MSM7715 View Datasheet(PDF) - Oki Electric Industry
Part Name
Description
MFG CO.
'MSM7715' PDF : 21 Pages View PDF
¡ Semiconductor
Setting the transmit signal level
MSM7715
VAO
C4
R1
R3
R2
AOUT
GAT2
4dB
RC-LPF
GAT1
DTO
DTMF
Generator
MOD
Modulator
R1, R2, R3 ≥ 20 kW, R1 £ R2, R1 £ R3
Figure 10 Setting the Transmit Signal Level
The modem’s modulated analog signal and DTMF signal are not transmitted at the same time.
The signal to be transmitted is determined by the selected operation mode. This device provides
the pins for individual setting of transmit signal levels.
VAOM: Level of the modem signal at the AOUT pin when R1 = R2 (dBm)
VAODT: Level of the DTMF signal at the AOUT pin when R1 = R3 (dBm)
When external resistors (R1, R2, R3) are changed, the signal level at AOUT is as follows:
VAO (modem) = 20 ¥ log (R1/R2) + VAOM
VAO (DTMF) = 20 ¥ log (R1/R3) + VAODT
However, to avoid distorted output, R1 ≤ R2 and R1 ≤ R3 are needed. In circuit design, R1 = R2
or R1 = R3 with 5% tolerance is permitted.
C4 is a component in the first order LPF for suppressing the out-of-band output noise. Select a
value C4 in such a way that cutoff frequency FC determined by R1 and C4 will be approximately
8 kHz.
C4 = 1/ (2p ¥ R1 ¥ FC)
17/21
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