Analog Encoded Chirp Transmission System Using Surface Acoustic Wave Filters

Abstract

Linear frequency modulation is implemented in encoding analog signals. Using surface acoustic wave (SAW) dispersive delay lines, the transmission system described operates with overlapping chirps that are position-modulated according to the sampled data. Demodulation in the receiver requires pulse compression in a matched fdter. The principle of analog encoding and decoding is explained. Ex- perimental results are provided on the performance of a modem de- signed for handling baseband signals with a spectral range from 300 Hz to 400 kHz. SAW linear FM filters, operating at a center frequency of 60 MHz with a time-bandwidth product of 180, are employed. The system shows good amplitude and phase characteristics and a dynamic range greater than 40 dB. tude of the sampled information signal. Investigating the basic types of simple analog pulse modulation (6) , pulse-amplitude modulation (PAM), is normally excluded in spread spectrum communications. Because of its easy demodulation, it tends to be at cross purposes with the intent of these systems. Vari- ation of the LFM, pulse-duration modulation (PDM), thus varying the bandwidth at the same time, would cause ampli- tude and duration modulation of the compressed pulse in the receiver and a compressive gain depending on the modulating signal. A possible but more complicated approach is the varia- tion of the center frequency of each chirp according to the instantaneous sampled signal amplitude. Here the receiver re- quires a high time bandwidth product in its compression filter, yielding pulses whose time separation corresponds to signal amplitude value. These considerations and the narrow correla- tion peak of the LFM waveform suggest the use of pulse- position modulation (PPM) with its significant simplicity. In the following section of the paper, encoding and decoding in a system transmitting overlapping PPM-chirps is explained. Section 111 provides experimental results on the performance of the SAW filters and the overall system.