[time-nuts] U-blox teaser

[Bob kb8tq]

Hi

The same 20 or so ns delay in a saw would also apply to the 
saw (or tight filter) in some timing antennas. It also would apply
to the saw(s) in some modules. Even if the tolerance is “only”
a couple ns on each of them, you *could* have 3 or more in the
chain.

Lots of numbers to crunch to get to 5 ns “absolute”. One could go 
grab a GPS simulator and start poking. First step would be to find
a simulator that is spec’d for a < 5 ns tolerance on the PPS into 
GPS out. I do believe that rules out the eBay marvels that some 
of us have lying around …..

Simpler answer would be a quick “clock trip” with your car full
of 5071’s …… hour drive over to NIST and then back home. 
That sounds practical for most of us :) :)

Bob

> On Feb 26, 2021, at 9:29 PM, John Ackermann N8UR <jra at febo.com> wrote:
> 
> A while ago I tried doing a decidedly non-anechoic measurement with a VNA exciter going to a 1500 MHz ground plane and the receiver connected to the antenna (with a known delay cable) and I got a similar result, but there was enough noise that I didn't think I could nail it down to within 10 ns.
> 
> I've also measured GPS antenna splitters and they tend to have 20-ish ns delays, mainly due to the SAW filters.  I did surgery on an HP splitter to remove the filters so it could be used for L1 and L2 and that dropped the delay down to only 1 or 2 ns.
> 
> So there's definitely lots of stuff to calibrate if you want to get accurate time transfer.
> 
> John
> ----
> 
> On 2/26/21 8:02 PM, Michael Wouters wrote:
>> Typical L1 antenna delays range from 20 to 70 ns.
>> I know of only one antenna for which a delay is given by the vendor and the
>> technique used was just to measure the electronic delay ie by injecting a
>> signal into the circuit. To do it properly, you need a setup in a microwave
>> anechoic chamber with transmitting antenna etc. The practical difference
>> may be small though, 1 or 2 ns ( sample of one antenna!).
>> Cheers
>> Michael
>> On Sat, 27 Feb 2021 at 11:42 am, John Ackermann N8UR <jra at febo.com> wrote:
>>> They're claiming "even better than" 5 ns for relative time, which given
>>> the 4 ns jitter seems at least sort-of reasonable.  But until someone
>>> shows me otherwise, I'm still thinking that getting better than 25 ns
>>> absolute accuracy is a pretty good day's work.
>>> 
>>> John
>>> ----
>>> 
>>> On 2/26/21 5:26 PM, Bob kb8tq wrote:
>>>> Hi
>>>> 
>>>> I can’t think of many antennas (multi band or single band) that claim to
>>> know their
>>>> delay to < 5 ns. Simply having a *differential* delay spec of < 5 ns is
>>> quite good.
>>>> Same thing with delay ripple, you see specs out to around 15 ns on a lot
>>> of antennas.
>>>> None of this is getting you to the actual total delay of the antenna.
>>> It’s a pretty good
>>>> bet that number is a bit larger than either of these.
>>>> 
>>>> Some of the ripple probably comes out in the standard modeling. I’m not
>>> sure that
>>>> the differential delay is taken out that way. Total delay, not taken out
>>> in any obvious
>>>> fashion ( at least that I can see). If the F9 has a built in antenna
>>> database, that’s not
>>>> mentioned in the doc’s. Any benefit from the corrections would have to
>>> be part of
>>>> post processing.
>>>> 
>>>> No, that’s not the same as talking about the F9 it’s self doing X ns,
>>> but it would be part
>>>> of any practical system trying to get close to 5 ns absolute accuracy.
>>>> 
>>>> 5 ns *relative* accuracy between two F9’s? I probably could buy that if
>>> the appropriate
>>>> one sigma / on a clear day / with the wind in the right direction sort
>>> of qualifiers are
>>>> attached.
>>>> 
>>>> Bob
>>>> 
>>>>> On Feb 26, 2021, at 4:27 PM, John Ackermann N8UR <jra at febo.com> wrote:
>>>>> 
>>>>> It's interesting that they talk about the F9 receivers offering 5 ns
>>> absolute time accuracy.  Does anyone know of tests confirming that, and
>>> what sort of care was required in the setup to get there?
>>>>> 
>>>>> John
>>>>> ----
>>>>> 
>>>>> On 2/26/21 9:34 AM, Robert LaJeunesse wrote:
>>>>>> FWIW. No detailed content, and a rather quick read. "Five key trends
>>> in GPS".
>>>>>> https://www.u-blox.com/en/blogs/insights/five-key-trends-gps
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