SDR | ShareTechnote

SDR(Software Defined Radio)

RTL2832/Matlab Example

Example 1 : Basic Capture and Visualize

One of the most tricky part of learning any new technology or new tool is to figure out the very first/the most basic connection method or command. When I was learning about electric circuit (a small embedded system) was to figure out how to turn on and off the small LED on the board. Everytime when I was trying to learn new computer language, the first thing I was trying to figure out is how to create a program to print out "Hello World".

Then what is the Hello World part of MatLab RTL ? It would be to get Matlab connected to the hardware and read a bunch of data (I/Q data) captured by the hardware. This is done by the simple two steps as below.

obj_hardwareHandle = comm.SDRRTLReceiver(...)

raw_data = step(obj_hardwareHandle);

The first line is to define which hardware it want to connect and what kind of configuration you want to apply to the hardware. This line itself does not do anything to the hardware, it is just definition of the configuration. The real work of communicating and controlling hardware is done by the second line. The step(obj_hardwareHandle) connect to the hardware and let it capture the signal as defined in the first line and read the captured data and store it to a variable called 'raw_data'.

This is all you need to know of Matlab RTL. Once you have the data stored in a variable (e.g, raw_data as in this tutorial), all the remaining procedure is up to you and knowledge about generic Matlab function and radio technology.

The outcome of the simple tutorial that I will show in this post is as show below.

Following is short descriptions on each of the plots shown above.

(A) : Frequency domain data from the captured data (the result of fft of raw_data)

(E) : Real values of raw data (I data) captured by the hardware. (real part of raw_data).

(F) : Real and Imaginary data of raw_data plotted in Cartesian coordinate (I/Q plane plot, Constellation)

(D) : Frequency domain representation of the digital filter.

(B) : The result of applying the digital filter (C,D) to data (A)

(G) : The result of applying the digital filter (C,D) to data (E)

(H) : The result of applying the digital filter (C,D) to data (F)

Following is the matlab source code that I used to get the result shown above. The green part is the critical part for Matlab to capture the data from the hardware and all the remaining part is just generic Matlab fuctions that you might already be familiar with.

clear all;

RTL_USB_ID = '0';

RTL_USB_CarrierFrequency = 91.5e6;

RTL_USB_Gain = 30;

RTL_USB_SamplingFrequency = 2.4e6;

RTL_USB_ppm = 0;

RTL_USB_FrameLength = 256*250;

RTL_USB_DataType = 'single';

Handle_RTL_USB = comm.SDRRTLReceiver(...

RTL_USB_ID,...

'CenterFrequency', RTL_USB_CarrierFrequency,...

'EnableTunerAGC', true,...

'SampleRate', RTL_USB_SamplingFrequency, ...

'SamplesPerFrame', RTL_USB_FrameLength,...

'OutputDataType', RTL_USB_DataType,...

'FrequencyCorrection', RTL_USB_ppm);

raw_data = step(Handle_RTL_USB);

a = 1;

b = firpm(100,[0,10e3,12e3,(240e3/2)]/(240e3/2), [1 1 0 0], [1 1], 40);

y = filter(b,a,raw_data);

block = 10000:14000;

subplot(2,4,1);

plot(10*log10(abs(fftshift(fft(raw_data)))));xlim([0 length(raw_data)]);

ylim([-50 50]);

subplot(2,4,2);

plot(10*log10(abs(fftshift(fft(y)))));xlim([0 length(y)]);

ylim([-50 50]);

subplot(2,4,3);

plot(b,'bo','MarkerFaceColor',[0 0 1],'MarkerSize',1);

xlim([1 length(b)]);

subplot(2,4,4);

plot(10*log10(abs(fftshift(fft(b,512)))));

xlim([0 512]); ylim([-50 0]);

subplot(2,4,5);

plot(block,real(raw_data(block)),'r-');

xlim([block(1) block(end)]);

ylim([-1 1]);

subplot(2,4,6);

plot(real(raw_data(block)),imag(raw_data(block)),'ro','MarkerFaceColor',[0 0 1],'MarkerSize',1);

axis([-1 1 -1 1]);

subplot(2,4,7);

plot(block,real(y(block)),'r-');

xlim([block(1) block(end)]);

ylim([-1 1]);

subplot(2,4,8);

plot(real(y(block)),imag(y(block)),'ro','MarkerFaceColor',[0 0 1],'MarkerSize',1);

axis([-1 1 -1 1]);

Example 2 : Playing WFM

My next trials was to write a Matlab code to do similar things as SDRsharp. I wanted to set the frequency as shown below (You would need to set other frequencies depending on where you are) and listen the Radio from Matlab. However, it was not as easy as I have thought. I spent a couple of days to figure out proper parameters values and routines that make this possible. The Reference [1] helped a lot, but I needed to put extra time and effort to find parameters values for this Radio and to create some additional filters for clearer sound. Even with all the efforts, the result was not as good as SDRsharp but at least I could listen and understand the broadcast with a little bit of noise.

Following is a snapshot of the real time audio spectrum and you would be able to listen the radio while this spectrum gets updated.

Following is my code to listen to the broadcase.

