[time-nuts] Re: 20210423: Introduction and Request for Spirent GSS4200 User Manual / Help

[Bob kb8tq]

Hi

> On Apr 25, 2021, at 10:13 AM, Lux, Jim <jim at luxfamily.com> wrote:
> 
> On 4/25/21 6:40 AM, Bob kb8tq wrote:
>> Hi
>> 
>> 
>>> On Apr 25, 2021, at 9:31 AM, Lux, Jim <jim at luxfamily.com> wrote:
>>> 
>>> On 4/25/21 6:02 AM, Bob kb8tq wrote:
>>>> Hi
>>>> 
>>>> The thing that I find useful about a GPS simulator is it’s ability to calibrate the
>>>> time delay through a GPS based system. In the case of a GPSDO, there may be
>>>> things beyond the simple receiver delay that get into the mix. Getting the entire
>>>> offset “picture” all at once is nice thing. Yes, that’s a Time Nutty way to look at it…..
>>>> 
>>>> So far, I have not seen anybody extending this sort of calibration to the low cost
>>>> SDR based devices. Without digging into the specific device, I’m not sure how
>>>> well a “generic” calibration would do. Indeed, it might work quite well. Without
>>>> actually doing it … no way to tell.
>>>> 
>>>> So if anybody knows of the results of such an effort, I suspect it would be of
>>>> interest to folks here on the list.
>>>> 
>>>> Bob
>>> 
>>> A double difference kind of relative measurement might be useful - compare two (or three) GNSS receivers.  Then the absolute timing of the test source isn't as important.
>> Well …. it is and it isn’t. If you are trying to get “UTC in the basement” (or even
>> GPS time)  to a couple nanoseconds, then you do need to know absolute delays
>> of a number of things. Is this a bit crazy? Of course it is :)
>> 
>> Bob
>> 
> Good point..
> 
> For many SDRs, it's tricky to get the output synchronized to anything - a lot were designed as an RF ADC/DAC for software SDR (like gnuradio). The software  SDRs are sort of a pipeline of software, with not much attention to absolute timing, just that the samples come out in the same order and rate as samples go in, but with a non-deterministic delay.  Partly a side effect of using things like USB or IP sockets as an interface. And, to a certain extent, running under a non-real time OS (where real time determinism is "difficult programming" - although clearly doable, since playing back a movie requires synchronizing the audio and video streams ).
> 
> If your goal is "write software 802.11" you don't need good timing - the protocol is half duplex in any case, and a millisecond here or there makes no difference.
> 
> A SDR that has a FPGA component to drive the DACs might work pretty well, if you can figure a way to get a sync signal into it.  One tricky thing is getting the chips lined up with the carrier - most inexpensive SDRs use some sort of upconverter from baseband I/Q, and even if the I/Q runs off the same clock as the PLL generating the carrier, getting it synced is hard.
> 
> The best bet might be a "clock the bits out and pick an appropriate harmonic with a bandpass filter".   If the FPGA clock is running at a suitable multiple of 1.023 MHz, maybe this would work?  Some JPL receivers use 38.656 MHz as a sample rate, which puts the GPS signal at something like 3/4 of the sample rate.
> I'd have to work it backwards and see if you could generate a harmonic that's at 1575...
> 

I think that coming out of a “one bit DAC” on the FPGA is the most likely way to generate
the signal. Run that at carrier / N and away you go. I don’t think you want to get N to 
terribly high. Having all those images running around may not be a good idea. Something
in the 5 to 10 range seems about right. Is there enough “harmonic” to make it work or do
you need some sort of SRD-ish thing to womp it up? ….. My guess is that there still will 
be a bunch of little delay ambiguities That pile up on you. 

The bigger issue is: do you want to make this a “lifetime project” ? It’s only one of many
things that need attention if you want to get to the final goal. Spend enough time on each
little step and this all becomes a “many decades” sort of adventure. 

In this case I’ll let Said worry about the details and just use his gizmo :)

Bob


> 
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