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MC145442P Datasheet(PDF) 8 Page - Motorola, Inc |
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MC145442P Datasheet(HTML) 8 Page - Motorola, Inc |
8 / 12 page MC145442 •MC145443 MOTOROLA 8 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 k Ω. 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 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 transformer (see Figure 7). When choosing resistor RTLA, keep in mind that – 9 dBm is the maximum output level al- lowed 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. Table 3. Transmit Level Adjust Output Transmit Level (Typical into 600 Ω) RTLA – 12 dBm ∞ – 11 dBm 19.8 k Ω – 10 dBm 9.2 k Ω – 9 dBm 5.5 k Ω THE LINE DRIVER The line driver is a power amplifier used for driving the telephone line. Both the inverting and noninverting input to the line driver are available for transmitting externally gener- ated 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: AV = – Rf RDSI where Rf ≈ 20 kΩ. Example: A resistor value of 20 k Ω for RDSI will provide unity gain. AV = – (20 kΩ/20 kΩ) = – 1 |
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