From: Jim Thompson
Subject: Re: Phase locked loops and confusion.
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Date: Sat, 23 Nov 2002 20:21:01 GMT
NNTP-Posting-Date: Sat, 23 Nov 2002 15:21:01 EST
Organization: Cox Communications
On Sat, 23 Nov 2002 12:08:48 -0800,
John Larkin ,
In Newsgroup: sci.electronics.design,
Entitled: "Re: Phase locked loops and confusion.",
Wrote the following:
|On Sat, 23 Nov 2002 07:41:37 -0000, "Kevin Aylward"
|>This don't make sense to me. I don't see any reason for the charge pump
|>to have a dead band. A charge pump is nothing more then a switched
|>source and sink of current. The dead band is usually in the phase
|>detector, that is, if steps are not taking to correct it.
|the 4046-type charge-pump circuit (widely copied) has the problem
|that, near balance, the pullup and pulldown current squirters are
|being asked to operate smoothly down to zero pulse width, which they
|can't. An HC4046, using the charge-pump detector, can add several ns
|of loop jitter because of this deadband. Interestingly, if you add a
|resistor from pin 13 to ground, this will force the pd off center
|(force a finite pulse width on the pullup side) and reduce jitter
Correct! That's why I like my dual-D version phase detector with the
quad-2-nand reset... forces a minimum pulse width.
| 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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