[USRP-users] How should content of struct 'tune_result_t' be understood?

freed.pointer at netcourrier.com freed.pointer at netcourrier.com
Wed Feb 6 12:05:52 EST 2019


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Hi,


I am using an Ettus USRP N210 with a LF RX daughterboard [1] to
*receive* signals.

After having tuned to a frequency of 1 MHz with this call (which I
understand corresponds to a full 'automatic policy' for the tuning):

  uhd::tune_request_t frequency{1.0};
  uhd::tune_result_t t_res;
  t_res = usrp->set_rx_freq(freq, chan);

I display results of the tuning request using 't_res.to_pp_string()'
and get:

Tune Result:
    Target RF  Freq: 1.000000 (MHz)
    Actual RF  Freq: 0.000000 (MHz)
    Target DSP Freq: -1.000000 (MHz)
    Actual DSP Freq: -1.000000 (MHz)

Now, this section of the documentation [2] stipulates:
<<
Note that the meaning of the DSP frequency's sign differs between TX
and RX operations. The target frequency is the result of target_freq =
rf_freq + sign * dsp_freq. For TX, sign is negative, and for RX, sign
is positive.
>>

I'm trying to make sense of these figures. But unless I am mistaken,
applying the formula above with a positive sign (since I am interested
in _receiving_ and have just set the RX frequency) leads to results I
fail to grasp properly:
- with the 'Target' figures: 1 + (-1) = 0
- with the 'Actual' figures: 0 + (-1) = -1

During the same execution, usrp->get_rx_freq() returns a result which
seems compliant to the tuning request: 1000000.000931 Hz.

So my question is: how should the figures found in the 'tune_request_t'
structure be read and understood?

I would be most grateful for any help you could provide.

Kind Regards,

Imad


[1] Details about the hardware I'm using:

$ uhd_usrp_probe
[INFO] [UHD] linux; GNU C++ version 7.3.0; Boost_106501; UHD_3.14.0.0-
229-gc34fa6d8
...
|       Device: USRP2 / N-Series Device
|     _____________________________________________________
|    /
|   |       Mboard: N210r4
|   |   hardware: 2577

|   |   serial: F44F5C
|   |   FW Version: 12.4
|   |   FPGA Version: 11.1
...
|   |   |       RX Dboard: A
|   |   |   ID: LF RX (0x000f)

[2] 

https://files.ettus.com/manual/structuhd_1_1tune__request__t.html#a616cd26b5d26bfa04909ab2304770492






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