Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DDED492.2B9873B0@webaccess.net> <3DDF16D8.90072F5@webaccess.net> <3DDF229B.B85180DD@ieee.org>
Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 23 Nov 2002 10:00:43 -0000
NNTP-Posting-Date: Sat, 23 Nov 2002 10:00:51 GMT
> Chuck Simmons, John Larkin wrote in part:
>>> Yeah, the charge pump adds another integrator to the loop. I don't
>>> know why the charge pump thing is so popular;
It makes the charge and discharge current to the capacitor equal and
independent of capacitor voltage. If the capacitor was charged from a
voltage source through a resistor the current would vary. For instance,
suppose that the input frequency was such that 1V was the steady state
voltage on the cap, and that the PD output was 0 to 5 volts. The current
would switch from 4/R to 1/R. If the input frequency now changed such
that the steady states voltage was 2V, the current would switch from 3/R
to 2/R. Now, what would be the result of this on the system?
most of them have
>>> nasty deadbands. A charge pump pd doesn't inherently drive the
>>> loop in the right direction when the freq error is large; it takes
>>> a bit of additional logic (like in the 4046) to do that.
Actually, I was a bit misleading on my dead band post, now I have woken
up a bit. The dead band, is of course, the result of the whole phase
detector/charge pump arrangement. I thought you were implying some sort
of x-over distortion type effect, which wouldn't really make sense at
all. So, if the phase detector puts out pulses two narrow they won't
make it through the charge pump to the filter cap, due to limited BW of
the pump. The fix for this, is to force the phase detector to generate
equal narrow +/- pulses when the input frequencies/phases are matched.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.