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Old 10-31-2007, 05:33 PM
John Larkin
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Default Re: Ping Jim: The PFD is dead!

On Wed, 31 Oct 2007 04:05:57 -0700, Jim Thompson
<[email protected]> wrote:

>On Tue, 30 Oct 2007 21:12:11 -0700, "Mike" <[email protected]> wrote:
>
>>Finally, proof positive! The PFD fails to detect phase! I'm sure you will be
>>happy to know this important information.
>>
>>http://rfdesign.com/vlf_to_uhf/time_...207/index.html
>>
>>To begin, the author does acknowledge that the PFD detects frequency (at
>>least you didn't get everything wrong):
>>
>>"When the frequency difference is small, one of the currrent sources will be
>>turned on more often than the other. Therefore, the PFD does function as a
>>frequency detector."
>>
>>So far, so good. At least you didn't mess everything up. But then, he
>>delivers the shocker! The PFD does NOT actually detect phase! It's true!
>>
>>"To test whether the PFD is a true phase detector, a fixed-frequency signal
>>can be supplied simultaneously to Fvco and to a variable-delay circuit whose
>>output feeds Fref. This will produce a variable phase delay. With the output
>>of the PFD connected to the LPF as before, the measured result shows that
>>the PFD is not a true linear phase detector, because the output of the LPF
>>can only stay at either the saturated high or low state, depending upon
>>which signal has a phase lead over the other."
>>
>>As if that's not devastating enough, he delivers his final conclusion about
>>the PFD:
>>
>>"The PFD is thus seen to be an arrival-time detector, instead of a
>>phase-frequency detector, because it provides a steady-state output that
>>provides both polarity and magnitude of the measured time difference."
>>
>>Wow! Can you believe it? I know I can.
>>
>>I've been using PFDs for many years, and thought I knew something about
>>them, but the author of this article has far more experience than I do, and
>>has even started a company to commercialize his discovery. He simply can't
>>be argued with!
>>
>>I'm sorry Jim, but after umpteen years, the jig is up. Forget the design
>>wins, and all the successful implementations. It didn't really work after
>>all.
>>
>>Better luck next time.
>>
>>-- Mike --
>>

>Sno-o-o-ort!
>
>Just goes to show you to what crap the USPTO will award a patent.
>
>I'll read it carefully upon return from hospital.
>
> ...Jim Thompson



I "invented" this last week. FPGAs aren't very good at implementing
the classic charge pump.

http://s2.supload.com/free/PFD.jpg/view/

The outputs here are just hard (not tri-state) logic outputs, driven
directly by the up/down flipflops in the pfd circuit. It's nicely
symmetric.

John




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