[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 12:56:46 EST 2019


Thank you very much for your prompt reply, and thank you for clarifying
the reason for the 'DSP Freq' values found in the 'tune_request_t'
struct, which are both equal to -1 MHz:

Target DSP Freq: -1.000000 (MHz)
Actual DSP Freq: -1.000000 (MHz)


However, I still cannot figure out how these values could be combined
with the two other values

Target RF  Freq: 1.000000 (MHz)
Actual RF  Freq: 0.000000 (MHz)

in the formula which appears in the documentation excerpt I quoted:
'target_freq = rf_freq + sign * dsp_freq'.

Does trying to do so make any sense? I was assuming that, in this
formula, 'target_freq' was the frequency I was interested into, and
tuning to, but maybe I am wrong.

Again, thank you for your help and insight on this.


Kind Regards,

Imad

On Wed, 2019-02-06 at 12:29 -0500, Marcus D. Leech via USRP-users
wrote:
> On 02/06/2019 12:05 PM, freed.pointer--- via USRP-users wrote:
> > [Note: I have already sent this message to the list, but before
> > subscribing to it, and it seems that for this reason, the message
> > has
> > not been delivered. I am making a second attempt after having
> > subscribed. Please accept my apologies in case you received it
> > twice.]
> > 
> > 
> > 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.
> > 
> 
> The LF RX has no local frequency-conversion hardware.  So, frequency 
> conversion down to complex-baseband is done entirely in the DSP
>    in the FPGA.
> 
> If you want to look at a 1MHz signal, the DSP has to bring it *down*
> by 
> 1MHz to baseband, so that is indicated by the negative DSP frequency
> value.
> 
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