November 4th, 2009 by Daylen

The basic principle behind a modem transmitter is to convert information received from the DTE in the form of binary digits to digitally modulated analog signals. The reverse process is accomplished in the modem receiver.

Serial Interface Circuit and Modular Circuit:

Interface the modem transmitter and receiver to the serial interface. The transmit section accepts digital information from the serial interface, convert it to the appropriate voltage levels, and then the direct the information to the modulator. The receive section receives digital information from the demodulator circuit, converts it to the appropriate voltage levels, and then directs the information to the serial interface. Receive digital information from the serial interface circuit. The digital information modulates an analog carrier, producing a digitally modulated analog signal. Inessence, the modulator converts digital changes in the information to analog changes in the carrier.

Bandpass Filter and Equalizer Circuit:

There are bandpass filter and equalizer circuits in both the transmitter and receiver sections of the modem. The transmit bandpass filter limits the bandwidth of the digitally modulated analog signals to a bandwidth appropriate for transmission over a standard telephone circuit. The receive bandpass filter limits the bandwidth of the signals allowed to reach the demodulator circuit, thus reducing noise and improving system performance. Equalizer circuits compensate for bandwidth and gain imperfections typically experienced on voice band telephone lines.

Telco Interface Circuit & Carrier and Clock Generation Circuit:

The primary functions of the telco interface circuit are to match the impedance of the modem to the impedance of the telephone line and regulate the amplitude of the transmit signal. The interface also provides electrical isolation and protection and serves as the separation point between subscriber equipment and telephone company-provided equipment. The telco line can be two-ire or four wires, and the modem can operate half or full duplex. When the telephone line is two wires, the telco interface circuit would have to perform four-wire-to-two-wire and two-wire-to-four-wire conversions. The carrier generation circuit produces the analog carriers necessary for the modulation and demodulation processes. The clock generation circuit generates the appropriate clock and timing signals required for performing transmit and receive function in an orderly and timely fashion.