Re: PLL convergence problem

yoni.baron@xxxxxxxxx wrote:

I have a "phase detector" for timing synchronization based on 4th order
non-linearity. I use a very narrow loop-filter - 400Hz when the
sampling rate is 10MHz.
The S-Curve for the detector shows nice curve (I use ~10000 averages
for every point in the S-Curve).
When I use the detector as is, the loop fails to converge.

To cause the loop to converge I'm using a simple leaky integrator with
the following structure y[n] = 0.9*y[n-1] + 0.1*x[n], to filter the
detector output before entering it into the loop filter.

I don't understand why the leaky integrator causes the loop to
converge. Basically, the leaky integrator turns the close loop filter
from second order to third order, but If I plot the frequency response
of the second and third order filter I see no much deference.

I also don't know how to investigate what is happening. I tried to use
the linear equivalent closed loop filter in matlab, but because the
filter is sow narrow, it's impulse response is not stable because the
poles of the filter are very close to the unit circle.

Thanks,
Yoni.

It sounds like your phase detector has a lot of noise that's throwing your loop out of whack. When you used your 10000 averages to find the detector characteristic did you look at how much random noise comes out? By running the signal through the low-pass filter you'd be cleaning up such noise.

What are you using for a loop filter? Is it a PID or just a PI filter? Even if it's just a PI filter knocking the instantaneous noise down by a factor of 10 must be doing something for you. Your gains must be very low indeed -- are you hitting a nonlinearity in your NCO or in your loop filter that's causing problems? Overflow, perhaps?

If you're really closing the loop at 400Hz then you should be able to filter this much more heavily. If you've already solved your problem then there's no need, of course, but if you've only partially solved it you should be able to bring your low-pass filter pole down to 4kHz fairly safely -- i.e. y[n] = y[n-1] + 0.0025(x[n] - y[n-1]). Note that with the pole value so low you may have underflow problems where you didn't before.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com
.

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