[USRP-users] 10us+ sample delay between daughterboards

Johannes Demel demel at ant.uni-bremen.de
Thu Jul 23 05:44:31 EDT 2020

Hi Marcus,

I tried to improve the situation. I had another look at [0,1].

 From [1] (N310 manual)
"[..] which can both be used as time- and clock references. The GPSDO 
will function as a reference even when there is no GPS reception."

This helps. I set
Sync: "Unknown PPS"
Clock/Time source: "O/B GPSDO"
and all streams are in sync. That's sort of a relief for N310s.

Since I don't have a GPS signal in the lab, I assume there's no point in 
trying to receive a GPS timestamp and set the device time on next PPS 
[0]. I would prefer to work with timestamps "since epoch" instead of 
"since start of flowgraph".

I need to figure out how to use GPSDO on those X310s though. Since the 
GPS LED does not light up, I don't think it detects a GPSDO at all. And 
I just hope it can use a GPSDO like an N310.

Since I work remotely, I can't just manually generate a sync pulse. But 
if I can find a device that works reliably I may switch to that. Thanks 
for the explanation and ASCII schematic.


[0] https://files.ettus.com/manual/page_gpsdo_x3x0.html
[1] https://files.ettus.com/manual/page_usrp_n3xx.html#n3xx_synchronization

On 23.07.20 09:59, Marcus Müller via USRP-users wrote:
> Hi Johannes,
> let me increas Marcusness of this by ~3dB.
> On 23.07.20 09:29, Johannes Demel via USRP-users wrote:
>> I don't have a PPS signal readily available. Would a 10MHz reference
>> suffice as well?
> Nope, that doesn't set a time register. You don't actually need a pulse
> per second – you need a single pulse.
> It's a trick I usually tell to impress physicists doing measurements:
> 1. call set_time_next_pps
> 2. connect 2× 1.5 V batteries (or similar) to get a voltage between
> something like 2.5 V and 5V.
> 3.             |
>               -----
>     ----------o   o----------+---------------> SMA inner
>    |   +                     |
> ------         push-       | | pull-
>   ---bat        button      |R| down
>    |                         |
>     -------------------------+---------------> SMA outer
> The pushbutton can effectively be anything that has a nice quick
> contacting mechanism to yield a nice edge, without ringing dangerously
> high.
>> Do I get rid of this issue if I use a GPSDO? We have the hardware
>> (antennas + modules) available but no GPS signal in our lab.
> Hmmmm I'd have to think hard here: the GPSDO does offer 10 MHz from its
> internal oscillator when it's not locked, but I think the PPS is really
> only present with GPS time
>> Besides, I was really hoping that one USRP would sync itself.
> Sounds fair; there's an internal PPS pulse, I've never tried it. Have
> you tried setting the pps source to "internal" and then setting on PPS?
> Cheers,
> Marcus the younger
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