[time-nuts] Digital PLL ICs, alternatives and digital loop filters

Stephan Sandenbergh stephan at rrsg.ee.uct.ac.za
Thu Dec 28 16:31:49 EST 2006


Hi Magnus,

Sorry for my delayed response, but I was a bit under resourced on e-mail
facilities during the festive season. 

> Go and get Floyd Garners "Phaselock Techniques". The reference and very
> worth reading. I have other books to recommend too. The popular BEST book
> isn't as great, but can sometimes be a good reminder too, but Garners book
> beats it in my opinion.

I haven't got any of the books you mentioned but I worked through Wolaver's
"Phase-Locked Loop Circuit Design" some time ago. It is probably a bit out
dated but it was the thinnest on the shelf :) Nonetheless, I thought it is
quite good and serve as a good intro to PLLs.

I've seen two books by William Egan: "Phase-Lock Basics" and "Frequency
Synthesis by Phase Lock" on our library's shelves. Are you familiar with
these? Are they worthwhile?

> I prefer phase detectors which is more continous such as mixers, XOR or
> S-R style phase detectors.

In this case the phase detector is purely digital so I guess the loop filter
could also be a purely digital IIR filter (or something?) I guess the only
limitation is the frequency at which the digital system could run.

Cheers,

Stephan Sandenbergh  





 

> -----Original Message-----
> From: Magnus Danielson [mailto:cfmd at bredband.net]
> Sent: 16 December 2006 10:02 PM
> To: time-nuts at febo.com; stephan at rrsg.ee.uct.ac.za
> Subject: Re: [time-nuts] Digital PLL ICs, alternatives and digital loop
> filters
> 
> From: "Stephan Sandenbergh" <stephan at rrsg.ee.uct.ac.za>
> Subject: [time-nuts] Digital PLL ICs, alternatives and digital loop
> filters
> Date: Sat, 16 Dec 2006 20:52:25 +0200
> Message-ID: <002401c72143$55b7c1e0$b1493d0a at mycomputer>
> 
> > Hi,
> 
> Hi Stephan,
> 
> > Another question for today - I have checked out some digital PLL ICs
> (more
> > specifically the one's from AnalogDevices. It seems as if they've got
> some
> > pretty neat stuff.) As I mentioned in my previous mail: I want to lock a
> > GPSDOs 10MHz to a 100MHz OCXO using some sort of PLL. I find the PLL ICs
> > convenient because they've got everything that is needed already built-
> in.
> > Also, the phase detector and dividers are all optimised for low jitter.
> 
> The type of phase detector is what I would worry about. See more below.
> 
> > Can anyone give me some pointers regarding this? If I need a low noise
> > 100MHz output, is the digital PLL and a 100MHz OCXO a good choice? I
> guess
> > that analog PLL techniques are better, but is it worth all that effort?
> > Maybe I should look at PLL ICs from other manufacturers?
> 
> Go and get Floyd Garners "Phaselock Techniques". The reference and very
> worth
> reading. I have other books to recommend too. The popular BEST book isn't
> as
> great, but can sometimes be a good reminder too, but Garners book beats it
> in
> my opinion.
> 
> > Also, I'm slightly worried about the thermal instabilities inherent in
> the
> > analog loop filter that is required by these PLL ICs. Has anyone
> > successfully chased the charge pumps output through a digital filter?
> Or, is
> > there a reason why this isn't done?
> 
> Unless you have a high comparision frequency I would avoid chargepumps.
> They
> (risk) delay the correction until it has become large enought. This means
> that
> it will balance around the intended frequency with related phase drift.
> I prefer phase detectors which is more continous such as mixers, XOR or S-
> R
> style phase detectors.
> 
> An active loop filter is also my preference, but for some case I go for
> passive
> lag since it works well enought for some applications. Passive lags has
> the
> inherent downside that the loop gain converts the difference between non-
> modulated frequency of the oscillator and tracked frequency into a phase
> difference. Changes in temperature will show up as phase modulations. For
> some
> applications I don't care, but when I do there is no alternative to active
> loop filter. An op-amp, two resistors and a capacitor isn't that bad
> solution
> when things comes around.
> 
> Cheers,
> Magnus




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