Digital FSK receiver capable of operating in high impulse noise environments

The Advanced Group Rapid Transit (AGRT) FSK communication link is required to operate with a bit error rate of 10^-5errors/bit. Inductive coupling between a wire loop embedded in the guideway and a rectangular coil antenna mounted beneath the vehicle provide the medium of communication. Susceptibility of such loops and antennas to noise has been a problem in the design of a reliable communication link. Past data taken on the Morgantown People Mover System and the Seattle Metro Trolley installation show that sinusoidal interference and Gaussian noise were negligible, but low PRF impulse noise from 1) the chopper controlled propulsion unit on board the vehicle and 2) the contact bounce of the power collectors is very severe. Cases were even observed where the signal to noise ratio within our FSK passband (100 - 150 KHz) fell as low as 0 dB. Due to the importance of meeting the bit error rate requirement even in such an environment, much time was spent in the development of a suitable receiver. The digital demodulation technique which detects frequency by digitally measuring the period (1/f) of the received signal was one of the concepts considered. The development of the Digital FSK Receiver using this technique is the topic of this paper. Initial development of this receiver was performed with a TTL logic implementation of the demodulator. The TTL logic implementation was then tested in severe impulse noise environments in order to evaluate the feasibility of this concept. The successful performance of this implementation prompted a Z8 microcomputer based design. The implementation of both receivers will be discussed along with the tests and test results obtained.