TRANSMIT SECTION

The transmit section input is an operational amplifier with

provision for gain adjustment using two external resistors

see

Figure 4 The low noise and wide bandwidth allow gains

in excess of 20 dB across the audio passband to be real-

ized The op amp drives a unity-gain filter consisting of RC

active pre-filter followed by an eighth order switched-ca-

pacitor bandpass filter clocked at 256 kHz The output of

this filter directly drives the encoder sample-and-hold circuit

The AD is of companding type according to m-law

(TP3054) or A-law (TP3057) coding conventions A preci-

sion voltage reference is trimmed in manufacturing to pro-

pulse controls the sampling of the filter output and then the

successive-approximation encoding cycle begins The 8-bit

proximately 165 ms (due to the transmit filter) plus 125 ms

(due to encoding delay) which totals 290 ms Any offset

voltage due to the filters or comparator is cancelled by sign

bit integration

RECEIVE SECTION

The receive section consists of an expanding DAC which

drives a fifth order switched-capacitor low pass filter

clocked at 256 kHz The decoder is A-law (TP3057) or

m

-law (TP3054) and the 5th order low pass filter corrects for

the sin xx attenuation due to the 8 kHz samplehold The

filter is then followed by a 2nd order RC active post-filter

power amplifer capable of driving a 600X load to a level of

72 dBm The receive section is unity-gain Upon the occur-

the end of the decoder time slot the decoding cycle begins

and 10 ms later the decoder DAC output is updated The

total decoder delay is E 10 ms (decoder update) plus

110 ms (filter delay) plus 625 ms(

frame) which gives

approximately 180 ms

4