[USRP-users] Phase drift issue of N310

Marcus D. Leech patchvonbraun at gmail.com
Mon Mar 18 18:10:09 EDT 2019

On 03/18/2019 01:26 PM, Damon wrote:
> 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
Since the LO on the daughterboards has no idea that you've changed input 
frequency, this is clearly a measurement thing, and it's up to you
   to understand what you're measuring, and why.

> 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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