On 04/18/2019 04:02 PM, Rob Kossler wrote:
Ok. I couldn't seem to duplicate with the stock "rx_samples_to_file",
but with the following minor mods, I see some of the bad behavior.
This sure looks like a UHD bug to me.
I simply added timed commands around the set_rx_freq() function (see
snippet below as well as attached source code). Is this the correct
way to do it? If so, then I believe there is a bug because I get
changing spectra just by changing the LO offset.
uhd::time_spec_t cmd_time = usrp->get_time_now()+0.1;
usrp->set_command_time(cmd_time);
usrp->set_rx_freq(tune_request, channel); // this line not modified
usrp->clear_command_time();
My command line was
rx_samples_to_file --nsamps 500000 --rate 50e6 --freq 2450e6
--lo-offset -10e6
If I change just the LO offset (always keeping "--freq 2450e6"), the
known CW tone that I supply as an input signal moves around the spectrum.
This is the correct spectrum achieved with --lo-offset 0. Note the CW
tone at 5 MHz which is correct because my sig gen is transmitting at
2455 MHz.
image.png
This is the spectrum received with --lo-offset -10e6. Note that the
tone (which did not change from the example above) shows up at 15 MHz
instead of the correct location at 5 MHz.
image.png
Rob
OK, so what happens if you use a *positive* LO offset?
On Thu, Apr 18, 2019 at 1:53 PM Rob Kossler <rkoss...@nd.edu
<mailto:rkoss...@nd.edu>> wrote:
I do see in the documentation that the tune result "target_rf_freq"
will match the tune request "rf_freq", which it does in my example
below. So, I suppose the output shown is as expected for my request.
Unfortunately, this doesn't solve my problem of the actual spectrum
being wrong. I may have to construct a test case to show that the
problem is in UHD....
On Thu, Apr 18, 2019 at 1:50 PM Marcus D. Leech via USRP-users
<usrp-users@lists.ettus.com <mailto:usrp-users@lists.ettus.com>>
wrote:
>
> 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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