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Design >> Analog Design >> PLL basic question.
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Message started by raja.cedt on Jul 17^th, 2010, 3:08am

Title: PLL basic question.
Post by raja.cedt on Jul 17^th, 2010, 3:08am

i have a basic question on PLL,

1.is it really locking phase locking or frequency locking. i feel it is making phase difference between ref and fb clock is constant at some value (normally it is zero), so frequency locking is happening. If so why Frequency Lock loop.

2. In loop filter we have RC series branch, in that r is doing phase lock and c is doing frequency locking, is this correct?

Thanks,
Rajasekhar.

Title: Re: PLL basic question.
Post by buddypoor on Jul 17^th, 2010, 3:47am

raja.cedt wrote on Jul 17^th, 2010, 3:08am:

1.) Of course, it depends on the kind of PD you are using.
In any case, when frequencies are different, it's a kind of frequency locking (pull-in process); if this is completed, phase locking takes over until the differece beween both phases is roughly constant (not always zero, dpends on PD).

2.) I don't understand the question. The rc combination is afilter which must attenuate unwanted products out of the PD.

Title: Re: PLL basic question.
Post by raja.cedt on Jul 17^th, 2010, 4:21am

hi buddypoor,

thanks for your ans. what i mean is in case loop filter with no shunt cap, there will be on RC series circuit, so resister is kind of proportional gain and cap is a kind of integral gain. So integral controller will change the frequency means cap will set the frequency. proportional controller will change the phase so resister will set phase.
Thanks.

Title: Re: PLL basic question.
Post by tkhan on Jul 17^th, 2010, 6:22am

Read Razavi's analog book chapter 15, it has a good explanation of the concepts you are interested in.

Title: Re: PLL basic question.
Post by Mayank on Jul 18^th, 2010, 8:53pm

+1 at tkhan

In a Locked State, Frequency's gotta b same as Fcomparison if you want a constant Phase Diff. / Phase Lock.

Quote:

[quote]what i mean is in case loop filter with no shunt cap, there will be on RC series circuit, so resister is kind of proportional gain and cap is a kind of integral gain. So integral controller will change the frequency means cap will set the frequency. proportional controller will change the phase so resister will set phase.

The ZERO from series RC provides proportional gain, while the Pole from C provides the integral gain. Nothing further.

--M

Title: Re: PLL basic question.
Post by love_analog on Jul 29^th, 2010, 6:37am

Raja
You are partially correct.

The cap acts as in integrator so it stores the frequency difference. In reality, if there was no stability thing you could just use a cap. Remember, DLL :-)

The resistor is a memory-less element not an accumulator.