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73K224L-IGT 데이터시트(PDF) 2 Page - TDK Electronics |
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73K224L-IGT 데이터시트(HTML) 2 Page - TDK Electronics |
2 / 32 page 73K224L V.22bis/V.22/V.21/Bell 212A/Bell 103 Single-Chip Modem 2 DESCRIPTION (continued) 2400 bit/s data communications over the 2-wire switched telephone network is desired. Its high functionality, low power consumption, and efficient packaging simplify design requirements and increase system reliability. The 73K224L is designed to be a complete V.22bis compatible modem on a chip. The complete modem requires only the addition of the phone line interface, a microcontroller for modem control and status monitoring, and RS-232 level converters for a typical system. Many functions were included to simplify implementation of typical modem designs. In addition to the basic 2400 bit/s QAM, 600/1200 bit/s DPSK and 300 bit/s FSK modulator/demodulator sections, the device also includes SYNCH/ASYNCH converters, scrambler/descrambler, call progress tone detect, DTMF tone generator capabilities and handshake pattern detectors. V.22bis, V.22, V.21 and Bell 212A/103 modes are supported (synchronous and asynchronous) and test modes are provided for diagnostics. Most functions are selectable as options and logical defaults are provided. OPERATION QAM MODULATOR/DEMODULATOR The 73K224L encodes incoming data into quad-bits represented by 16 possible signal points with specific phase and amplitude levels. The baseband signal is then filtered to reduce intersymbol interference on the bandlimited telephone network. The modulator transmits this encoded data using either a 1200 Hz (originate mode) or 2400 Hz (answer mode) carrier. The demodulator, although more complex, essentially reverses this procedure while also recovering the data clock from the incoming signal. Adaptive equalization corrects for varying line conditions by automatically changing filter parameters to compensate for line characteristics. DPSK MODULATOR/DEMODULATOR The 73K224L modulates a serial bit stream into di-bit pairs that are represented by four possible phase shifts as prescribed by the Bell 212A/V.22 standards. The base-band signal is then filtered to reduce intersymbol interference on the bandlimited 2-wire PSTN line. Transmission occurs on either a 1200 Hz (originate mode) or 2400 Hz carrier (answer mode). Demodulation is the reverse of the modulation process, with the incoming analog signal eventually decoded into di-bits and converted back to a serial bit stream. The demodulator also recovers the clock which was encoded into the analog signal during modulation. Demodulation occurs using either a 1200 Hz carrier (answer mode or ALB originate mode) or a 2400 Hz carrier (originate mode or ALB answer mode). Adaptive equalization is also used in DPSK modes for optimum operation with varying line conditions. FSK MODULATOR/DEMODULATOR The FSK modulator produces a frequency modulated analog output signal using two discrete frequencies to represent the binary data. The Bell 103 standard frequencies of 1270 and 1070 Hz (originate mark and space) and 2225 and 2025 Hz (answer mark and space) are used when this mode is selected. V.21 mode uses 980 and 1180 Hz (originate, mark and space) or 1650 and 1850 Hz (answer, mark and space). Demodulation involves detecting the received frequencies and decoding them into the appropriate binary value. The rate converter and scrambler/descrambler are automatically bypassed in the FSK modes. PASSBAND FILTERS AND EQUALIZERS High and low band filters are included to shape the amplitude and phase response of the transmit and receive signals and provide compromise delay equalization and rejection of out-of-band signals. Amplitude and phase equalization are necessary to compensate for distortion of the transmission line and to reduce intersymbol interference in the bandlimited receive signal. The transmit signal filtering corresponds to a 75% square root of raised Cosine frequency response characteristic. ASYNCHRONOUS MODE The Asynchronous mode is used for communication with asynchronous terminals which may communicate at 600,1200, or 2400 bit/s +1%, -2.5% even though the modem’s output is limited to the nominal bit rate ±.01% in DPSK and QAM modes. When transmitting in this mode the serial data on the TXD input is passed through a rate converter which inserts or deletes stop bits in the serial bit stream in order to output a signal that is the nominal bit rate ±.01%. This signal is then routed to a data scrambler and into the analog modulator where quad-bit/di-bit encoding results in the output signal. Both the rate converter and scrambler can be bypassed for handshaking, and |
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