[USRP-users] Phase drift issue of N310

Marcus D. Leech patchvonbraun at gmail.com
Wed Mar 20 13:07:23 EDT 2019

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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