Receiving while transmitting: erratic transmission

[fdavies]

I now have much better results, based on the advise of the inestimable bpadalino. He said that I should transmit and receive at different frequencies if transmitting and receiving at the same time to avoid the tx and rx PLLs interfering with each other. I am transmitting and receiving 6 MHz apart, with a sample rate of 24 MHz.

I got the following results, which are quite satisfactory. Now to buy a directional coupler so that I can measure S11, and make a cavity with a decent Q factor.

spec_1.png

Here is the Octave script that I used:

Code: Select all

%  by fdavies
% based on a file from https://github.com/Nuand/bladeRF/wiki/bladeRF-CLI-Tips-and-Tricks
%  modified
%

GUARD_BAND= 0.5e6;  % separate the frequency sweep zone from the DC spikes
SAMPLE_RATE = 24e6;
NUM_SAMPLES = 131072;  % 
NUM_SECONDS = NUM_SAMPLES/SAMPLE_RATE;

SCAN_WIDTH = (SAMPLE_RATE/4-GUARD_BAND*2);

SCAN_START = 2600e6;
SCAN_STOP = 3600e6;

freq_list = SCAN_START:SCAN_WIDTH:SCAN_STOP;

START_FREQ_RAD = (GUARD_BAND/2) * 2 * pi;  % it will be a complex frequency
STOP_FREQ_RAD = (SAMPLE_RATE/4-(GUARD_BAND/2)) * 2 * pi;

RX_TX_DELTA = SAMPLE_RATE/4;  % to prevent rx PLA and tx PLA mutual interference (pos for rx>tx)

DELTA= (1/2)*(STOP_FREQ_RAD - START_FREQ_RAD)/NUM_SECONDS;
t = [ 0 : (1/SAMPLE_RATE) : ((NUM_SECONDS) - 1/SAMPLE_RATE) ];
tx_signal = (0.8+(0.075/NUM_SECONDS)*t) .* exp(1j .* (START_FREQ_RAD+DELTA.*t) .* t);

save_sc16q11('/media/ramdisk/tx_waveform.sc16q11', tx_signal);

%  set up the bladeRF with a bunch of system command line calls
  %system(["cal lms"]);
  system(["bladeRF-cli -e 'set samplerate rx ",num2str(SAMPLE_RATE),"'"]);
  system(["bladeRF-cli -e 'set bandwidth rx ",num2str(SAMPLE_RATE),"'"]);
  system(["bladeRF-cli -e 'set bandwidth tx ",num2str(SAMPLE_RATE),"'"]);
  system(["bladeRF-cli -e 'set samplerate tx ",num2str(SAMPLE_RATE),"'"]);
  system(["bladeRF-cli -e 'set lnagain 0'"]); % 0 3 6
  system(["bladeRF-cli -e 'set rxvga1 5'"]); % [5, 30]
  system(["bladeRF-cli -e 'set rxvga2 0'"]); % [0, 30]
  system(["bladeRF-cli -e 'set txvga1 -4'"]); %  [-35, -4]
  system(["bladeRF-cli -e 'set txvga2 25'"]); % [0, 25]
  system(["bladeRF-cli -e 'tx config delay=0'"]); %

result=[]; % for when I do many in a row
freq_result=[];
rx_signal=[];

% so, it works by transmitting the sweep signal 3 times while activating 
% the receive.  This is necessary because I do not know how to synchonize
% the transmit and receive well

% so, transmit a frequency sweep from tx_freq+START_FREQ_RAD to tx_freq+STOP_FREQ_RAD
% note that this includes guard bands so it is wider than SCAN_WIDTH
% the part of the frequency sweep that will actually end up in the result is
%    tx_freq+GUARD_BAND to tx_freq+GUARD_BAND+SCAN_WIDTH
% receive it with a rx base frequency that is offset from the tx base frequency

%%%   now for the loop
for fr=freq_list
  % set the transmit and receive frequencies
  tx_freq = fr - GUARD_BAND;
  rx_freq = tx_freq+RX_TX_DELTA;
  system(["bladeRF-cli -e 'set frequency rx ",num2str(rx_freq),"'"]);
  system(["bladeRF-cli -e 'set frequency tx ",num2str(tx_freq),"'"]);
  system("bladeRF-cli -s script_H_1.txt"); % see below for contents of script_D_1.txt
  f = fopen("/media/ramdisk/rx_sample", "r", "ieee-le");
  samples = fread(f, Inf, "int16");
  fclose(f);
  samples_i = samples(1:2:end, :)';
  samples_q = samples(2:2:end, :)';
  rx_signal = (samples_i + j * samples_q);
  % the spectrum will go from -1/SAMPLE_RATE to 1/SAMPLE_RATE in frequency
  spectrum=20*log10(abs(fftshift(fft(rx_signal)))/NUM_SAMPLES);
  trim_start = floor(NUM_SAMPLES/4+NUM_SAMPLES*(GUARD_BAND/SAMPLE_RATE));
  trim_stop = floor(NUM_SAMPLES/2-NUM_SAMPLES*(GUARD_BAND/SAMPLE_RATE));
  trim_width = trim_stop-trim_start+1;
  trimmed_spectrum=spectrum(trim_start:trim_stop);
  result=[result trimmed_spectrum]; % add a chunk to the result
  freq_result = [freq_result fr:SCAN_WIDTH/trim_width:fr+SCAN_WIDTH-(SCAN_WIDTH/trim_width)];
endfor

clf;
rx1=real(rx_signal);
h4=plot(freq_result,result);  %  here is where we see the result
axis([freq_result(1) freq_result(end) -50 30]);
h6=title('Spectrum assembled from pieces');grid on
h7=xlabel('Frequency');
h8=ylabel('dB');
grid on;
%set(gca(),'xtick',min(freq_list):1e6:max);
%set(gca(),'ytick',-30:5:30);
%saveas(h1,"t.png",'png');

% ***************************************
%  this is the script file used by the above
%
% tx config delay=0
% tx config file=/media/ramdisk/tx_waveform.sc16q11 format=bin
% rx config file=/media/ramdisk/rx_sample n=131072
% tx config repeat=3
% tx start 
% rx start
% tx wait
% rx wait
%

[bpadalino]

Awesome! Glad I could have helped. Sorry for the inconvenience it caused, but this is great progress for sure.

Have you thought about windowing and overlapping your results? You may be able to get a better noise floor and possibly see other artifacts?

If you ever make a blog post about this, let us know. This is very cool stuff.