about phase frequency detector in PLL

Thanks for your reply. This is a charge pump pfd...the refrence frequ is 212.5MHz and fout is

4GhZ, divison ratio is 20... They are very narrow spikes, not pulses.. and are or equal width and at regular intervals.... I can send you the jpeg file if that ok with you, pl let me know..

Meghana

Andrew Holme wrote:

Is the loop locked?

snipped-for-privacy@yahoo.com wrote: (top posting fixed)

You are most likely seeing exactly what you should.

Phase detectors aren't perfect, and neither are charge pump filters, so there's always some offset or leakage that the phase detector has to null out with little spikes from the "up" or "down" output. You just have to design your PLL to perform well in the face of the noise.

--

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

Suck thumb? Check the VCO spectrum?

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

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Narrow spikes are exactly what you should see when the loop is in lock...

try to detune the VCO and you will see the pulses created to correct the error.

Mark

What interval? At the reference / comparison frequency?

Are the up and down spikes of equal width, and coincident in time?

Are they very narrow spikes? That might be OK. Or are they more pulse than spike??

What PFD are you using?

Is the loop locked?

Thanks for your reply Mark

Can you pl elaborate how I should detune the VCO?

depends on what type of vco it is

an LC VCO can be perterbed by bringing your finger near the coil

xtals can be pertrubed a little by squeexing them or adding come capacitnace to one lead

voltage or temperature changes can work too.

almost anything that tries to change the frequency of the vco will require the PD to generate a correction...thats the point of a PLL afterall..

Mark

Mark

In practice any real charge pump can only produce pulses down to a certain minimum width. Below that width, the peak current of the pulse is never achieved, and the relationship between pulse width and integrated charge in the pulse becomes non-linear. It is desirable always to operate in the region where the charge pump is linear, particularly in Fractional-N synthesisers. In order to achieve this, a certain minimum time is often applied to both the up and down currents. Provided the up and down currents are well matched, the only disadvantage of the scheme is slightly increased noise, but this is usually small in comparason to the thermal noise in the loop filter resistors etc.

Chris

You need to disconnect the VCO tune input from the loop filter and apply an external voltage to the vco. Vary the voltage and monitor the PFD to see if the phase switches. I've seen a few engineers connect the PFD polarity ass-backwards try that too. Which PFD are you using anyways?