Phase measurement with LimeSDR mini in GNU Radio

hanc · 24 October 2019 16:07

Hi all,

I am trying to measure phases of the five sinusoids in a loopback configuration with LimdSDR mini in GNU Radio environment.
The flowgraph is described below.

I set the FFT size as 2048, and the five carriers are spaced with integer multiples of FFT bin spacing, thus resulting the period of the combined signal as 128 samples.
I monitor the phase of the FFT bin at the carrier frequency to extract the phase of the carrier.
This perfectly gives the phases of five carriers in a simulation.
However, when I utilize the LimeSuite Sink blocks, there are unpredictable RX timing jumps over time, which screws the carrier phases except the DC carrier.
Please see the captured measurement screen below.

The upper scope shows the received waveform, the middle one shows the phase of the 4th carrier (FFT bin near 6.4 us indicates the phase), and the lower one shows the magnitude of the signal FFT.
Does anyone have good idea for solving this problem?
I want the phase to be static during the measurement.

Regards,
hanc

Garmus · 25 October 2019 13:05

Hey hanc,

Are you using ubuntu and if so did you compile gr-limedsr from source? If so you could edit these lines to see if any dropped packets happened.
Change ‘false’ to ‘true’:

github.com

myriadrf/gr-limesdr/blob/master/lib/sink_impl.h#L36






static const pmt::pmt_t TIME_TAG = pmt::string_to_symbol("tx_time");


namespace gr {
namespace limesdr {
class sink_impl : public sink {
    private:
    lms_stream_t streamId[2];


    bool stream_analyzer = false;


    int sink_block = 2;


    pmt::pmt_t LENGTH_TAG;
    lms_stream_meta_t tx_meta;
    long burst_length = 0;
    int nitems_send = 0;
    int ret[2] = {0};
    int pa_path[2] = {0}; // TX PA path NONE

github.com

myriadrf/gr-limesdr/blob/master/lib/source_impl.h#L36






static const pmt::pmt_t TIME_TAG = pmt::string_to_symbol("rx_time");


namespace gr {
namespace limesdr {
class source_impl : public source {
    private:
    lms_stream_t streamId[2];


    bool stream_analyzer = false;


    int source_block = 1;


    bool add_tag = false;
    uint32_t pktLoss = 0;


    struct constant_data {
        std::string serial;
        int device_number;
        int channel_mode;

Recompile and reinstall. I’m guessing you are dropping RX packets there and might need to lower the sampling rate.

Also, since you are using hardware throttling( by using a physical board) you need to remove ‘Throttle’ block from your flowgraph.

DOCDAILEY · 6 November 2019 19:10

Remove throttle from both tx and rx or just rx? I have never used a device tx sink is why I ask.

Garmus · 7 November 2019 09:54

Throttle should not be used when any stream(RX or TX) is connected to hardware. You can read more here.