[USRP-users] Phase drift issue of N310

Marcus D. Leech patchvonbraun at gmail.com
Tue Mar 19 22:40:27 EDT 2019

On 03/19/2019 09:12 PM, Damon wrote:
> Hi Marcus,
> The phase  responses of two channels of different dboards (ubx-160) 
> in  a X310  are very consistent. When the frequency of transmitting 
> signal changes from 460.01MHz to 460.03Mhz, the phase difference 
> between two RX channels of different dboards  in X310 remains 
> unchanged. But for the phase difference between two RX channels of 
> different dboards of  N310,  there are dozens or hundreds degree of 
> changes.
> Can you try to reproduce the test and help me to figure out how to 
> solve it?
> I'm not sure if it's a hardware bug, a driver bug or USRP setup problem.
I cannot imagine a hardware or driver bug that could produce this 
behavior.  It would mean that the receiver was somehow changing LO
   frequency when the TX frequency changed.

Does this happen with *other* frequency changes, or just 
460.01<--->460.03.  I wonder if you have an interfering signal that is being
   "uncovered" by TX frequency change, and you're simply measuring that 
interfering signal?

The only other thing that occurs to me is filtering that is extremely 
non-linear phase.  But that would create such a mess that most applications
   would likely not work--clearly they do.

I can try to reproduce in my lab tomorrow, but, like the last tests I 
did, I very much expect to not be able to reproduce.

> Best regards,
> Damon
> On 2019/3/19 上午6:10, Marcus D. Leech wrote:
>> 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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