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MC145442BDW bảng dữ liệu(PDF) 8 Page - LANSDALE Semiconductor Inc.

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Giải thích chi tiết về linh kiện  Single-Chip 300-Baud Modem
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LANSDALE Semiconductor, Inc.
ML145442, ML145443
CARRIER DETECT TIMING ADJUSTMENT
The value of a capacitor, CCDT at CDT (pin 4) determines
how long a received modem signal must be present above the
minimum threshold level before CD (pin 3) goes low. The
CCDT capacitor also determines how long the CD pin stays
low after the received modem signal goes below the mini mum
threshold. The CD pin is used to distinguish a strong modem
signal from random noise. The following equations show the
relationship between tCDL, the time in seconds required for
CD to go low; tCDH, the time in seconds required for CD to
go high; and CCDT, the capacitor value in µF.
Valid signal to CD response time:
tCDL≈ 6.4 X CCDT
Invalid signal to CD off time:
tCDH
≈ 0.54 X CCDT
Example:
tCDL ≈ 6.4 X 0.1 µF ≈ 0.64 seconds
tCDH ≈ 0.54 X 0.1 µF ≈ 0.054 seconds
CARRIER DETECT THRESHOLD ADJUSTMENT
The carrier detect threshold is set by internal resistors to
activate CD with a typical –44 dBm (into 600
Ω) signal and
deactivate CD with a typical –47 dBm signal applied to the
input of the hybrid circuit. The carrier detect threshold level
can be adjusted by applying an external voltage on CDA (pin
7). The following equations may be used to find the CDA volt-
age required for a given threshold voltage. (Von and Voff are in
Vrms.)
VCDA = 244 x Von
VCDA = 345 x Voff
Example (Internally Set)
Von = 4.9 mV
≈ –44 dBm: VCDA = 244 x 4.9 mV = 1.2 V
Voff = 3.5 mV
≈ –47 dBm: VCDA = 345 x 3.5 mV = 1.2 V
Example (Externally Set)
Von = 7.7 mV
≈ –40 dBm: VCDA = 244 x 7.7 mV = 1.9 V
Voff = 5.4 mV
≈ –43 dBm: VCDA = 345 x 5.4 mV = 1.9 V
The CDA pin has an approximate Thevenin equivalent volt-
age of 1.2 V and an output impedance of 100 k
Ω. When using
the internal 1.2 V reference, a 0.1 µF capacitor should be con-
nected between this pin and VSS (see Figure 5).
TRANSMIT LEVEL ADJUSTMENT
The power output at TxA (pin 17) is determined by the value
of resistor RTLA that is connected between TLA (pin 20) to
VDD (pin 6). Table 3 shows the RTLA values and the corre-
sponding power output for a 600
Ω load. The voltage at TxA is
twice the value of that at ring and tip because TxA feeds the
signal through a 600
Ω resistor RTx to a 600 Ω line trans-
former (see Figure 7). When choosing resistor RTLA, keep in
mind that –9 dBm is the maximum output level allowed from a
modem onto the telephone line (in the U.S.). In addition, keep
in mind that maximizing the power output from the modem
optimizes the signal–to–noise ratio, improving accurate data
transmission.
THE LINE DRIVER
The line driver is a power amplifier used for driving a tele-
phone line. Both the inverting and noninverting input to the line
driver are available for transmitting externally generated tones.
Exl (pin 18) is the noninverting input to the line driver and
gives a fixed gain of 2 (Ri = 50 kΩ). The average signal level
must be the same as VAG to maintain proper operation. This
pin should be connected to VAG if not used.
The driver summing input (DSI, pin 1) may be used to con-
nect an external signal, such as a DTMF dialer, to the phone
line. When applying a signal to the DSI pin, the modulator
should be squelched by bringing SQT (pin 14) to a logic high
level. DSI must be left open if not used.
In addition, the DSI pin is the inverting side of the line driv-
er and allows adjustable gain with a series resistor RDSI (see
Figure 6). The voltage gain, AV, is determined by the equation:
where Rf
≈ 20 kΩ.
Example: A resistor value of 20 k
Ω for RDSI will provide
unity gain.
AV = – (20 kΩ/20 kΩ) = –1 .
Table 3. Transmit Level Adjust
Output Transmit Level
(Typical into 600
Ω)
RTLA
–12 dBm
–11 dBm
19.8 k
–10 dBm
9.2 k
–9 dBm
5.5 k
AV = –
Rf
R DSI
Legacy Applications Information
Issue b


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