TCM8000

CELLULAR TELEPHONE AUDIO PROCESSOR

SCTS041A – D4086, AUGUST 1992 – REVISED APRIL 1993

POST OFFICE BOX 655303

4

Terminal Functions (continued)

PIN

NAME

NO.

I/O

DESCRIPTION

NAME

FN

FR

VBTX

44

5

Buffered midsupply for all transmit sections, nominal output VCC /2

VCC

1

6

Most positive supply voltage, 5 V

±5%

XT0, XT1

21, 20

26, 25

Crystal oscillator terminals for connection to 3.58-MHz crystal resonator

detailed description – transmit path

input conditioning

A pair of uncommitted operational amplifiers is provided at the main audio signal input. These components can

be configured using external resistors to adjust the gain as required to suit particular microphones and external

preamplifiers. The TCM8000 has been designed to provide 100 mVrms at the outputs of the preamplifier at pins

M1O and M2O. The switch MICSW allows the selection of either amplifier input for hands-free or handset

microphones. A digitally controlled gain/attenuation block, MICTRIM, allows adjustment of the signal level into

the compressor in sixteen 0.5-dB steps. A digitally controlled switch, TDSW, allows alternative signals from other

sources such as audio or DTMF tones to be injected into the transmit signal path from TDI. The output of TDSW

is ac coupled at TXVO through an external capacitor into the compressor.

compressor

The compressor provides a 2:1 dynamic range compression of its input signal, converting an input range of

60 dB to 30 dB at its output. The test circuit diagram (Figure 2) shows the external components required to set

time constants and to limit gain under idle channel conditions. The envelope time constant is determined by

the value of a capacitor attached to CTC with 220 nF providing attack time of 3 ms approximately and decay

times of approximately 13 ms. The compressor can be bypassed under control of the serial interface. The output

from this stage, CO, is externally ac coupled into the transmit filter at TXVI and into the compressor rectifier input

at CMPR.

TX filtering and limiter

The transmit band limiter is located between a pair of band-pass filters. Filter TXF1 is located before the limiter

and provides 6 dB per octave preemphasis; filter TXF2, located after the limiter, features a phase-equalized

response to control overshoot. The frequency responses of TXF1 and TXF2 are switchable between 3 kHz and

3.4 kHz cut off to meet the requirements of AMPS/TACS and NMT systems, respectively. Frequency response

templates and typical responses are shown in Figures 3 through 6.

The limiter is provided to meet maximum deviation specifications, and the limit level is controlled by the dc

voltage applied to TXLIM. The output signal from POUT is ac coupled into the limiter input to eliminate

asymmetrical limiting that could arise as a result of dc offsets.

output conditioning

The voice output signal from TXLP2 is passed to a three-input summing circuit, whose other inputs come from

the TX data path and from TXSUM. Each input to the summing circuit can be enabled or muted under control

of the serial bus. The overall deviation level is controlled by a digitally controlled gain/attenuation block that

provides sixteen 0.5-dB steps of output level adjustment. The transmit supervisory audio tone (SAT) is applied

to TXSUM. TXSUM can also be used as an input for transmit data, allowing implementation of alternative data

filtering and conditioning to that implemented on the TCM8000.

transmit data

Transmit data, which is voice-band FSK coded for NMT and wide-band Manchester coded for AMPS and TACS

(8K-baud TACS, 10K-baud AMPS), is applied to TXDA. The signal is filtered by the TX DATA filter, whose

response can be switched from a band-pass to a low-pass configuration to meet the requirements of NMT and

AMPS/TACS, respectively. Frequency templates and typical responses are shown in Figures 7 and 8.