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MC145442B

Single–Chip 300–Baud Modem

Motorola => Freescale 

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 µF.

Valid signal to CD response time: tCDL ≈ 6.4 × CCDT

Invalid signal to CD off time:

tCDH ≈ 0.54 × CCDT

Example: tCDL ≈ 6.4 × 0.1 µF ≈ 0.64 seconds

tCDH ≈ 0.54 × 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

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 kΩ. When

using the internal 1.2 V reference, a 0.1 µF 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 Ω load. The volt-

age 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 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 Ω)

–12 dBm

–11 dBm

–10 dBm

–9 dBm

RTLA

∞

19.8 kΩ

9.2 kΩ

5.5 kΩ

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.

AV = –

Rf

RDSI

Example: A resistor value of 20 kΩ for RDSI will provide

unity gain.

AV = – (20 kΩ/20 kΩ) = –1 .

MC145442B•MC145443B

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