[USRP-users] UBX coherence between TX and RX

Michael R. Freedman mfreedman at ll.mit.edu
Wed Mar 27 15:39:09 EDT 2019


Marcus,


I tried a 10MHz LOOffset with no change.  I also tried it at 1155.0MHz 
and 1160.0MHz.  At 1155 the phase rolls... at 1160 it does not.


Mike


On 03/27/2019 11:10 AM, Marcus D. Leech via USRP-users wrote:
> On 03/27/2019 10:41 AM, Michael R. Freedman via USRP-users wrote:
>>
>> Any thoughts on this?
>>
>>
>> Mike
>>
> Given that in this case, the incoming carrier will be fighting 
> DC-offset removal rather hard, I wonder if this is a weird artifact of 
> DC-offset
>   removal?
>
> If you use offset tuning on the RX, do you still see a significant 
> phase roll?
>
>
>>
>> On 03/25/2019 01:04 PM, Michael R. Freedman via USRP-users wrote:
>>>
>>> Marcus,
>>>
>>>    190Hz is what we calculated.  I have attached a text file with 
>>> the data we used.  This is a single UBX40 tuned to 155MHz sampling 
>>> at 2MHz.
>>>
>>>
>>> Mike
>>>
>>>
>>> On 03/25/2019 12:34 PM, Nick Foster wrote:
>>>> Well, according to your description, you could transmit a carrier 
>>>> from TX to RX (through an attenuator) with both sides set to the 
>>>> same frequency. Your received signal should look like a sine wave 
>>>> at the frequency of the offset.
>>>>
>>>> On Mon, Mar 25, 2019 at 9:16 AM Michael R. Freedman via USRP-users 
>>>> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>>>>
>>>>     Hello,
>>>>
>>>>     Do you have any suggestions as to how to measure the frequency
>>>>     delta between the transmit channel and the receive channel?
>>>>
>>>>     As I sat down to do this, I realized I have no real way to do that.
>>>>
>>>>
>>>>     Mike
>>>>
>>>>
>>>>
>>>>
>>>>
>>>>     On 03/24/2019 03:23 PM, Marcus D. Leech via USRP-users wrote:
>>>>>     On 03/24/2019 02:39 PM, Freedman, Michael - 1008 - MITLL via
>>>>>     USRP-users wrote:
>>>>>>     It is not just a phase offset. It is a frequency offset. The phase drifts between the two. I can tolerate a phase offset at startup. A freq offset however is causing problems.
>>>>>>
>>>>>>     Mike
>>>>>>
>>>>>>     Sent from my iPhone
>>>>>>
>>>>>>     On Mar 24, 2019, at 4:28 AM, Marcus Müller<marcus.mueller at ettus.com> <mailto:marcus.mueller at ettus.com>  wrote:
>>>>>>
>>>>>>     Can you elaborate on what "not coherent" means to you – the relative
>>>>>>     phase should be constant after each tune, but if you don't tune
>>>>>>     simultaneously, i.e. with timed commands, random at each tune.
>>>>>>
>>>>>>     Best regards,
>>>>>>     Marcus
>>>>>     Also, what version of UHD?  What hardware rev of UBX cards?
>>>>>
>>>>>
>>>>>>     On Sat, 2019-03-23 at 17:06 -0400, Michael R. Freedman via USRP-users
>>>>>>     wrote:
>>>>>>>     Hello,
>>>>>>>
>>>>>>>
>>>>>>>     I have an issue where I tune both the TX and RX side of a UBX40 card
>>>>>>>     in
>>>>>>>     an X310 to the same frequency and find that the transmitted signal
>>>>>>>     and
>>>>>>>     what is received are not coherent.  I am using an external 10MHz
>>>>>>>     reference and have tried the documented suggestions.
>>>>>>>
>>>>>>>     at 150MHz it is coherent.
>>>>>>>
>>>>>>>     at 155MHz it is NOT coherent.
>>>>>>>
>>>>>>>
>>>>>>>     I have tried setting the dboard_clock_rate to 20MHz.  This made the
>>>>>>>     problem appear at 150MHz as well.  I've tried integer_n tuning.
>>>>>>>
>>>>>>>     I have verified that the ref_lock and lo_lock are both true.
>>>>>>>
>>>>>>>
>>>>>>>     Any suggestions?
>>>>>>>
>>>>>>>
>>>>>>>     Mike
>>>>>>>
>>>>>>>
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>>>>>>
>>>>>>
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>>>>>
>>>>>
>>>>>
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>>>>
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>>>
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