[USRP-users] Tune Request for B210

Marcus Müller marcus.mueller at ettus.com
Mon Jul 3 09:13:53 EDT 2017


Dear Altug,

> There is a transmitter and I try to receive the transmitted signal
> with two antennas of the B210. As the signal is transmitted from the
> same source, I would expect same signals at the receiver side.
> Nevertheless, at the output (after the DSP part of the FPGA) these two
> very same signals become phase-shifted. 

That can have multiple reasons, among these:

  * different distances between the transmitter and the phase center of
    the receive antennas
  * physically different phase length of cables, connectors, and the
    analog part of the receiver.

These would be a static offset, and totally normal, and also,
inevitable. You'll need to calibrate these out.

The DSP tuning part would only exhibit a different phase if the first
DSP chain was tuned at a different time than the second. You can avoid
that by always using timed commands for tuning. The actual frequency you
tune to, be it 0 Hz Offset or not, doesn't matter, as the two CORDICs
are absolutely identical and will stay identical if started with the
same phase at the same time.

Best regards,
Marcus

On 07/03/2017 03:06 PM, altuğ kaya wrote:
> Dear Marcus,
>
> Thank you for your answer.
>
> There is a transmitter and I try to receive the transmitted signal
> with two antennas of the B210. As the signal is transmitted from the
> same source, I would expect same signals at the receiver side.
> Nevertheless, at the output (after the DSP part of the FPGA) these two
> very same signals become phase-shifted. Thus, I want to by-pass the
> DDC Chain in order to observe weather phase-shift is caused by DSP
> modules or not.
>
> If you have any suggestions, I would keen to hear them.
>
> Best regards,
> Altug
>
> On Mon, Jul 3, 2017 at 2:52 PM, Marcus Müller via USRP-users
> <usrp-users at lists.ettus.com <mailto:usrp-users at lists.ettus.com>> wrote:
>
>     You can do this with uhd.tune_request_t in python, if you're
>     writing GNU Radio Python code, but otherwise, yes, that should be
>     part of your C++ UHD program; see the examples that ship with UHD
>     [1] under host/examples.
>
>     Anyway, as I tried to illustrate before, I don't think you're
>     chasing the right problem, but I might still be misunderstanding
>     you. Could you maybe explain why you think setting the DSP
>     frequency to zero solves your problem, and what exactly that
>     problem is, and why it is a problem? Thanks!
>
>     Best regards,
>
>     Marcus
>
>     [1] https://github.com/EttusResearch/uhd
>     <https://github.com/EttusResearch/uhd>
>
>
>     On 07/03/2017 01:01 PM, altuğ kaya via USRP-users wrote:
>>     Hello everyone,
>>
>>     I would like to tune the DDC Chain of my B210. When I search the
>>     net and ask to this forum, I learned that I have to equate the
>>     sampling rate to the master clock in order to by-pass the filters
>>     and I have to eliminate the CORDIC in order to add/subtract 0
>>     Degrees from IQ data. Changing the sampling rate is rather easy,
>>     as it is done from cmd by entering an additional parameter.
>>     However, I did not understand how to manipulate CORDIC.
>>
>>     In the MailArchive of USRP[1], I found the explanation below:
>>
>>         A tune request similar to this will disable the CORDIC:
>>         uhd::tune_request_t tune_req(target_frequency_in_hz, desired_lo_offset);
>>         tune_req.dsp_freq_policy = tune_request_t::POLICY_MANUAL;
>>         tune_req.dsp_freq = 0.0;
>>
>>     But, I really cannot figure out where to put these. Should I
>>     write a C++ code and include tune_request.hpp?
>>     I am looking forward to your reply,
>>     Altug
>>     [1]https://www.mail-archive.com/usrp-users@lists.ettus.com/msg01105.html
>>     <https://www.mail-archive.com/usrp-users@lists.ettus.com/msg01105.html>
>>
>>
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