[USRP-users] UBX coherence between TX and RX

Marcus D. Leech patchvonbraun at gmail.com
Wed Mar 27 11:10:01 EDT 2019


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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