[USRP-users] Phase drift issue of N310

Tillson, Bob (US) robert.tillson at baesystems.com
Mon Mar 18 16:13:05 EDT 2019

Same Daughterboard is same LO, right, so things should be exactly the same on ch0-ch1 and ch2-ch3...

-----Original Message-----
From: USRP-users <usrp-users-bounces at lists.ettus.com> On Behalf Of Damon via USRP-users
Sent: Monday, March 18, 2019 1:26 PM
To: Marcus D. Leech <patchvonbraun at gmail.com>; usrp-users at lists.ettus.com
Cc: Damon Qiu <qiu.guowang007 at gmail.com>
Subject: Re: [USRP-users] Phase drift issue of N310

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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,


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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