[time-nuts] Digital Phasae Lock Loops

Alex Pummer alex at pcscons.com
Sat Oct 17 09:17:11 EDT 2015

actually, that is a ketch 22, if the loop bandwidth is to low, you will 
have low noise , but it may will not lock at all, an other way to try to 
filter out the noise, also you may make the loop filter digital, but 
leave the the PLL analog, that could have  the possibility to have the 
advantage to be able to change the loop bandwidth  increase for locking, 
and reduce after the detected locking

On 10/17/2015 4:57 AM, Tom Van Baak wrote:
> 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?
> /tvb
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