[time-nuts] Good clean-up oscillators

[Mark Goldberg]

I'm using a Wenzel Phase Lock Oscillator as a "clean up" oscillator for my
Bodnar GPSDO. I lack the capability to measure if it really can do as they
advertise, but the 1 kHz away phase noise is claimed to be in the -170
dBc/Hz range. The ADC noise floor keeps me from measuring anywhere near
that low. The loops are in the .1 - 10Hz range, depending on which one you
get. I bought one that was made for telcom with proprietary input and
output frequencies that just happened to meet my needs. There was no
official documentation but Wenzel did send me documentation for the
standard one it was similar to. The one I got was a little strange,
unlocked it was way off frequency and it would only lock if it started well
above the desired input frequency. If you adjusted the free running
frequency to be close to ideal, it would not lock reliably. I have no idea
if that was the intended functionality, but since it did lock up fine if it
starts a kHz or so off, I decided to use it. To figure out the input
frequency, I had to slowly sweep a signal generator while measuring the
output and see when it locked.

I wish I had access to a correlating phase noise measurement system, but no
longer do. The phase noise measurement I can do with this system close in
matches very well with what I got when I did have access and measured some
TCXOs.

Regards,

Mark


On Wed, Jan 23, 2019 at 4:00 PM Bob kb8tq <kb8tq at n1k.org> wrote:

> Hi
>
> If you take a look at typical TCXO’s and XO’s in a real world environment,
> their noise is higher than a similar
> OCXO even at 100 Hz. If you are cleaning up something like a Rb, your loop
> may well be below 1Hz.
>
> Like everything else, it becomes a “that depends” sort of thing. If your
> cleanup loop bandwidth is in
> the KHz … it really does not matter. In the 30 to 300 Hz range it’s a bit
> of a tossup. By the time you go
> below 10 Hz, a good OCXO will always win.
>
> Yes, this is all based on the Rb cleanup example that would be in the 10
> MHz vicinity. Go up to 300 MHz
> and everything shifts a bit. Yet another “depends”.
>
> Bob
>
> > On Jan 23, 2019, at 3:43 PM, Magnus Danielson <magnus at rubidium.se>
> wrote:
> >
> > Fellow time-nuts,
> >
> > While we often look at long-term stability, ADEV etc. we consider
> oscillators in their free-running properties. This is all fine and dany,
> but when we want to use an oscillator as a clean-up oscillator, the
> servo-loop with surpress much of the low-frequency/high-tau properties and
> replace that with that of the reference.
> >
> > Consider for instance that we have a LPRO rubidium and we want to use a
> clean-up oscillator to provide better phase-noise, what would be a good
> selection?
> >
> > While there is plenty of choices, it is a bit different from that which
> is relevant when considering it as a free-running oscillator. The white
> noise which dominated far-out would dominate, and only some of the flicker
> phase becomes relevant.
> >
> > I could easily get a OCXO of good quality, but really, what would be
> interesting choices? Have someone measured this enough to get a kind of
> rough idea at least?
> >
> > Cheers,
> > Magnus
> >
> >
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