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74VHC943N Fiches technique(PDF) 4 Page - National Semiconductor (TI)

[Old version datasheet] Texas Instruments acquired National semiconductor.
No de pièce 74VHC943N
Description  300 Baud Modem (5V Supply)
Download  12 Pages
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Fabricant  NSC [National Semiconductor (TI)]
Site Internet  http://www.national.com
Logo NSC - National Semiconductor (TI)

74VHC943N Fiches technique(HTML) 4 Page - National Semiconductor (TI)

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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
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


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