% Define Parameters for RTL Device.

% The tricky part for me was to find proper value for RTL_USB_FrameLength. If it is too short, I could

% hear a kind of knocking sound. If it is too long, you would see too slow update for audio spectrum.

% In addition, Maximum frame length is just a little bit ver 300,000bits and this number should be

% a integer multiples of Decimation Factors.

RTL_USB_ID = '0';

RTL_USB_CarrierFrequency = 91.5e6;

RTL_USB_Gain = 30;

RTL_USB_SamplingFrequency = 2.4e6;

RTL_USB_ppm = 0;

RTL_USB_FrameLength = 256*20*50;

RTL_USB_DataType = 'single';

AudioFrequency = 48e3;

PlayTime = 20;

RTL_USB_FrameTime = RTL_USB_FrameLength/RTL_USB_SamplingFrequency;

% Define a Handle to SDR RTL Device

% A parameter you need to set carefully would be 'EnableTunerAGC'. In my example,

% I set it as 'true' meaning that the Gain is set automatically by the RTL hardware.

% you can set a specific Gain value if you like, but in that case the signal level

% coming into RTL antenna should be good and should not fluctuating too much.

Handle_RTL_USB = comm.SDRRTLReceiver(...

RTL_USB_ID,...

'CenterFrequency', RTL_USB_CarrierFrequency,...

'EnableTunerAGC', true,...

'SampleRate', RTL_USB_SamplingFrequency, ...

'SamplesPerFrame', RTL_USB_FrameLength,...

'OutputDataType', RTL_USB_DataType,...

'FrequencyCorrection', RTL_USB_ppm);

% Define a Handle to an Audio Player

Handle_Audio = dsp.AudioPlayer(AudioFrequency);

% Define a Handle to Spectrum Analyzer

Handle_AudioSpectrum = dsp.SpectrumAnalyzer(...

'Name', 'Spectrum Analyzer - Audio',...

'Title', 'Audio Spectrum',...

'SpectrumType', 'Power density',...

'FrequencySpan', 'Full',...

'SampleRate', AudioFrequency);

% design a filter for the captured signal (raw data)

% With the current Antenna and location, the demodulated audio came in with a lot of noise

% I tried to filter out the noise at Audio stage only, but the result was not satisfactory.

% So I designed a filter to reduce the noise from the raw data.

% You may play with this filter parameters for your setup

fir_a = 1;

fir_b = firpm(100,[0,15e3,20e3,(240e3/2)]/(240e3/2), [1 1 0 0], [1 1], 40);

% design a filter for audio signal (demodulated data)

% This is the filter to reduce the noise at Audio stage.

% You may play with this filter parameters for your setup

[iir_b,iir_a] = butter(4,(2500)/(AudioFrequency/2));

run_time = 0;

while run_time < PlayTime

% Capture I/Q Data from Hardware and Store it to RTL_USB_RawData

RTL_USB_RawData = step(Handle_RTL_USB);

% apply the filters to the captured data

RTL_USB_Filtered = filter(fir_b,fir_a,RTL_USB_RawData);

% Calculate the Phase Differences

RTL_USB_Filtered_Delay_1 = circshift(RTL_USB_Filtered,1);

RTL_USB_Filtered_Conjugate = conj(RTL_USB_Filtered);

RTL_USB_Filtered_PhaseDifference = ...

angle(RTL_USB_Filtered_Conjugate .* RTL_USB_Filtered_Delay_1);

% decimate the data by the factor of 50.

% This convert 2.4Mhz sample to 48 Khz (2.4 Mhz/50) sample

RTL_USB_Filtered_PhaseDifference_Decimated = ...

decimate(double(RTL_USB_Filtered_PhaseDifference),50,8);

% Apply the iir filter to the audio data

Audio_Filtered = ...

filter(iir_b,iir_a,RTL_USB_Filtered_PhaseDifference_Decimated);

% Plot the audio data on Spectrum

step(Handle_AudioSpectrum, Audio_Filtered);

% Play the Audio data

step(Handle_Audio,Audio_Filtered);

run_time = run_time + RTL_USB_FrameTime;

end

Example 3 : Capturing RKE Signal

One of the difficulties I had with playing with RTL device was it is Reciever only hardware.. it does not have any transmitter functionality. The easiest function you can play with would be Radio Broadcast, but the motivation would not last long. Next stage would be to try with signals other than Radio Broadcast like GPS and others, but in those case you may need speciall antenna being installed out side of home or office. I was looking for some other signal that I can play with without any extra investment. Then.. one day my car key caught my attention and I decided to try using my Car key as a signal generator. The result was very satisfactory and this example is about playing with my Car Key. Since I am in North America, the frequency of the RKE is expected to be 315 Mhz, but I wanted to check the exact frequency first. Each of the Key may have a little bit of variations of all. So I confirmed on the frequency with SDRSharp as described in SDRSharp tutorial : Capturing RKE(Remote Key Entry) Signal.

Next trial was to use the Matlab code from Example 1 without any modification just to capture the signal and display it. The purpose of this part is just to capture the I/Q Data. So the meaningful part would be (A),(E),(F) in this example.