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Design >> RF Design >> Question on PD (phase detector)
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Message started by tulip on Dec 3^rd, 2018, 7:21pm

Title: Question on PD (phase detector)
Post by tulip on Dec 3^rd, 2018, 7:21pm

In PLL, PFD( phase frequency detector) are mostly used. PD are usually used in DLL. I have some questions on the usage of PD, which I could not find answers in books and papers.

what is the locking range of a PD PLL, for example, a mixer used as PD. I tried to analyze the locking range of PD PLL, but failed.
what is the advantage and disadvange of using PD instead of PFD? For example, we generally use PD in a DLL, why do not use PFD in a DLL?

Thanks.

Title: Re: Question on PD (phase detector)
Post by vroy_92 on Dec 29^th, 2018, 4:23pm

what is the locking range of a PD PLL, for example, a mixer used as PD.
-- It depends on your loop filter bandwidth as well. PLLs are a chaotic system and I doubt if a closed loop expression for it exists. I remember Gardner's book had some analysis (with approximations of course).

A PFD also detects the frequency. (The average output of the PFD+CP is a function of the frequency as well; its not a linear expression). What is means is that if the reference and feedback clock have different frequencies, the PFD (along with CP) will keep driving control voltage higher or lower.

In DLL, you assume that you already have the correct frequency and you just need to align the phase. Why put a PFD?

Title: Re: Question on PD (phase detector)
Post by tulip on Jan 2^nd, 2019, 10:59pm

Thank you, I will read the Gardner's book to see if it can help me clearify the concepts.
In DLL, we can use PD or PFD, I wonder if there are disadvantages of using PFD.