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

Johannes Demel demel at ant.uni-bremen.de
Thu Jul 23 03:29:27 EDT 2020


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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