[SI-LIST] Re: Phase/Frequency Detector!
- From: Paul Levin <levinpa@xxxxxxxxxxxxx>
- To: parthsv@xxxxxxxxx, SI-List Reflector <si-list@xxxxxxxxxxxxx>
- Date: Tue, 19 Aug 2003 23:45:48 -0700
Dear Partha,
As you indicated, with a phase detector all one knows
is the phase error. Whether that is a voltage resulting
from pulse width modulation or a voltage resulting from
charge pumps, that is all you know. In either case, if
the frequencies are way off, you get a sinusoidal varying
voltage whose average voltage is zero (give or take the
offset.) This gives you no information as to which way to
move the VCO. That is why a straight phase detector has
a limited acquisition range; a PLL based on a phase detector
must be able to "catch" the frequency during a fraction
of a beat period.
On the other hand, a phase-frequency detector gives you
information about which of the two frequencies is higher.
When the two frequencies are way off, the output voltage,
however created, will have a real dc content indicating
which way the VCO should move to get closer. As the two
frequencies get closer together, the PFD reverts to being
a phase detector, so you can phase-lock the loop.
The simplest way of thinking of a PFD is to imagine a two
bit up-down counter. Every time you get a reference pulse,
the counter moves one way (let's say up), and every time
you get a VCO pulse, the counter moves the other way (call
this down.) This counter also saturates at 00 and 11 so
that if the counter is 00 and you get a VCO pulse, the
counter will remain at 00 or if the counter is 11 and you
get a reference pulse, the counter will remain at 11. The
PFD output is the more significant bit. If you get two
reference pulses in a row, the VCO is too slow and you
want to speed it up. Since the counter now oscillates
between 10 or 11, and the MSB is a solid 1. Thus, the VCO
is driven to a higher frequency. Once the VCO gets high
enough (and actually just a bit too high,) there will be
two VCO pulses in a row, and the counter will settle into
a pattern between 01 and 10. Since the MSB is now oscillating,
you have the equivalent of an XOR output between the two
signals, i.e., a classic phase detector.
Hope that this helps.
Regards,
Paul
____________________
Parthasarathy Sampath wrote:
> Hi All,
> Whats the difference between Phase detector and
> Phase/Freq Detector?
>
> Why is PFD advantageous to PF?
> - Normally PF has low acquisition range limited by
> usage of Low Pass Filter. In PFD we use charge pump
> instead of LPF. Is that the main reason?
>
> Thanks in Advance,
>
> Regards,
> Partha!
>
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--
Paul Levin
Senior Principal Engineer
Xyratex Storage Systems
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