MAS9191A datasheet(17/36 Pages) MAS

17 / 36 page

DA9191A.000

July 31, 1997

FUNCTIONS

◆ RX Audio

The RX audio block starts with switch S1. The switch

is controlled by bit S1 of register 0EHEX. The input

ALP is used for enabling the audio loopback mode.

In normal operation RX is used. Behind the switch is

amplifier GC1 with adjustable gain from -3dB to

+3dB. The gain is controlled with bits 0..3 in register

0EHEX.The amplifier output is connected to the

second order lowpass filter F1. The cutoff frequency

of the filter is 50 kHz and the gain at 1kHz is 0dB.

The filter output is connected to data comparator C1

and to the SAT bandpass filter F2 and to the rest of

the RX audio block. The polarity of the received data

can be inverted with bit INVRX, which is located in

connected to the DPLL and Manchester decoder

blocks. The filter F2 is a 6kHz bandpass filter for

supervisory audio tone. The filter is a second order

SC filter. The filter output is connected to SAT

comparator C2 and the SAT detection block.

The signal from F1 is connected to filter F3 through a

switch which is used by the internal logic when the

AUMUT bit of register 13HEX is active. In this case if

the data is received on the voice channel the

lowpass filter F3 input is grounded automatically with

this switch. With switches S6 and S7 the received

audio and transmitted audio can be summed. The

control bits of the switches are located in register

03HEX. This signal is fed to F4 and to the DTMF

receiver. The function of the DTMF receiver is

described in the next section RX de-emphasis filter

F4 has a -6dB/octave (±1dB) frequency response in

the range 300Hz ... 3kHz. The filter can be bypassed

with S2-RXTST0-RXTST1. Switch S2-RXTST0-

RXTST1 is connected to the DEOUT pin. The

performance of C1, C2 and F2 can be monitored with

switch S2-RXTST0-RXTST1. Bits 5 and 6 in register

02HEX and bit 5 in register 0EHEX control this switch.

An external capacitor is needed between DEOUT

and RBPFIN. The RBPFIN input is connected to filter

F5, which is a 6th order bandpass filter. The gain of

this filter is 0dB at 1kHz. The filter can be bypassed

with switch S4. After S4 the signal is connected to

the RBPFOUT pin. An external capacitor is used to

connect the signal to the EXPIN input.

1:2

GC1

RX data, SAT

RXMUTE

MICOUT

ALP

DTMF

Receiver

C1 C2 F2

RXTST0

RXTST1

EXPANDER

GC2

S11

S10

EXTERP

EARP1

EARP2

BUZFB

BUZOUT

SIDEFB

SIDETONE

Audio Receive Path

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