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

Marcus Müller marcus.mueller at ettus.com
Thu Jul 23 03:59:33 EDT 2020


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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