[USRP-users] Phase drift issue of N310

Damon qiu.guowang007 at gmail.com
Mon Mar 18 13:26:11 EDT 2019


Hi Marcus,

Sorry, I can't reproduce the first observation in this discussion 
thread. A new problem about phase response has arisen.
I am testing the phase coherence performance of four receiving channels 
of N310. A B200 is transmitting single tone continuous wave to a one to 
four splitter. The 4 outputs of the splitter are connected to 4 RX 
channels of N310. Attached please find the GRC file of this test.

The RX frequency of 4 channels of  N310 is set to 460MHz, and keep 
running in the test.

The TX frequency of B200 is set to 460.02MHz first, and then to 
460.03MHz. I thought the phase difference between different dboards 
would change very little when the signal frequency difference is very 
small, similar to the performance of X310. However, the fact is that the 
phase difference between the two dboards of N310 varies considerably 
with the signal frequency transmission. For example, in the attached 
picture, when the signal frequency is 460.01MHz, the phase difference 
between channel 2 and channel 0 is -118 degrees, the phase difference 
between channel 1 and channel 0 is 0 degrees; when the transmission 
frequency is adjusted to 460.03MHz, the phase difference between channel 
2 and channel 0 is 117 degrees, and the phase difference between channel 
1 and channel 0 is 0 degrees. It is very difficult to understand that 
the phase difference of two receiving channels of two different dboards 
has changed by 235 degrees with the signal frequency change of 20 KHz. 
The phase difference of two receiving channels of the same dboard is 
basically unchanged.

Best regards,

Damon



On 2019/3/16 上午7:47, Marcus D. Leech wrote:
> On 03/14/2019 04:37 PM, Damon wrote:
>> Hi Marcus,
>>
>> The UHD Version is v3.14.0.0-rc1.
>>
>> Best regards,
>>
>> Damon
>>
> I don't see this issue at all, using v3.14.0.0-rc3
>
> How are you measuring phase, what does you flow-graph look like? Have 
> you increased the gain enough to assure that the inherent system
>   noise is not dominating your phase measurements?
>
>
>>>> Hi Ali,
>>>>
>>>> The daughterboards have their own clock generators, but they are not
>>>> exactly 'independent'. At least they don't have to be, as they 
>>>> share the
>>>> same reference clock. Look at the block diagram:
>>>>
>>>> https://kb.ettus.com/images/9/9d/USRP_N310_N300_DB_Schematic.pdf
>>>>
>>>> and "Ref Clock" block. I don't have N310 and I know that reality 
>>>> can be
>>>> a bit far from expectations (i.e. look at my "What makes sense and 
>>>> what
>>>> doesn't in the way carrier frequency is set for TwinRX currently?" 
>>>> post).
>>>>
>>>> But maybe the daughterboards can be configured to use that 
>>>> reference clock.
>>>>
>>>>
>>>> Best Regards,
>>>> Piotr Krysik
>>> The LMK clock generator uses the reference clock from the mainboard, so
>>> there should not be any mutual phase-jitter/drift issues.  I can 
>>> test this
>>>     on my N310 in the coming day or two.
>>>
>>> What version of UHD is in use?
>>
>
>
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