msm7715 Oki Semiconductor, msm7715 Datasheet - Page 17

• Manufacturer Part Number: msm7715
• Description: Multi-function Telecommunication Lsi
• Manufacturer: Oki Semiconductor

Setting the transmit signal level
¡ Semiconductor
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.
V
V
When external resistors (R1, R2, R3) are changed, the signal level at AOUT is as follows:
V
V
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 F
8 kHz.
C4 = 1/ (2p ¥ R1 ¥ F
AOM
AODT
AO
AO
(modem) = 20 ¥ log (R1/R2) + V
(DTMF) = 20 ¥ log (R1/R3) + V
: Level of the modem signal at the AOUT pin when R1 = R2 (dBm)
C4
R2
: Level of the DTMF signal at the AOUT pin when R1 = R3 (dBm)
VAO
R3
R1
C
)
Figure 10 Setting the Transmit Signal Level
AOUT
GAT2
GAT1
DTO
MOD
AODT
AOM
4dB
C
determined by R1 and C4 will be approximately
R1, R2, R3 ≥ 20 kW, R1 £ R2, R1 £ R3
RC-LPF
Modulator
Generator
DTMF
–
+
MSM7715
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