A Simple Adaptive FSK Modem

Abstract

<p> A simple and relatively inexpensive equalizer has been developed in conjuction with a FSK modulator and demodulator (modem) for the transmission of digital data over time dispersive channels. </p> <p> The transmitter structure consists of a digital modulator followed by two filters, the first of which is a binary transversal filter (BTF), the second a conventional two-stage R-C low-pass filter. </p> <p> In the receiver structure a phaselock loop (PLL) is employed as a discriminator. Synchronization is achieved with a digital phaselock loop (DPLL). Considerable attention is placed on the error rate optimization of the receiving structure. </p> <p> The equalizer uses binary transversal filter comprising a tapped delay line, realized by a long string of metal-oxide semiconductor (MOS) flipflops, with a variable gain circuit associated with each tap in the delay line. The operation of the equalizer is adaptive in that the tap gains are adjusted rom the received data, consequently the tap gains are continually optimized. The algorithm employed for setting the tap gains is simple and can be quite easily implemented using a combination of analog and digital techniques. </p> <p> Experimental results are presented which illustrate the important features of the system implemented under a variety of operating conditions. </p>