[time-nuts] Re: Which GPSDO for an accurate, low phase noise 10 MHz?

[Michael Wouters]

Hello Ed

Cryogenic sapphire oscillators (CSOs) aren’t metallized - confinement of
the microwaves is by total internal reflection of the propagating waves.

They cost about a million dollars but you get short term stability better
than 1E-15 off the shelf.

Regards
Michael

On Wed, 30 Apr 2025 at 6:22 am, Ed Marciniak via time-nuts <
time-nuts at lists.febo.com> wrote:

> A while back I recall seeing an Australian company that offered a sapphire
> “whispering gallery” (in this context simply meaning the cavity equivalent
> of a waveguide operating in a higher order mode with lower losses)
> oscillator with a regenerative divider for direct 10GHz output. I assume
> they probably took a silver plated copper cavity and shrink fitted it onto
> the sapphire, or metallized the sapphire and then fitted it into a housing.
> I have no idea how they’d tune it.
>
> Unfortunately, I’m going to assume it was far more costly than would work
> for me or my use cases.
>
> For the rest of us, the best reasonable cost ~10GHz solution available is
> likely a DRO with a sampling phase detector locked to a ~100-200MHz
> oscillator, which succeeded the 70-80s technique of an VHF ~100MHz SC cut
> crystal locking an ~1.6 GHz cavity oscillator (with a sampling phase
> detector), followed by an SRD (Frequency west/CTI/MA-COM brick). It’s
> either that or tolerate synthesizer phase noise and noise sidebands.
>
> ________________________________
> From: Bob Camp via time-nuts <time-nuts at lists.febo.com>
> Sent: Tuesday, April 29, 2025 7:02:33 AM
> To: Discussion of precise time and frequency measurement <
> time-nuts at lists.febo.com>
> Cc: Bob Camp <kb8tq at n1k.org>
> Subject: [time-nuts] Re: Which GPSDO for an accurate, low phase noise 10
> MHz?
>
> Hi
>
> Any source at 10 MHz will have issues if multiplied directly to 10 GHz.
>
> Your phase noise goes up be 20 log N. You are multiplying by a thousand.
> The
> net result is phase noise that is 60 db worse at 10 GHz than at 10 MHz.
>
> Even your “pretty good” 10811 will be suffering when you do that.
>
> The answer is to go up to 10 GHz in stages. One of about a billion
> approaches:
>
> 1) lock up a quiet 100 MHz crystal oscillator to your 10 MHz source set
> the PLL
> bandwidth to maybe 50 Hz. Outside that bandwidth the quieter 100 MHz source
> “rules”.
>
> 2) Do the same thing to get to (maybe) 1 GHz. Lock up a DRO there. Same
> idea
> with PLL bandwidth.
>
> 3) Now step up to 10 GHz and lock up a third quiet oscillator.
>
> In each case you are making a tradeoff. The phase noise at some offset
> from carrier
> on each of the 100, 1G, 10G sources is below that of the “multiplied up”
> source you
> are locking to. Closer in to carrier that’s not the case.
>
> GPSDO’s are no different.
>
> GPS “as delivered” is a very noisy signal. The typical module adds to this
> noise.
> This means it is not a good source to use for at all “offsets”. What gets
> confusing
> is that we now talk about offsets with units of time rather than
> frequency. That’s
> a pretty common thing as you go below 1 Hz.
>
> Averaged over a second, your GPS is very poor compared to just about any
> crystal
> oscillator you might pick. Compared to your 10811 it might be poor out to
> 100’s of
> seconds.
>
> The same process applies here as to the 10 GHz setup. You very much have
> to pick
> that crossover point between the lousy GPS signal and your great local
> standard.
>
> Why even do this? Eventually GPS wins. Go out long enough in time and it
> will out
> perform anything you likely have hooked to your radio. Some GPS’s do
> better than
> others. They will “win more” long term. It should be no big surprise that
> those modules
> cost more than the ones that don’t perform as well.
>
> These days, the eBay or typical home brew GPSDO uses a simple GPS module
> for
> cost reasons. That’s not a bad idea as long as the end result meets your
> needs.
> Don’t spend $3,000 if $100 will solve the problem ….
>
> Bob
>
>
> > On Apr 28, 2025, at 11:17 PM, Bill Katz via time-nuts <
> time-nuts at lists.febo.com> wrote:
> >
> > Hello.
> > I found this group last year when I was working on my microwave rover
> > station (up to 10GHz currently, 24 GHz soon) when I ran into a challenge.
> >
> > First I bought a small Rubidium standard, hoping that would keep my
> system
> > accurate. (Symmetricom x72 from ebay).  It looked fine with some
> > transverters, but with one brand the phase noise was so bad, one couldn't
> > make out the primary tone on the spectrum analyzer.  The next step was a
> > club member sent me an HP 10811A, which has great phase noise, but showed
> > some drift when I fed my counter from the rubidium standard.
> >
> > Oh, and once I repaired the (apparently somewhat common) temperature fuse
> > inside the second 10811A inside the counter, I now had two oscillators
> that
> > almost agree.  Almost.
> >
> > So I'd like to get something that can run the instruments in my house,
> and
> > allow me to calibrate the OCXO in my rover equipment.  And tweak in the
> > various other OCXO standards.
> >
> > At the high end, there are used HP Z3801As that seem to have excellent
> > phase noise, but are somewhat expensive.  At about half the price are
> used
> > Thunderbolt units.  and there are a myriad of other options.  What are
> the
> > pros/cons of some of these units?
> >
> > Thank you,
> > Bill Katz
> > KA1TZ
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