and RTL-SDR Wireless Systems

Transcription

1 Laboratory 4 FM Receiver using MATLAB and RTL-SDR Wireless Systems TLEN 5830 Wireless Systems This Lab introduces the working of FM Receiver using MATLAB and Software Defined Radio This exercise encompasses 3 important sections: 1. FM-Receiver Introduction 2. Software Defined Radio Introduction 3. MATLAB interfacing SDR FM- Receiver Introduction: FM receiver (also called radio receiver) is an electronic device that receives radio waves and converts the information carried by them to a usable form. In U.S, the FM frequency band is between 88 MHz 108MHz. Receiver Block Diagram TLEN 5830 Wireless Systems Lab-04 FM Receiver using MATLAB and SDR Page 1 of 5

2 1. RF amplifier amplifies the received signal 2. Mixer/Oscillator used to generate the intermediate frequency (IF) 3. Filter selects the desired frequency 4. IF amplifier amplifies the generated IF signal 5. Limiter suppresses the noise in the signal 6. Frequency demodulator used to demodulate the received signal 7. AF amplifier amplifies the audio frequency to make sure it has the power to operate the speaker Software Defined Radio Introduction: It is a radio communication system in which the components that have been typically implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of a software on a personal computer or an embedded system. This exercise uses a RTL-SDR and it is one of the cheapest SDRs available. Few other examples are Airspy, NESDR-Smart, USRP B210 and N210. Details about RTL-SDR: 1. It is Rx-only SDR. 2. Operational Frequency: 50 Hz to 1.7 GHz 3. Sampling Rate: 2.4 Ms/s 4. Bandwidth of each channel: 3.2MHz 5. Compatible with LABVIEW, MATLAB and GNU Radio. 6. Support all digital TV standards: DVB-T, ATSC, DTMB and ISDB-T. 7. The RTL-SDR dongle features the Realtek RTL2832U chip, which provides I-Q samples through the USB interface. TLEN 5830 Wireless Systems Lab-04 FM Receiver using MATLAB and SDR Page 2 of 5

3 MATLAB Interfacing RTL SDR For this exercise, you need a matlab library rtlsdr_book_library, to make use of the de-emphasis filter. This will be provided along with the manual. To integrate the library to Matlab, below steps need to be followed. 1. Install the Communications System Toolbox support Package for RTL-SDR Radio. 2. Google ZADIG RTL SDR and click on the 1st link to install rtf sdr drivers. 1) select 'Set path' under HOME tab 2) point to the downloaded SDR-Book library 3) Refresh the project. Important Components to Account for: 1. The RTL-SDR Receiver module. Interfaces the SDR with MATLAB 2. FIR Decimator The data received from the RTL-SDR is of very high frequency, which means that there are lots of samples or cycles per second. MATLAB might not be able to read the data from the SDR at such high speeds. So, the data rate needs to be scaled down. Please note that along with the reduction in sample speed, a FIR decimator also acts as a lowpass filter and helps in reducing the noise in the spectrum. This needs to be added just before the audio output, to hear the audio without the noise. TLEN 5830 Wireless Systems Lab-04 FM Receiver using MATLAB and SDR Page 3 of 5

4 3. De-Emphasis Filter Generally, the audio waves are amplified before being transmitted. This is done to ensure that the signal is picked up with sufficient strength by the receivers. At the receiver end, this signal is scaled down in amplitude and processed. This scaling down of amplification is called de-emphasis and this is done by the de-emphasis filter. 4. Audio Output This interfaces MATLAB to the system speakers. Unless this is added, no sound will be generated from the system. Procedural Flow: TLEN 5830 Wireless Systems Lab-04 FM Receiver using MATLAB and SDR Page 4 of 5

5 Brief Explanation: The first line indicates the signal received by the RTL-SDR. The signals are generally considered as complex variables as it is easier to work with complex numbers as compared to individual cosines. The product of a complex conjugate and its delay gives the power spectral density and the angular part of the PSD gives the output baseband signal. As opposed to the general convention of extracting the amplitude, we extract the angular part of the PSD as the information is frequency modulated. Output Verification: Tune into KBCO and check the broadcast. TLEN 5830 Wireless Systems Lab-04 FM Receiver using MATLAB and SDR Page 5 of 5