RFX144V24-S23 and RFX96V24-S23 Modem Designer’s Guide
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14-1
14. MODEM INTERFACE CIRCUIT
14.1 CIRCUIT AND COMPONENTS
The modem is supplied in a 100-pin PQFP (MDP) and a 28-pin PLCC (XIA) for design into OEM circuit boards.
The recommended modem interface circuits shown in Figure 14-1 illustrate the connections and components required to
connect the modem to the OEM electronics.
A typical line interface circuit is shown in Figure 14-2.
A typical line interface using an external hybrid is shown in Figure 14-3. Resistors R7 and R9 can be used to trim the
transmit level and receive threshold, respectively, to the accuracy required by the OEM equipment. For a tolerance of ±1
dBm, the 1% resistor values shown are correct for more than 99.8% of the units.
Figure 14-4 shows a recommended microphone circuit.
Figure 14-5 shows a typical speaker circuit.
The corresponding parts list is shown in Table 14-1.
14.1.1 Crystal/Oscillator
Specifications
Table 14-2 specifies parameters for crystals or oscillators identified in these schematics.
14.1.2 Transmit Level and Receive Threshold Trimmers
Resistors R7 and R9 can be used to trim the transmit level and receive threshold to the accuracy required by the OEM
equipment. For a tolerance of ±1 dBm, the 1% resistor values shown are correct for more than 99.8% of the units.
14.1.3 Echo Canceller Working Condition
Maximum echo return gain from TXA1 or TXA2 to RIN must be under +6 dB at all passband frequencies.
14.1.4 Room Acoustic Echo Canceller Working Condition
Maximum echo return gain from TXA1 (SPKHI) or TXA2 (SPKLO) to RIN (MICOUT) must be under +12 dB at all passband
frequencies.
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