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

Magnus Danielson cfmd at bredband.net
Sat Dec 16 20:02:14 UTC 2006


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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