[time-nuts] LPFRS phase ripple?

Bob kb8tq kb8tq at n1k.org
Tue May 23 07:28:26 EDT 2017


It could be a DDS based coarse tune loop and you just happen to be unlucky on one unit.
You can (with bad luck) get a fairly significant spur out of a DDS quite close to carrier. That
will give you “ripples” in the ADEV plot.


> On May 23, 2017, at 3:40 AM, Matthias Jelen <Matthias.Jelen at gmx.de> wrote:
> Hello Time-Nuts,
> i´ve been playing with some Rbs recently.
> I compared two LPFRS-01 to a PRS-10. One seems to behave quite well (compared to the specs given here: http://www.spectratime.com/uploads/documents/isource/iSource_LPFRS%20Spec.pdf) - short term stability specs are met), the other one shows quite some phase ripple (appx. 1 ns RMS with a frequency around 0.007 Hz) which spoils ADEV in the 60 - 200s range, very similiar to the typical GPSDO bump. Long term stabilty is OK.
> Unfortunatly I didn´t find any schematics on the web. Has anyone encountered a similiar problem with this unit? Any idea were this might come from? My first guess would be a problem in the control loop locking the VCXO to the Rb, but I have no clue which time constants are used in the PLL.
> Any hints are appreciated.
> Regards,
> Matthias
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