Software defined radio based FM demodulator design

The present thesis includes the design and implementation of the FM demodulator stuructures which provide obtaining information signal from the FM signal which has widely used in communication systems. The designed demodulator structure is based on software defined radio. Digital FM demodulator structures are examined at the design stage and the demodulator design is completed considering the previous studies. The simulation of the system is performed with the help of MATLAB Simulink and Modelsim simulation tool. After this, the design is successfully implemented with using VHDL. Upon the developments in recent years, the spread of low-cost digital signal processing ICs has gain significant importance for the design of software defined radio systems. The need for low-cost products has increased with the rapid development in telecommunication technology, especially for the cellular communication. Nowadays, traditional radio communication systems have given its way to the software defined radio systems. Software defined radio is formed by performing the hardware functions in a software base in the radio systems. Some basic communication units such as modulation, demodulation, IQ discrimination of complex signals, encoding, decoding and filtering are traditionally implemented as hardware based. However it is difficult to perform these with analog design, these units could also be implemented as software based. Thus, the communication system functions can be changed by updating existing software. In this context, the use of fixed electronic circuit are decreased in the software defined radio system and flexible structure can be obtained. Frequency modulation has widespread usage in communication systems. With the development of software defined radio technology and its advantages, frequncy modulation, which is an analogue type of modulation, can be designed as a software based in the new generation communication systems. Frequency modulated RF signals are digitally sampled and demodulated in real time using signal processing techniques. Baseband delay demodulator, phase-adapter demodulator, phase-locked loop and mixed demodulator structures are used in the digital FM demodulation. The simulations of these demodulator structures are made with the help of MATLAB Simulink. Considering signal quality, processing power and the area covered, mixed demodulator structure is preferred. Mixed demodulator structure consists of two main blocks – a quadrature mixer and a demodulator. At the quadrature mixer block, FM signal sampled by the ADC is multiplied by the sine and cosine signals created by NCO and then passed through a low pass filter. The obtained real and imaginary signals at the output of this block, constitute the input of demodulator block. Demodulator block consists of unit delay, multiplication, addition and CORDIC blocks. The information signal is obtained from the output of demodulator block after formatting with the aid of low pass FIR filter. After confirming the design of the FM demodulator architecture with Modelsim simulation tool and SignalTap debugging tool, it is implemented in Altera DE0 Nano development board. With the aid of Rohde & Schwarz vector signal generator, FM signal having 1MHz carrier signal is produced and the design is tested and verified with different frequency deviations. The implemented system uses 1893 out of 22320 logical element in Altera DE0 Nano develeopment board EP4CE22F17C6 device. Power consumption of the system is 113.56 mW and delay time is 5.84 ns.