Can PLL remove or reduce the jitter?

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gingerjiang

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hi all

i'm designing a pll
i want to know if the reference input clock has a jitter, can pll remove or reduce the jitter?

thanks in advance!
 

Re: function of pll

sure the PLL act as low pass filter for the input phase noise "jitter" , so it will minimize the input jitter , but if ur PLL have a divider be aware that the phase noise of the reference clock will be amplified by the value of the divider , coz the N will be in the loop gain
khouly
 

    gingerjiang

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Re: function of pll

my pll has not divider, so if pll is designed properly, input reference jitter will be reduced, it's sure?
thanks
 

Re: function of pll

yeah , if ur loop filter is narrow it will attenuate the phase noise from the reffrence co it act as low pass filter

khouly
 

    gingerjiang

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Re: function of pll

khouly said:
yeah , if ur loop filter is narrow it will attenuate the phase noise from the reffrence co it act as low pass filter

khouly
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hi khouly

yeah, you'r right
in pll, the vco contributes to output's jitter, and the jitter is also suppressed by loop filter, i remember that larger is bandwidth, smaller is vco's jitter, so in order to reduce the reference jitter, vco's jitter must be small enough, and the loop filter's bandwidth has a critical point, it's right?
 

Re: function of pll

yeah
the loop band width is very critical in jitter and phase noise optimization , so u need it optimize it well
coz if the BW is high it will attenuate the jetter or the phase noise from the VCO but not from the reference ,
and if it is low BW the jetter from the reference will be attenuated

khouly
 

    gingerjiang

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Re: function of pll

There is an optimum loop filter bandwidth that provides a minimum integrated jitter. If you are only looking for minimum jitter, then you may get an improvement. Vauchers PLL book has the details, but you can use ADISim to do it manually. I've written an Excel PLL spreadsheet that could do most of it, with a little more work.


The minimum jitter is dependent on the Reference Oscillator phase noise, the VCO phase noise, and the loop filter bandwidth.

Dave
www.keystoneradio.com
 

    gingerjiang

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function of pll

yes ofcours b coz the vco needs only the control voltage as its input which is generated by comparision of reference input and the vco output and the control voltage is itself a dc which is given to vco after passing it trough a filter so there is no reason why the pll wont remove the input reference clock jitter
 

    gingerjiang

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Re: function of pll

The key, as some have alluded to, is the phase noise of the VCO.

If you have a lot of reference noise, you can use a low phase noise VCO to clean it up. You would use a SAW or CRYSTAL VCXO and a narrowband loop filter, maybe set at 100 Hz. Then the VCO acts as a flywheel to the perterbations of the reference, but in a slow time frame the reference keeps the VCO on frequency.

If you try to use a low Q VCO, you will find that the phase noise in your VCO might be higher than the reference phase noise, so the PLL no longer cleans up the phase jitter, no matter what the loop bandwidth.
 

    gingerjiang

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