[time-nuts] Re: GPSDO replacement recommendation

[Zdenek Chaloupka]

Hi,

it is very difficult to tame Mosaic-T's sawtooth (clock quantisation error) as the correction data for the sawtooth are reported from Mosaic-T only after the pulse is out (as oppose to, for example u-blox’s ZED-F9T/NEO-F10T, or Furuno’s GT-100). Also note that simply cleaning up sawtooth with a PLL is also not that easy, as the sawtooth is not uniformly, or normally distributed at times, that means it introduces a significant bias which needs to be cleaned up by PLL. Therefore you would need a PLL with 5mHz bandwidth or lower to clean up the sawtooth (and bias) properly. OCXO’s won’t do, but then a good Rb may be enough to keep PLL stable at those low bandwidths.

In general I would go with a good low cost receiver (my recommendation is NEO-F10T, which you can buy cheap here: https://gnss.store/54-neo-f10t-timing-gnss-modules) and a reasonably good PLL to clean up the sawtooth. The sawtooth is reported from ubx receivers before the pulse, so you can correct it directly in the PLL, if it supports it (supported by Si5383 or AD9545, for example). For some of those PLLs a reasonably cheap (300$) evaluation boards exist, so you don’t have to design your own PCB.

Cheers
Zdenek


> On 18 Apr 2024, at 15:04, Bob Camp via time-nuts <time-nuts at lists.febo.com> wrote:
> 
> Hi
> 
> Yup, various folks have indeed done that sort of thing. Some have done it on multiple Rb’s. 
> 
> The “filter” in these Rb’s (many of them have that option) is not ideal for a GPSDO. It does not handle the “corner cases” very well. Unfortunately > 90% of the “smarts” in a good GPSDO involves those corner cases. Yes, that makes the term corner case a bit suspect …. still, it’s the term that gets commonly used. The “why?” is simply that the feature is primarily intended for “quick calibration" against a Cs or something similar. 
> 
> The PRS10 has a bit of a bump in its ADEV curve around 10 seconds. It is a result of their fancy mag field management approach. That also (at least to me) makes it a less than perfect choice for this sort of thing. A 10 second gate time on a frequency counter is a pretty common thing …..
> 
> On the plus side, you can hook up something like a Mosaic-T and have a pretty fancy drive into the device. To get the best performance, you would need to come up with a way to apply the sawtooth correction data to the pps before it goes over to the Rb. That certainly can be done, it just adds another layer of excitement to the setup. 
> 
> Fun !!
> 
> Bob
> 
>> On Apr 17, 2024, at 6:34 PM, Jürgen Appel via time-nuts <time-nuts at lists.febo.com> wrote:
>> 
>> Hi, 
>> 
>> P,erformance wise I imagine a SRs PRS10 rubidium clock connected with its 1pps input to the 1pps output of a ublox zf9t will be hard to beat for approx 2. 2kEur with respect to many requirement profiles.
>> Has anyone here tried that out? 
>> 
>> Cheers,
>> Jürgen 
>> 
>> On 14 April 2024 15:53:33 CEST, Bob Camp via time-nuts <time-nuts at lists.febo.com> wrote:
>>> Hi
>>> 
>>> Cost wise, the surplus UCCM Trimble’s (or the Symmetricom version) still seem be the one to go with. At the same price, I’d go for the Trimble version. Both appear to work pretty well. 
>>> 
>>> If cost is no object, there are lots of crazy choices.  Each of them has it's own unique advantages and (usually) unique issues. The SRS FS740 is one example. It does indeed have its quirks. Not exactly cheap, and still far from the most expensive option. 
>>> 
>>> https://www.thinksrs.com/products/fs740.html
>>> 
>>> At least two list members have one of them. 
>>> 
>>> In this era of multi-band GNSS as well as anti-spoofing / anti-jam there are a lot of ways to run up the price :). 
>>> 
>>> How crazy do you want to get?
>>> 
>>> Bob
>>> 
>>>> On Apr 14, 2024, at 8:24 AM, Attila Kinali via time-nuts <time-nuts at lists.febo.com> wrote:
>>>> 
>>>> Moin,
>>>> 
>>>> My house GPSDO (a Trimble UCCM) died recently and I am now looking for a
>>>> replacement. As I have been a bit out of touch with the current state
>>>> of affairs regarding GPSDOs, I'd like to ask what the current recommendation
>>>> for getting a GPSDO is, these days.
>>>> 
>>>> Thanks in advance
>>>> 
>>>> 			Attila Kinali
>>>> 
>>>> -- 
>>>> Science is made up of so many things that appear obvious 
>>>> after they are explained. -- Pardot Kynes
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