From: Jim Thompson
Subject: Re: Phase locked loops and confusion.
References: <3DDED492.2B9873B0@webaccess.net> <3DDFDC40.DBB1240D@webaccess.net>
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Date: Sat, 23 Nov 2002 20:17:57 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 15:17:57 EST
Organization: Cox Communications
On Sat, 23 Nov 2002 19:51:51 GMT,
Chuck Simmons ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: Phase locked loops and confusion.",
Wrote the following:
|Jim Thompson wrote:
|> On Sat, 23 Nov 2002 01:06:45 GMT,
|> Chuck Simmons ,
|> In Newsgroup: sci.electronics.design,
|> Article: <3DDED492.2B9873B0@webaccess.net>,
|> Entitled: "Phase locked loops and confusion.",
|> Wrote the following:
|> Phase detectors don't have integrators, but VCO's are represented by
|This was the problem in that the designers, cribbing from working analog
|circuits produced a more or less exact digital equivalent. The circuits
|they cribbed from collapsed an integrator into the phase detector.
|> Thus, if you add any filter at all, it *must* be lead-lag, or it's
|Not true. A single integrator loop is stable and doesn't need a
|stabilizing filter. Two integrators require at least a 1st order
You still misunderstand. A VCO *is* an integrator all by itself. Add
a filter and you've added another pole... you need a zero in there
|> Why don't you post the complete PLL system requirements, type of input
|> signal, type of output signal, frequencies; is it 1:1, or is there a
|> divider in the loop?
|It's a clock recovery system. There are two PLLs actually involved. One
|has inputs of 44kHz to a couple of megaHertz using various divide ratios
|and has requirements on bandwidth that depend on the data modulation
|method which may be FM or phase. The output frequency may be a bit over
|200MHz. The other is harmonically locking on an RLL code to recover the
|read clock and design goals require at least 200MHz output and more than
|double that in a year (assuming a process change).
You can do that all digitally.
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| Jim-T@analog_innovations.com Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |
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