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74VHC943N Fiches technique(PDF) 4 Page - National Semiconductor (TI) |
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74VHC943N Fiches technique(HTML) 4 Page - National Semiconductor (TI) |
4 / 12 page Functional Description INTRODUCTION A modem is a device for transmitting and receiving serial data over a narrow bandwidth communication channel The 74VHC943 uses frequency shift keying (FSK) of audio fre- quency tone The tone may be transmitted over the switched telephone network and other voice grade chan- nels The 74VHC943 is also capable of demodulating FSK signals By suitable tone allocation and considerable signal processing the 74VHC943 is capable of transmitting and receiving data simultaneously The tone allocation used by the 74VHC943 and other Bell 103 compatible modems is shown in Table I The terms ‘‘originate’’ and ‘‘answer’’ which define the frequency allo- cation come from use with telephones The modem on the end of the line which initiates the call is called the originate modem The other modem is the answer modem TABLE I Bell 103 Tone Allocation Originate Modem Answer Modem Data Transmit Receive Transmit Receive Space 1070 Hz 2025 Hz 2025 Hz 1070 Hz Mark 1270 Hz 2225 Hz 2225 Hz 1270 Hz THE LINE INTERFACE The line interface section performs two to four wire conver- sion and provides impedance matching between the mo- dem and the phone line THE LINE DRIVER The line driver is a power amplifier for driving the line If the modem is operating as an originate modem the second har- monics of the transmitted tones fall close to the frequencies of the received tones and degrade the received signal to noise ratio (SNR) The line driver must thus produce low second harmonic distortion THE HYBRID The voltage on the telephone line is the sum of the transmit- ted and received signals The hybrid subtracts the transmit- ted voltage from the voltage on the telephone line If the telephone line was matched to the hybrid impedance the output of the hybrid would be only the received signal This rarely happens because telephone line characteristic im- pedances vary considerably The hybrid output is thus a mixture of transmitted and received signals THE DEMODULATOR SECTION The Receive Filter The demodulator recovers the data from the received sig- nals The signal from the hybrid is a mixture of transmitted signal received signals and noise The first stage of the receive filter is an anti-alias filter which attenuates high fre- quency noise before sampling occurs The signal then goes to the second stage of the receive filter where the transmit- ted tones and other noise are filtered from the received sig- nal This is a switch capacitor nine pole filter providing at least 60 dB of transmitted tone rejection This also provides high attenuation at 60 Hz a common noise component The Discriminator The first stage of the discriminator is a hard limiter The hard limiter removes from the received signal any amplitude modulation which may bias the demodulator toward a mark or a space It compares the output of the receive filter to the voltage on the 01 mF capacitor on the FTLC pin The hard limiter output connects to two parallel bandpass filters in the discriminator One filter is tuned to the mark frequency and the other to the space frequency The out- puts of these filters are rectified filtered and compared If the output of the mark path exceeds the output of the space path the RXD output goes high The opposite case sends RXD low The demodulator is implemented using precision switched capacitor techniques The highly critical comparators in the limiter and discriminator are auto-zeroed for low offset Carrier Detector The output of the discriminator is meaningful only if there is sufficient carrier being received This is established in the carrier detection circuit which measures the signal on the line If this exceeds a certain level for a preset period (ad- justable by the CDT pin) the CD output goes low indicating that carrier is present Then the carrier detect threshold is lowered by 3 dB This provides hysteresis ensuring the CD output remains stable If carrier is lost CD goes high after the preset delay and the threshold is increased by 3 dB MODULATOR SECTION The modulator consists of a frequency synthesizer and a sine wave synthesizer The frequency synthesizer produces one of four tones depending on the OA and TXD pins The frequencies are synthesized to high precision using a crystal oscillator and variable dual modulus counter The counters used respond quickly to data changes introduc- ing negligible bit jitter while maintaining phase coherence The sine wave synthesizer uses switched capacitors to ‘‘look up’’ the voltages of the sine wave This sampled sig- nal is then further processed by switched capacitor and continuous filters to ensure the high spectral purity required by FCC regulations 4 |
Numéro de pièce similaire - 74VHC943N |
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Description similaire - 74VHC943N |
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