Hi Marcus, This situation occurs with both our laboratory RB oscillator (it's also a GPSDO so provides a 1pps aligned to UTC and a 10 MHz output) and with the Ettus research GPSDO modules purchased here: https://www.ettus.com/all-products/gpsdo-kit/ [https://www.ettus.com/wp-content/uploads/2019/01/GPSDO_650x650-650x630.jpg]<https://www.ettus.com/all-products/gpsdo-kit/> GPSDO Kit for USRP N200/N210 | Ettus Research, a National Instruments Brand | The leader in Software Defined Radio (SDR)<https://www.ettus.com/all-products/gpsdo-kit/> USRP GPS-Disciplined Oscillator Kit . Includes GPS locked reference oscillator for USRP N200/N210 series (rev 4 and above). www.ettus.com The RB oscillator has a shorter PPS high duration than the USRP specifies as the minimum duty cycle, but again since this is happening with both this timing source and the ettus supplied GPSDO sources, we think the fault is in either the software or the USRP.
-Aiden ________________________________ From: Marcus D. Leech <[email protected]> Sent: 10 September 2022 16:50 To: Aiden Morrison <[email protected]>; [email protected] <[email protected]> Subject: Re: [USRP-users] Re: N210 USRP synchronization performance 4x worse than expected - suspect configuration problem On 2022-09-10 06:06, Aiden Morrison wrote: Hi Marcus, The timestamps returning from the radios are always in perfect agreement in the metadata between the two USRPs. We are checking the synchronization by doing a correlation on the captured sample blocks and observing the peak shifting by 0-3 samples. If the front-ends were actually synchronized at the ~100 ns level, this would be +/-1. To be clear, this is with your laboratory setup, with an Rb refclock, and some kind of shared 1PPS signal? What are the characteristics of your 1PPS signal in this case? Rise time, pulse duration, voltage, etc. ________________________________ From: Marcus D. Leech <[email protected]><mailto:[email protected]> Sent: 08 September 2022 16:47 To: [email protected]<mailto:[email protected]> <[email protected]><mailto:[email protected]> Subject: [USRP-users] Re: N210 USRP synchronization performance 4x worse than expected - suspect configuration problem On 2022-09-08 04:54, Aiden Morrison via USRP-users wrote: Hello, I am attempting to do synchronized multi-site reception using USRP N210 radios. Our first attempt was to make use of 2x radios with ettus GPSDO modules installed, feeding the radios with a common PRN sequence and correlating the outputs to evaluate timing offset. The Ettus GPSDO radios say they provide +/-60ns timing accuracy, so even in the worst case where one of the radios goes to each side of the limit, at 10 MHz we would have slightly more than 1 sample of offset between them, but in testing we observed +0 to +3 samples of offset. To isolate the problem we repeated the test with two different N210 radios fed by a common external 10 MHz and PPS signal from a GNSS disciplined Rubidium source, and observed exactly the same +0 to +3 samples of shift between the captured sequences. Seeing the same error when using a common clock indicates the problem is very likely to be in our test software. In our software we follow https://files.ettus.com/manual/page_sync.html<https://eur03.safelinks.protection.outlook.com/?url=https%3A%2F%2Ffiles.ettus.com%2Fmanual%2Fpage_sync.html&data=05%7C01%7CAiden.Morrison%40sintef.no%7Cb3cb5353e62b4b076f0408da933bde15%7Ce1f00f39604145b0b309e0210d8b32af%7C1%7C0%7C637984182589926129%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=l0fbP8XnZ5khyJaJC3zBR5%2B3CAlOVb4LHGrs%2BorfQN0%3D&reserved=0> for synchronization: 1. We create two USRP objects, one for each radio. I would repeat this experiment, but using a single multi-usrp object. UHD will time-align samples across radios in the same multi-usrp container object. But not across objects. 1. 2. We command the USRPs to use the external synchronization (radios report back that they are synchronized to the GPSDO in the former configuration) 3. We use a common time in the time spec commands for each configuration change (e.g. setting LO frequencies) 4. For each burst capture of 50,000 samples we set a time_spec for each USRP a long time in the future (1.0 seconds since the last capture) To be clear you are requesting streaming start at the same time? 1. 2. Step 4 is repeated 1000 times giving us 1000 bursts for evaluating the synchronization The only factor I can see is that we are using WBX radio modules, and we are uncertain if this is somehow a factor in what we are seeing. Thanks in advance for feedback. -Aiden Also, how are you checking time alignment? Are you actually looking at the timestamps on the sample frames?
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