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參數(shù)資料
| 型號(hào)： | MC145443BP |
| 廠商： | MOTOROLA INC |
| 元件分類： | 調(diào)制器/解調(diào)器 |
| 英文描述： | Single-Chip 300-Baud Modem |
| 中文描述： | 0.3 kbps DATA, MODEM, PDIP20 |
| 封裝： | PLASTIC, DIP-20 |
| 文件頁數(shù)： | 8/12頁 |
| 文件大小： | 111K |
| 代理商： | MC145443BP |
MC145442B
MC145443B
8
MOTOROLA
APPLICATIONS INFORMATION
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 re-
quired for CD to go low; tCDH, the time in seconds required
for CD to go high; and CCDT, the capacitor value in
μ
F.
Valid signal to CD response time: tCDL
≈
6.4
×
CCDT
Invalid signal to CD off time:
Example: tCDL
≈
6.4
×
0.1
μ
F
≈
0.64 seconds
tCDH
≈
0.54
×
0.1
μ
F
≈
0.054 seconds
tCDH
≈
0.54
×
CCDT
CARRIER DETECT THRESHOLD ADJUSTMENT
The carrier detect threshold is set by internal resistors to
activate CD with a typical –44 dBm (into 600
) 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
voltage required for a given threshold voltage. (Von and Voff
are in Vrms.)
VCDA = 244
×
Von
VCDA = 345
×
Voff
Example (Internally Set)
Von = 4.9 mV
≈
–44 dBm: VCDA = 244
×
4.9 mV = 1.2 V
Voff = 3.5 mV
≈
–47 dBm: VCDA = 345
×
3.5 mV = 1.2 V
Example (Externally Set)
Von = 7.7 mV
≈
–40 dBm: VCDA = 244
×
7.7 mV = 1.9 V
Voff = 5.4 mV
≈
–43 dBm: VCDA = 345
×
5.4 mV = 1.9 V
The CDA pin has an approximate Thevenin equivalent
voltage of 1.2 V and an output impedance of 100 k
. When
using the internal 1.2 V reference, a 0.1
μ
F capacitor should
be connected 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 corresponding power output for a 600
load. The volt-
age at TxA is twice the value of that at ring and tip because
TxA feeds the signal through a 600
resistor RTx to a 600
line transformer (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, improv-
ing accurate data transmission.
Table 3. Transmit Level Adjust
Output Transmit Level
(Typical into 600
)
RTLA
∞
19.8 k
9.2 k
5.5 k
–12 dBm
–11 dBm
–10 dBm
–9 dBm
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 k
). 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:
AV = –
Rf
RDSI
where Rf
≈
20 k
.
Example: A resistor value of 20 k
for RDSI will provide
unity gain.
AV = – (20 k
/20 k
) = –1 .
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