From: "Stephan Sandenbergh" <[EMAIL PROTECTED]> Subject: [time-nuts] Digital PLL ICs, alternatives and digital loop filters Date: Sat, 16 Dec 2006 20:52:25 +0200 Message-ID: <[EMAIL PROTECTED]>
> 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 _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
