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Design of Audio Signal FM Modem based on digital PLL.
1. Scope of works include: design, prototype and test Modulator and Demodulator of frequency-modulated signal based on digital PLL principle.
2. Carrier signal is frequency adjustable sinus wave. Carrier frequency has to be variable (under s/w control) between 1.5MHz and 5.0MHz at both transmitter and selective input receiver. For prototype purposes testing will be required at three carrier frequencies: 2.0MHz, 3.0MHz and 4.0MHz.
3. Modulation signal is audio range (20Hz-18kHz) signal. For prototype purposes testing at following modulation sinus-wave frequencies will be required: 20 Hz, 50Hz, 100Hz,. 200Hz, 500Hz, 1kHz, 2kHz, 5kHz, 10KHz, 12kHz, 15kHz, 18kHz.
4. Total harmonic distortion level of demodulated analogue signal at the receiver output has to be less than 0.5% within 20Hz-10kHz range and less than 1.0% within 10Hz-18kHz range.
5. Frequency deviation of FM signal may be set between +/-75kHz and +/-150kHz.. This can be discussed further if required.
6. We prefer to implement both modulator and demodulator using Altera CPLD EPM240T100C5. Structure of the 74HC297 may be used as a basis for FM signal demodulator. Structure of the AD9850 or AD9851 may be used as a basis for FM signal modulator.
7. Please note this project requires also design of analogue front-ends for both transmitter and receiver. It will be required to design, prototype and test a full signal path from analogue audio signal input of modulator, through FM sinus-wave signal transmission line, up to audio signal output of demodulator.
8. The output signal level at transmitter has to be variable (under s/w control) and must not exceed 2.95Vac_peak. In real-life implementation the FM signal will share the same transmission line with other various signals (FM, FSK, audio signals, DC voltage). Frequency range of 100kHz-1.2MHz is assigned to all FSK signals. Frequency range of 1.5MHz-5.0MHz is assigned to all FM signals. There might be more than one FM signal transmitted simultaneously within this frequency range on the same transmission line. Therefore selective filtering has to be employed at receiver analogue front-end. Length of transmission line can be up to 1,000m over one twisted pair of shielded wires.