[USRP-users] usrp b210 and mimo

Ian Buckley ianb at ionconcepts.com
Fri Dec 5 11:18:33 EST 2014


Juha,
Is the B200 timing offset always before (or after) the N200, or does it randomly bracket the N200?

There are a couple of things I'll mention up front that come to mind because of what you are doing…but they don't explain your issue:
1)N2x0 will always sample the PPS with a 100MHz clock, B200 will sample PPS with whatever rate is configured as the master clock rate. If that value is 50MHz then there's a 10nS ambiguity introduced …if 100/master_rate is not an integer then the sampling clock edges are going to constantly shift phase w.r.t each other.
2) Group delay through the DSP's is quite different…it will in general be larger for B200 I suspect…but it will depend heavily on what the configured decimation is.

Neither of these however produces an offset of ~100uS.

-Ian


On Dec 5, 2014, at 7:53 AM, Juha Vierinen via USRP-users <usrp-users at lists.ettus.com> wrote:

> Starttime is just a unix second about 10 seconds into the future. 
> 
> PPS is TTL from fanout buffer. I think the 10 MHz is +10 dBm. This is the device: http://www.fei-zyfer.com/cs2.htm
> 
> The same clock source works well with the N200. The B210 is kind of synchronized, but there is ~100 microseconds of random offset between restarts. 
> 
> juha
> 
> On Fri, Dec 5, 2014 at 3:22 PM, Marcus D. Leech via USRP-users <usrp-users at lists.ettus.com> wrote:
> On 12/05/2014 08:50 AM, Juha Vierinen via USRP-users wrote:
>> I've slowly gathered the courage to try out the B210 devices. I was impressed by the fact that I could get 50 MHz of bandwidth out of it. However, I immediately ran into problems with absolute timing. 
>> 
>> I've been using the B210 for mimo with USRP N200 devices for testing. The N200 devices all nicely synchronize themselves to the external PPS and after multiple restarts, their timing will be accurate. However, the starting time of the B210 seems to have a random component with a standard deviation of about 100 microseconds. Every time I restart sampling, the offset between the N200 and the B210 is slightly different. There is no change in the timing offset between the N200 devices. 
>> 
>> Is this a known issue, or am I doing something wrong?  
>> 
>> This is how I configure the devices:
>> 
>> u.set_clock_source("external")
>> u.set_time_source("external")
>> tt = time.time()
>> while tt-math.floor(tt) < 0.2 or tt-math.floor(tt) > 0.3:
>>     tt = time.time()
>>     time.sleep(0.01)
>> print("Latching at "+str(tt))
>> u.set_time_unknown_pps(uhd.time_spec(math.ceil(tt)+1.0))
>> time.sleep(1)
>> u.set_start_time(uhd.time_spec( starttime ) )
>> 
>> juha
>> 
>> _______________________________________________
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>> USRP-users at lists.ettus.com
>> http://lists.ettus.com/mailman/listinfo/usrp-users_lists.ettus.com
> Where and how does 'starttime' get set?
> 
> How are you distributing your 1PPS and 10Mhz clocks?
> 
> 
> -- 
> Marcus Leech
> Principal Investigator
> Shirleys Bay Radio Astronomy Consortium
> http://www.sbrac.org
> 
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> 
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