73K222AU Datasheet, PDF (3/40 Pages) TDK Electronics – Single-Chip Modem Modem with UART

PSK MODULATOR / DEMODULATOR
The 73K222AU modulates a serial bit stream into
dibit pairs that are represented by four possible
phase shifts as prescribed by the Bell 212A or V.22
standard. The baseband signal is then filtered to
reduce intersymbol interference on the band limited
2-wire PSTN line. Transmission occurs using 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 dibits and converted back to
a serial bit stream. The demodulator also recovers
the clock which was encoded into the analog signal
during modulation. The demodulator decodes either
a 1200 Hz carrier (originate carrier) or a 2400 Hz
carrier (answer carrier). The 73K222AU uses a
phase-locked-loop coherent demodulation technique
that offers inherently better performance than typical
DPSK demodulators used by other manufacturers.
FSK MODULATOR/DEMODULATOR
The FSK modulator frequency modulates the analog
output signal using two discrete frequencies to
represent the binary data. In Bell 103, the standard
frequencies of 1270 Hz and 1070 Hz (originate mark
and space) and 2225Hz and 2025 Hz (answer mark
and space) are used. V.21 mode uses 980 Hz and
1180 Hz (originate, mark and space) or 1650 Hz and
1850 Hz (answer, mark and space). Demodulation
involves detecting the received frequencies and
decoding them into the appropriate binary value.
PASSBAND FILTERS AND EQUALIZERS
A high and low band filter is included to shape the
amplitude and phase response of the transmit signal
and provide compromise delay equalization and
rejection of out-of-band signals in the receive
channel. Amplitude and phase equalization is
necessary to compensate for distortion of the
transmission line and to reduce intersymbol
interference in the band limited receive signal. The
transmit signal filtering approximates a 75% square
root of raised Cosine frequency response
characteristic.
AGC
The automatic gain control maintains a signal level
at the input to the demodulators which is constant to
within 1 dB. It corrects quickly for increases in signal
which would cause clipping, and provides a total
dynamic range of >45 dB.
73K222AU
Single-Chip Modem
with UART
SPECIAL DETECT CIRCUITRY
The special detect circuitry monitors the received
analog signal to determine status or presence of
carrier, call-progress tones, answer tone, and weak
received signal (long loop condition). An
unscrambled mark signal is also detected when the
received data out of the DPSK demodulator before
the descrambler has been high for 165.5 mS ±13.5
mS. The appropriate status bit is set when one of
these conditions changes and an interrupt is
generated for all monitored conditions except long
loop. The interrupts are disabled (masked) when the
enable interrupt bit is set to a 0.
DTMF GENERATOR
The DTMF generator will output one of 16 standard
dual-tones determined by a 4-bit binary value and
TX DTMF mode bit previously loaded into the tone
register. Tone generation is initiated when the DTMF
mode is selected and the transmit enable (CR0 bit
D1) is changed from a 0 to a 1.
TEST FEATURES
Test features such as analog loopback (ALB),
remote digital loopback, local digital loopback, and
internal pattern generators are also included.
LINE INTERFACE
The line interface of the 73K222AU consists of a
two-to-four wire hybrid, and an off-hook relay driver.
The two-to-four wire converter has a differential
transmit output and requires only a line transformer
and an external impedance matching resistor. Four-
wire operation is also available by simply using
either of the transmit output signals.
The relay driver output of the 73K222AU is an open
drain signal capable of sinking 20 mA, which can
control a line closure relay used to take the line off
hook and to perform pulse dialing.
AUDIO MONITOR
An audio monitor output is provided which has a
software programmable volume control. Its output is
the received signal. The audio monitor output can
directly drive a high impedance load, but an external
power amplifier is necessary to drive a low-impedance
speaker.
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