A 1200 Bit/s QPSK full duplex modem

An NMOS integrated circuit is described that provides all the modulation, demodulation-filtering, and data-buffering functions for a 1200-b/s full-duplex voice-band modem. This modem is compatible with the Bell 212A modem, the Racal-Vadic 2400 modem, and the CCITT V.22 recommendation. All of the modems, options, and alternatives supported by these modems are also supported by the integrated circuit. The integrated circuit uses switched-capacitor signal processing circuit technology. This technique uses charge storage on-ratioed capacitors and allows the design of filters, amplifiers, automatic gain control elements, voltage-controlled oscillators, Hilbert filters, phase-locked loops, and a fully adaptive equalizer to be implemented on the same integrated circuit as the digital functions, which include the clock generation circuits and the transmit and receive buffers. Performance measurements show that the IC meets the bit error rate standard at 12-dB signal-to-noise ratio.