[time-nuts] Digital Phasae Lock Loops

Tom Van Baak tvb at LeapSecond.com
Sat Oct 17 07:57:22 EDT 2015

Martyn Smith wrote:
> All we want to do is lock a 10 MHz ULN OXCO to a rubidium.
> So basically a clean up loop.
> Then we can provide an ULN output from the ULN OXCO and long term stability from the rubidium.
> The 10 MHz ULN OXCO has phase noise of –115 to -120 dBc/Hz @ 1 Hz with a –174 dBc noise floor.
> The rubidium’s phase noise at 1 Hz is about –105 dBc.
> So for the PLL to remove the poor rubidium phase noise I need a loop BW of less than 0.2 Hz.
> I have tried digital PLL’s from other companies.  One you can specify the bandwidth down to 1 mHz.
> But they are very unreliable, subject to flicking out of lock now and then.
> At the moment we use an analog PLL with a loop bandwidth around 0.2 Hz.
> That works well for my ULN OXCO’s that make about –113 dBc at 1 Hz.
> But now we are getting even lower phase noise at 1 Hz (-115 dBc and below), I need
>    a smaller loop BW, and we aren’t able to get that with an analog PLL.

Ok, thanks for the clarification. I'll leave it up to those with more experience to recommend a solution. I'd be surprised if using a digital PLL helps here. Are you sure you've reached the limit of what you can do with analog?

BTW, if your existing clean up oscillator is "very unreliable" consider that it may be your oscillator and not the PLL. For this you do not want to use ADEV or L(f) statistics. A one-time phase jump can cause loss of lock and averaging statistics will not tell you this. Instead you may want to look at the raw phase data and quantify the jumps. Occasional large jumps may not show up at all in an ADEV plot but can cause trouble for a PLL. Magnus, would MTIE be appropriate here?


More information about the time-nuts mailing list