The CMX469A is a single-chip CMOS LSI circuit which operates as a full-duplex 1200, 2400 or 4800 baud FFSK/MSK modem. The mark and space frequencies are 1200/1800, 1200/2400 and 2400/4800 Hz respectively. Tone frequencies are phase continuous; transitions occur at the zero crossing point. A common Xtal oscillator with a choice of two clock frequencies (1.008MHz or 4.032MHz) provides baud-rate, transmit frequencies, and Rx and Tx synchronization. The transmitter and receiver operate entirely independently, including the individual section powersave functions. The CMX469A includes on-chip circuitry for Carrier Detect and Rx Clock recovery, both of which are made available as output pins. Rx, Tx and Carrier Detect paths contain bandpass filters to optimise signal conditions in each section of the modem. The CMX469A demonstrates good sensitivity and bit-error-rate under adverse signal conditions. The Carrier Detect time constant is set by an external capacitor, so that the product's performance can be optimised in high noise environments. This low-power device operates from a single supply between 2.7V and 5.5V, requires few external components and is available in a wide variety of plastic packages.