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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3DDED492.2B9873B0@webaccess.net> <3DDF16D8.90072F5@webaccess.net> <3DDF229B.B85180DD@ieee.org> <3DDF7754.5FEB9C8C@webaccess.net>
Subject: Re: Phase locked loops and confusion.
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Sat, 23 Nov 2002 12:52:41 -0000
NNTP-Posting-Date: Sat, 23 Nov 2002 12:52:49 GMT
Chuck Simmons wrote:
> Kevin Aylward wrote:
>> analog wrote:
>>> 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.
> Yes, we are aware of that one. Translating this into digital
> equivalents was not to difficult. I haven't seen the logic but it
> simulates well. The point that came up that I mentioned is that of
> deciding the phase detector structure and the type of filter needed
> seems confusing to some because of the charge pump and cap being
> popular but not very usual in some of the papers. Actually, we have
> to have an integrator in the loop but we have choices on how it is
> introduced (how we add up phase error and introduce the sum into the
> output). The structure of the loop filter is pretty much dictated by
> Actually, after one of the engineers printed the paper he had read,
> the design cleared up rapidly after I pointed out that I was counting
> one more integrator than he was and that I had no argument with the
> paper. The additional integrator simply required a greatly different
> filter design.
But I still dont follw what you were actually saying here.
The proposed loop had a phase detector,
charge pumps and capacitor which is Kp/s. The article used a phase
detector that was Kp. So the article loop has Kp*Kf/s while the proposed
loop has Kp*Kf/s^2. A low pass is perfectly fine for the article loop
but not workable for the proposed loop.
I can't see the architecture of the two systems that you are describing.
The general VCO + (charge pump with capacitor) is a second order system,
that requires compensation e.g. a resistor in series with the charge
What are you comparing this to?
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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