72 Circuit Descriptions TM8100 Mobile Radio Service Manual
May 2004 © Tait Electronics Limited
Auxiliary CODEC The auxiliary CODEC handles transmitter power control, receiver gain
control, auxiliary audio output and general analogue monitoring
functions. The digital section communicates with this CODEC via a
three-wire synchronous serial interface (
ASFS, ASDI and ASDO balls). The
DAC used for receiver gain control (
IDACOUT ball) is a current output type.
Current-to-voltage conversion is performed by R238. The full-scale
output of 1.2 V is amplified by IC201 to approximately 3 V as required
by the receiver.
Audio Circuitry The audio circuitry performs four functions:
■ output of audio signal for speaker
■ input of microphone audio signal
■ input of auxiliary audio signal
■ output of auxiliary audio signal
The sections of the circuitry concerned with these functions are
described below.
Audio Signal
for Speaker
The audio signal for the speaker is generated by IC204 (VOUTAUXP ball).
This signal is post-volume-control and has a pre-emphasised frequency
response. The signal is then processed by R218, R217 and C205 to
restore a flat frequency response and reduce the signal level to that
required by the audio power amplifier.
Summing Circuit The top of C205 is where side tones are summed in and the CDC RX AUD
signal is obtained. C201 and R211 pre-emphasize and attenuate the
side-tone signal to give a flat side-tone frequency response and reduce the
input to an appropriate level.
Buffer Amplifier IC201 (pins 8 to 10) amplifies the signal at the top of C205 by 19 dB and
drives the
CDC RX AUD system interface line via C212 and R225. The
capacitor C212 provides AC output coupling and R225 ensures stability.
The DC bias for this amplifier is derived from IC204.
Audio Power Amplifier The signal at the top of C205 is fed via C204 to the audio power
amplifier IC202. IC202 has 46 dB of gain and a differential output
configuration. C209, C211, R252 and R253 ensure stability of the
amplifier at high frequencies. When operational, the output bias voltage
for IC202 is approximately half the radio supply voltage. When not
operational, the output becomes high impedance.
Control of Audio
Power Amplifier
Power up, power down, and muting of IC202 is controlled by two signals
from the digital section,
DIG AUD PA EN1 and DIG AUD PA EN2. The network
consisting of Q200, Q201, R200 to R206, R210 and R250 converts the
two digital signals to the single three-level analogue signal required by
IC202.
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