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PLL out of lock after several seconds

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golohoyeah

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I am testing a PLL which I designed with VCO oscillates at around 400MHz.

I find the lock on time is much slow than that what I simulated from around 150us to several seconds,
and what's more, after the PLL lock to a desired frequency 400MHz, it will stay there for around 10 seconds, and move out the lock to much higher frequency, and stay that frequency forever.

I am wondering what is the possibilities for such result to happen, I am suspecting the supply noise, and driving it with an external source, seems the jitter perform better than before, but the locking issue still there.

I am out of idea now, hope someone can give some hints or suggestion.

best regards,
golohoyeah
 

By any chance is your frequency lock determined by a resistor-capacitor combination? It may help for you to try various R:C ratios, yet which still create your desired RC time constant.

Example, very low R, high C: requires greater current possibly more than the IC can source/ sink.

Very high R, low C: high impedance circuitry is more likely to be affected by external EMI, tv broadcast, etc.
 

Check out the Crystal stability as well, the Crystal in the circuit may be unstable, which can cause that kind of issue.
 

You have to check the output of R and N divider. It could be the level of the rf signal sent back to the pll is too low. Also investigate if the charge pump is working properly.
 

it could be a number of things.
I would hook up a voltmeter to the VCO control line, and see if the DC voltage is nearing a supply rail....if say you have a 3.3 V supply rail, and you need 3.0 volts to lock up the VCO...maybe the op amp or charge pump can not put out that much voltage, and it falls out of lock.

I would also suspect that the PLL is unstable. You need phase margin and gain margin to have a stable PLL control loop. Often times this means adding a control loop "zero" to the feedback circuit. Otherwise you have two poles (the VCO is one pole, and the integrator op amp a 2nd pole) and you have an inherently unstable loop without adding a zero. Another common term for this is to add a "lead-Lag network".

You might have higher order poles, like due to shunt RF bypass capacitors in the tuning part of the VCO, that further degrade the PLL stability, and need to be compensated. This is a common problem if you are using a really big control loop bandwidth...like 1 MHz wide.

You might have not taken into account "Transport Lag" in your control loop design. If you have a big divisor ratio N....what happens is some digital counter counts N RF input pulses, and when N is achieved, it puts out ONE digital pulse. So if N=600,345.....it takes T = (600345) * (the VCO period) to get an output from the divider. THAT can be a significant time delay. A big time delay has the same effect as having poor phase margin in your control loop design.

HOW TO TEST for stability? I like to add a small digital phase shifter to the clock port, and then look at the phase response of the locked VCO. If you step the clock 2 degrees in phase, and the VCO has a nice damped phase change that settles out quickly...you probably have a good control loop circuit. If you put your clock phase step into the system, and the VCO phase goes crazy with all sorts of ringing and instability...you have a marginally stable, or even unstable, PLL design.

HOW TO TEST if you are actually phase locked? Sometimes you THINK it is phase locked, but you are kidding yourself. Move the clock frequency a few Hz. If the VCO moves N x a few Hz....it is locked. It needs to be EXACTLY N times! (N being the divisor, or fractional divisor ratio you think you are programming in)
 
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The above nice guys have given you a lot of good ideas. I just want to give you some extra comments:
1. Which frequency does the PLL stays at? Is it the harmonics of 400MHz or some strange frequency?

2. Try to feed the VCO control voltage with several different level, and see if the output frequency is what the datasheet says;

3. Measure the VCO control voltage when the PLL is locked to the strange frequency. Oscilloscope with high impedance probe is preferred;

4. If you have several prototype cards, just see if they all have the same problem.

Good luck!
 

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