Can PLL control any frequency deviation of VCO?

[Muhammad Furqan]

PLL

can PLL control any frequency deviation of VCO or it is only sensitive to phase
if it can control the frequency deviation than how much frequency deviation it can control (approx value) if VCO is in s-band

 

[biff44]

Re: PLL

That is a good question.

A PLL locks phase, not frequency. If you change the frequency, the phase slews, so by definition it can not be phase locked.

But, you can fool the PLL into thinking it is locked. One technique quite commonly used in FSK communications is to use a very narrow band PLL that is DC coupled to the tune port of a VCO. You then AC couple the digital voltage onto the tune port of the VCO. The Digital voltage sweeps the VCO frequency + or - about the average, while the PLL tries to keep the average frequency in lock.

Caveats: you must use a large divide by N in the PLL, so that the instantaneous phase shift due to frequency tuning is much less than Π at any time.

Also, if you send lopsided data (like all zeroes for some time), the apparent carrier frequency will drift. This is why many simple systems use Manchester coding for the data.

 

[Muhammad Furqan]

Re: PLL

basically i am designing a PLL loop for a S band oscillator at 2.0Ghz
the critical thing is frequency because this is used as a local oscillator at receivers end (multiplier for demodulation)
now i am using IC from analog devise "ADF4007" which has a funtion of phase detector and a divider (divide by 64)
output is fead to the loop filter and finally to VCO
i have read the datasheet of ADF4007 after that i am confused that is it suitable for my need i.e. frequency stability or not ?
can u plz help me in this regard

 

[RFDave]

Re: PLL

Frequency Stability generally refer's to a long term stability, measured in terms of seconds. The frequency stability is determined by the frequency stability of the reference oscillator, and so long term, the stability of the Synthesizer output in term's of PPM offset (Parts per million) will be the same as the stability of the Reference.

For example, using a 10 MHz reference with a +/- 1 PPM stability, the output of the reference oscillator will vary by +/- 10 Hz. If you use this as a reference for a syntheizer with an output frequency of 1 GHz, your frequency stability could be as good as +/- 1 PPM, or +/- 1 kHz.

I hope this help's with your question.

Dave