On 04/18/2019 01:44 PM, Rob Kossler via USRP-users wrote:
Hi,
I am seeing some strange (and unrepeatable) behavior with an
X310/UBX-160 and the multi_usrp->set_rx_freq() function. I call this
function with a tune_request that I create with a desired target
frequency & lo_offset. The output below shows a "pp_string" of the
request and the result (for the request, I had to create my own
"pp_string" function but for the result, I used the existing Ettus
function).
I can tell from the resulting spectrum (not provided) that the
frequencies / offsets are often not getting set correctly because I
have a CW tone at a known frequency coming in. But, what I am
presently interested in is the output shown below. In this example,
I requested a target frequency of 2450 MHz with a 5 MHz LO offset.
The tune request looks like I would expect. However, for the tune
result, shouldn't the "Target RF freq" match the "Target freq" from
the request (i.e., 2450 MHz rather than 2455 MHz)?
Rob
// This is essentially the code executed
tune_request_t tune_req(2450e6,5e6);
tune_result_t tune_res = usrp->set_rx_freq(tune_req,0);
// here is the output showing to_pp_string()
Tune request
RF Policy: M
Target freq: 2450.000000 (MHz)
RF freq: 2455.000000 (MHz)
DSP Policy: A
DSP freq: 0.000000 (MHz)
args:
Tune Result:
Target RF Freq: 2455.000000 (MHz)
Actual RF Freq: 2455.006105 (MHz)
Target DSP Freq: 5.006105 (MHz)
Actual DSP Freq: 5.006105 (MHz)
The "Target RF Frequency" is the frequency setting of the LO, which if
you are requesting a 5MHz LO offset, will be somewhere around 5MHz away from
your target "sky" frequency. The DSP arranges to "mop up" the
difference.
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