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

Johannes Demel demel at ant.uni-bremen.de
Wed Jul 22 12:39:10 EDT 2020

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)


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