MT8840AS datasheet(3/8 Pages) MITEL

3 / 8 page

ISO2-CMOS

MT8840

5-5

Functional Description

The

MT8840

contains

the

modulator

and

demodulator circuitry for 32 kHz ASK signalling as

well as a two-tone warbler (tone caller) to replace the

function of the mechanical telephone ringer.

A 32 kHz carrier is 100% amplitude modulated by the

digital bit stream applied to input TxDI. This results in

an amplitude shift keyed (ASK) 32 kHz carrier. A

logical high at TxDI disables the carrier and a logical

low enables it. The digitally modulated waveform is

shaped by the Tx BANDPASS FILTER and smoothed

by the Tx POST FILTER. The signal then enters the

routing block where it is transferred to the TxO

output.

The modulated 32 kHz receive signal is applied to

RxI. With a logical low applied to LOOP and a logical

high applied to RxE, receive signals are routed to the

Rx PREFILTER. High frequencies are removed by

the Rx PREFILTER to prevent aliasing in the

switched capacitor Rx BANDPASS FILTER. Voice

signals are removed by the bandpass filter which is

followed by an AGC circuit. This provides a dynamic

range of 20dB for the receiver. An external 1µF

capacitor connected from CRx to VSS is required to

control the AGC attack and decay time constants.

Data is recovered from the received signal in the

demodulator. The minimum voltage level to which

the demodulator responds may be adjusted by

connecting a resistor from DET to VDD or VRef. Since

DET is the input to a comparator, noise should be

kept to a minimum at this pin. The recovered receive

data is synchronized to the leading edge of the 32

kHz clock (available at CK32) before appearing at

RxDO.

When in loop around mode, the Rx PREFILTER

input is internally disconnected from the RxI input pin

and connected to TxO. The transmitter output is still

available at TxO.

A two tone warbling audio signal is available at TCO

when the tone caller enable input (ETC) is high. TCO

is internally clamped to VRef when the tone caller is

disabled. The tone output can be attenuated by 20dB

if a logical high is applied to the tone caller mute

input (MTC).

Applications

Figures 3 through 5 show how the MT8840 may be

utilized to transfer data and voice simultaneously

over a single pair of wires in digital or analog PABXs

and "smart" telephone sets. In all three figures a

microprocessor sends/receives data to/from

the

MT8840 via a UART which converts the data format

from parallel-to-serial or serial-to-parallel for

the

transmit and receive directions, respectively. In the

receive direction the MT8840 has on-board filters to

reject voice-band signals leaving only the 32 kHz

carrier. This carrier is then demodulated to recover

the received data. In the transmit direction the data

to be sent is modulated and passed on to a summing

circuit which sums the modulated 32 kHz carrier and

voiceband

signals

for

transmission

over

the

telephone line. In the PABX the Filter/Codec has

filters which reject the 32 kHz carrier from the

received composite voice and data signal allowing

only voiceband signals to pass through which are

then PCM encoded for digital switching. However, in

both the analog PABX and smart telephone set,

lowpass filters could be included to bandlimit the

received signal leaving only voice signals to be

passed on to the switch array or handset earpiece.

MT8840AS Datasheet(PDF) 3 Page - Mitel Networks Corporation

MT8840AS Datasheet(HTML) 3 Page - Mitel Networks Corporation

Html Pages

1 2 3 4 5 6 7 8

Datasheet Download

Link URL