[time-nuts] Re: Comparison/evaluation of u-blox timing receivers

[Bob kb8tq]

Hi

The simple answer is to put a cleanup loop on the output of the GPSDO 
if you are looking for a low noise signal. In the microwave case, something
running at (maybe) 100 MHz with a good OCXO is a pretty bulletproof 
solution. 

Indeed, if you need low phase noise at 10 MHz, the same basic answer 
applies there. Lock up a good / clean OCXO through a reasonable loop and 
use its output.

The problem isn’t really GPSDO specific. Any time you pack a whole bunch
of high speed digital stuff in tightly with a source, the digital is going to bleed 
over. There will be this and that spur or noise bump showing up. Simply isolating
from the digital world is part of the “why” for the cleanup oscillator. 

No solution is without its problems …. if your objective is time, locking up another
oscillator in a chain is not a great idea. You now have yet another loop with
all of its drift issues. It may not be the greatest thing if you are after good ADEV
at “all points”. There *will* be a hump added in your ADEV plot …

Bob

> On Aug 24, 2021, at 9:05 AM, Avamander <avamander at gmail.com> wrote:
> 
> Hi,
> 
> I hope you don't mind me starting a "newcomer" thread based on your sentence: "with a GPSDO, finding one that’s got a clean output is not at all easy".
> 
> Are there any up-to-date resources that would provide such information about a larger selection of GPSDOs? 
> 
> Are there any GPSDOs that people really recommend purchasing right now, costing say up to $500? Or is the "just build your own" approach the most cost-effective way?
> 
> P.S: "Study John Vig's Introduction to Quartz Frequency Standards" link on http://www.leapsecond.com/time-nuts.htm <http://www.leapsecond.com/time-nuts.htm> is broken
> 
> On Tue, Aug 24, 2021 at 3:20 PM Bob kb8tq <kb8tq at n1k.org <mailto:kb8tq at n1k.org>> wrote:
> Hi
> 
> The problem with the approach used in the basic GPS / GNSS modules is
> that the TCXO is not tuned on frequency. Even with an integer relation to the
> internal oscillator, you still see the pulse drop / pulse add into the divider. The
> result is that they are noisy.
> 
> There are a couple of modules that *do* tune the frequency. ( as noted in the
> paper). There the very coarse DAC used is what gets you in terms of noise. 
> Again, it’s regardless of output frequency. The signal *is* cleaner, it’s just not
> as clean as what most folks are looking for. 
> 
> Even with a GPSDO, finding one that’s got a clean output is not at all easy.
> Yes, this all depends a lot on just what you mean by clean. It normally comes
> up in the context of folks wanting to run radios at microwaves. Indeed the 
> normal answer there is that you use multiple oscillators in your multiplier
> chain.
> 
> Bob
> 
> > On Aug 24, 2021, at 7:18 AM, Marek Doršic <marek.dorsic at gmail.com <mailto:marek.dorsic at gmail.com>> wrote:
> > 
> > Hello,
> > 
> >  thank you for sharing valuable comprehensive comparisions. 
> > 
> > In the high timepulse rate experiment, have you also investigated other rates than the standard 10 MHz? Most Ublox receivers use 48MHz internal clock thus the 10MHz timepulse must be missing some of the clock pulses which generates additional jitter. 8MHz or 12MHz rates which are integer divisions might have cleaner output.
> > 
> >  .marek
> > 
> >> On 24 Aug 2021, at 03:51, John Ackermann N8UR <jra at febo.com <mailto:jra at febo.com>> wrote:
> >> 
> >> In 2020 I did an extensive evaluation of the timing ability of the u-blox LEA-M8F, NEO-M8N, NEO-M8T, NEO-M9N, ZED-F9P, and ZED-F9T.  The work was made possible by support from the HamSci consortium (https://hamsci.org <https://hamsci.org/>) under NSF grants supporting HamSci activities.
> >> 
> >> I was sure I'd posted about the paper on time-nuts, but I can't find any record that I did, so this is a belated announcement.  It's available for download from
> >> 
> >> https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf <https://hamsci.org/sites/default/files/publications/2020_TAPR_DCC/N8UR_GPS_Evaluation_August2020.pdf>
> >> 
> >> As BobC says, "Lots of fun!"
> >> 
> >> John
> >> 
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