[USRP-users] Phase drift issue of N310

Damon qiu.guowang007 at gmail.com
Thu Mar 21 07:08:31 EDT 2019

Hi Marcus,

Thank you for your reply.

I compared the USRP configuration of your grc file with that of mine. I 
found that the key to solve this issue is set_time_next_pps or 
set_time_unknow_pps should be called.

I don't know why   those two functions have an effect on phase error. I 
haven't found any special description of these two functions in the N310 

Best regards,


On 2019/3/21 上午1:07, Marcus D. Leech wrote:
> On 03/20/2019 12:55 PM, Damon wrote:
>> Hi Marcus,
>> Thanks for your reply.
>> Yes, it happen with any other frequency changes, not only 460.01M and 
>> 469.03Mhz.
>> I'm building a 4-channel coherent receiving system with n310. It 
>> needs to compensate the phase errors of multiple receiving channels. 
>> So I need to measure the phase errors first and then compensate them. 
>> But in the range of 100MHz bandwidth (from center_freq-50MHz to  
>> center_freq+50MHz), the phase difference of two receiving channels of 
>> different dboards varies too much with frequency, so it's very 
>> difficult to compensate the phase errors. As a contrast, the phase 
>> errors of the two receiving channels of X310 with ubx vary very samll 
>> with frequency, so it is easy to compensate the phase difference.
>> Looking forward to your test results
>> Best regards,
>> Damon
> I used the attached script.
> Now, I'm only looking at +/- 2MHz, rather than your 100Mhz bandwidth.
> I found that the phase noise and offset did not change noticably 
> tuning across the entire 4Mhz band.  I don't have a machine fast 
> enough here to
>   sweep my TX across 100Mhz, but with the N310 RX at a fixed tuning, 
> and sweeping the TX (in this case, a Marconi transceiver test set), I 
> did not see
>   any significant phase shift or change in mutual phase noise as the 
> TX swept across the 4MHz band.
>> On 2019/3/20 上午10:40, Marcus D. Leech wrote:
>>> 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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