Design and Performance of a Digital Multifrequency Receiver

An experimental multifrequency receiver for recognition of digitally encoded multifrequency signaling was designed, constructed and tested. The receiver is based on a quadrature detection technique that consists of digital demodulation followed by second-order, lowpass digital filtering. The post filtering processing produces an estimate of the amplitude of each of the six multifrequency tones and provides suitable information for thresholding and timing measurements. The receiver performs correctly even when subjected to severe environmental conditions including an analog signal range of 23 dB, 10 ms signal interruptions ('hits') and 20 ms signal spacings. The receiver's operation demonstrates the robustness of the digital signal processing techniques employed. The design of the receiver exploits the use of subsampling techniques to increase the efficiency of the hardware through greater multiplexing. When using subsampling, 128 multifrequency receivers with 16-bit words are realized with 6.5 dual-in-line packages per receiver; commercial TTL logic circuits, a 4-bit serial-parallel pipeline multiplier circuit, serial data and a 16.384 MHz clock are assumed.