Hi Marcus,
thanks for your prompt answer.
I don't have a PPS signal readily available. Would a 10MHz reference
suffice as well?
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.
Besides, I was really hoping that one USRP would sync itself.
A note on documentation: All the info etc. I could find in old mailing
list posts and EttusKB always refer to multiple USRPs but never mention
multiple daughterboards on one USRP. It would be really helpful to add a
more prominent note there.
Cheers
Johannes
On 22.07.20 19:00, Marcus D. Leech via USRP-users wrote:
On 07/22/2020 12:39 PM, Johannes Demel via USRP-users wrote:
Hi all,
I have an issue with multiple USRP streams. If they are on separate
daughterboards, but on the same motherboard aka USRP, those streams
are not time aligned. They show a time offset of more than 10us. I use
one USRP source block in GNU Radio and configure it with multiple
streams.
At first, I observed this behavior with N310s but now I see it on an
X310 as well. I use a GNU Radio flowgraph where I set the clock to "PC
clock". I'd expect all streams to be in sync because they are handled
by the same object. At least that's what I hope.
On an X310 I have 2 RX streams that observe a signal from across the
room. On an N310, I tried different configurations and I could observe
this delay between daughterboards. They are in sync for one
daughterboard.
My flowgraph handles frequency offsets etc. But constant time offsets
between RX streams are problematic. And I suppose they should not be
there.
I'd like to use GPSDOs at some point but I don't have a GPS signal in
the lab. And I don't have an octoclock or smth similar. But all of
this shouldn't matter since I only use on USRP for RX. Or am I missing
smth?
USRP: N310/X310(with 2x UBX160)
UHD: 3.15LTS
OS: Ubuntu 20.04
GNU Radio 3.9 (aka master)
Cheers
Johannes
Here's something you need to know about the time-keeping architecture in
both X310 and N310. There are, effectively TWO time-keeping
registers.
When you use "Sync to PC", those registers each get updated as a
*separate* operation across the wire, so there can be (as you observe) a
several-microsecond inconsistency between them.
The whole thing about the 1PPS pulse when doing time-setting is that no
matter how many timekeeper registers there are, they all get
set at the same instant, due to the 1PPS pulse processing.
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