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Pluto TX-RX with Undesirable Phase Shift

Question asked by MarcusW on Dec 27, 2017
Latest reply on Dec 28, 2017 by IPrium

I'm trying to understand why a waveform uploaded to the Pluto's internal memory and then transmitted continuously would result in a received waveform with an apparent arbitrary phase shift.

 

Below is a Matlab plot showing the transmitted waveform using the transmitRepeat() function and the resulting received waveform.  The waveform simply consists of two CW tones at 10 and 20 KHz summed together.  It appears as though the received waveform is correct, but that a phase shift was added to one of the tones.

 

Pluto_TX_RX_Image

 

Matlab code I used to configure the radio and generate the figure above is shown below

 

% Clear Environment

clear all;

clc;

 

% Set Properties

CarrierFreq = 1e9;      % 1 GHz

samples = 10e3;         % Number of total samples

fs = 1e6;                     % 1 Msps

f1 = 10e3;                   % 10 KHz tone

f2 = 20e3;                   % 20 KHz tone

 

% Define Timebase

t = 0:1/fs:1/fs*(samples-1);

 

% Build a complex waveform from two CW tones

cw1 = (0.1 * (cos(2*pi*f1*t) - j*sin(2*pi*f1*t)))';

cw2 = (0.1 * (cos(2*pi*f2*t) - j*sin(2*pi*f2*t)))';

tx_waveform = cw1 + cw2;

 

% Initialize SDR

sdrdev('Pluto');

 

% Initialize SDR Receiver Functionality

radio_rx = sdrrx('Pluto',...

    'RadioID',                             'usb:0',...

    'CenterFrequency',              CarrierFreq,...

    'BasebandSampleRate',      fs,...

    'GainSource',                       'Manual',...

    'Gain',                                   10,...

    'OutputDataType',                'double',...

    'SamplesPerFrame',            samples);

 

% Initialize SDR Transmit Functionality

radio_tx = sdrtx('Pluto',...

    'RadioID',              '               usb:0',...

    'CenterFrequency',               CarrierFreq,...

    'BasebandSampleRate',       fs,...

    'Gain',                                    -10);

 

% Trigger transmit of on device stored waveform

transmitRepeat(radio_tx,tx_waveform);

 

% Command Window feedback

disp('Radio Transmitting')

 

% ---- RUN LOOP ----

 

% Create Figure

figure

 

runtime = tic;

while toc(runtime) < 60     % run for 1 minute

 

    % Receive Radio Samples

    rx_waveform = radio_rx();

    

    % Plot snippet of TX and RX waveform

    subplot(1,2,1)

    plot(real(tx_waveform(2000:2500)))

    ylabel('real(IQ) value');

    xlabel('real(IQ) sample');

    title('Transmitted Waveform')

 

    subplot(1,2,2)

    plot(real(rx_waveform(2000:2500)))

    xlabel('real(IQ) sample');

    ylabel('real(IQ) value');

    title('Received Waveform')

    

    % Wait 1 second

    pause(1);

    

end

% ------------------

 

% Release Pluto resources and exit

release(radio_tx);

release(radio_rx);

disp('Done');

return;

 

Test Setup:

  • I've tested this in two configurations.  First with direct loop back with the supplied coax cable hooking up TX to RX directly on the Pluto.  Second, I've run the TX through a splitter with one output going directly back to the Pluto RX port, and the other going into a spectrum analyzer.  The spectrum analyzer shows two steady tones, but I can't tell if the phase shift is present or not.

 

Questions

  • If I'm literally copying the same sample used to generate the Matlab plot into the Pluto's memory to generate the waveform, how could a phase shift be introduced?  I can understand if the amplitude would be different, or if there was added noise, but I can't explain this.

 

Attachments

  • 'testScript.m" - A simple matlab script demonstrating this.  Hook up the Pluto to a computer and connect a coax between the TX and RX port.  The script will run for approximately 1 minute and the received waveform will update at about once a second and you'll (presumably) see the same thing in the picture above.

 

I appreciate any feedback or suggestions you may have.  Thank You,

Marcus

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