74VHC943WM datasheet(4/12 Pages) NSC

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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

1070 Hz

Mark

1270 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

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