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Re: [Discuss-gnuradio] Time syncing between SDRs on different computers

From: Marcus Müller
Subject: Re: [Discuss-gnuradio] Time syncing between SDRs on different computers
Date: Wed, 16 Aug 2017 22:27:29 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.8.0

10 ms timing certainty is actually pretty hard to achieve across USB2!
But: as far as I can see, it'd be rather easy to say, tune both sticks
to DVB-T, cross-correlate to find the relative offset, tune to the
frequency of interest and work from there. That is, if the delay spread
in the cellular channel < 10ms, but considering that would be equivalent
to a path length difference of 3000 km...

I don't think the tuning process itself would interrupt the sampling,
right (Sylvain)?


On 08/16/2017 07:40 PM, address@hidden wrote:
> Say we need +-10ms accuracy in timestamps. Precision Time Protocol in LAN 
> should sync PC's clocks with enough accuracy. 
> And then using USRP-like methods - set current/start time - get first sample 
> in known time? Of course USB will add
> undetermined delays but no external GPS required.
> On Wed, 2017-08-16 at 16:57 +0100, Derek Kozel wrote:
>> It should be pointed out that the hardware based timestamping is only needed 
>> if you need time alignment better than a
>> half second or so. With USB transfers, various buffers, NTP based alignment 
>> of the host computer's time, and some
>> extra code on the host side you could do a coarse time alignment, probably 
>> with less than a half second of error.
>> You could also time align the streams if both radios receive the same signal 
>> and you know the distance (and other
>> details depending on the precision needed) between the transmitter and your 
>> two receivers. This is done by many
>> protocols like the cellular standards to create a distributed timebase, but 
>> quickly becomes non-trivial.
>> The B200 does timestamping in the FPGA and can use a 1PPS signal to align to 
>> GPS time. It is more expensive than the
>> HackRF, but much less than the $3000 mentioned above.
>> On Wed, Aug 16, 2017 at 4:20 PM, Sylvain Munaut <address@hidden> wrote:
>>> On Wed, Aug 16, 2017 at 5:11 PM,  <address@hidden> wrote:
>>>> What type of hardware? I thought from hardware point of view only precise 
>>>> clock is required and all the other
>>> things in
>>>> firmware. I've naively thought i could modify hackrf firmware to get this 
>>>> feature.
>>> Mostly a FPGA and a PPS input from a GPS receiver.
>>> Each individual sample has a timestamp of when it has been received.
>>> And you can also reset the timestamp on the next  PPS edge.
>>> Technically it would be possible to modify the hackrf firmware and
>>> repurpose some GPIOs and have all samples transmitted to the host be
>>> in timestamped packets and implements timestamping in the on-board
>>> ARM.
>>> For additional hardware you'd only need an external GPS receiver (or
>>> some other way to have both a single freq reference + single sync
>>> pulse).
>>> Cheers,
>>>     Sylvain
>>> _______________________________________________
>>> Discuss-gnuradio mailing list
>>> address@hidden
>>> https://lists.gnu.org/mailman/listinfo/discuss-gnuradio
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