[time-nuts] crunching numbers from XOR phase detector

[Brian Justin]

Be very careful of the FPD in the 4046. It has a dead-zone when the phase error
is at or very close to zero.  Some versions of the chip claim to have "improved"
that dead-zone. But it's still there to some degree, at least in all the 
versions I have tried.

-Brian, WA1ZMS





----- Original Message ----
From: Chris Albertson <albertson.chris at gmail.com>
To: Discussion of precise time and frequency measurement <time-nuts at febo.com>
Sent: Wed, January 4, 2012 12:21:06 PM
Subject: Re: [time-nuts] crunching numbers from XOR phase detector

On Wed, Jan 4, 2012 at 12:58 AM, Hal Murray <hmurray at megapathdsl.net> wrote:

>
> Neat.  Thanks for sharing.
>
> With a XOR, you can't tell which input is higher frequency.  I think you
> can
> fix that with a second XOR and a delay line.


I think you can build a PFD or Phase Frequency Detector with three or four
flip flops.  This avoid using the delay line and works over a wide range.

If I'm not mistaken there is a PFD inside the 74HC4046.   It uses flip
flops.  Be sure to look at the 74HC type not the cd4046.   The 75hc type
works up to 18MHz.


Chris Albertson
Redondo Beach, California
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