not 90 deg more.

Thanks for your advice, I will try the other Phase comparator.

 

[biff44]

Are you talking about a little phase error offset that is constant, or a big phase error that moves around if you breath on the circuit. In the later case, you should suspect that the PLL is not truly locked.

In the former case, yes, there is a small static phase error unless you have either an integrating op amp, or some sort of a charge pump to charge up a capacitor with the average voltage needed to maintain frequency lock. The problem is that that static phase error that keeps sending pulses to keep the VCO locked up also gives rise to big RF sideband spurs. To reduce the sideband spurs, you want the width of the phase error pulses to go to ~ zero. To do that, you need an integrator somewhere.

Rich