Rx Audio Mute Gate; Audio Power Amplifier; Noise Squelch Attenuator; Vox Circuit Operation - Motorola GP350 Service Manual

Theory of Operation
Frequency Generation Circuitry
15k-ohm impedance. P1-7 is always unmuted, not affected
by the receiver with squelch circuit. Option RX audio input
to the GP350 radio is available at P1-6, with a sensitivity of
100 mVrms at less than 200 ohm output impedance from
option board.

RX Audio Mute Gate

PNP transistor Q406, the RX audio mute gate, with resistors
R458 and R459, and capacitors C432 and C433, provide
receiver audio muting. The RX audio mute gate circuit func-
tions in a similar manner to Q409, the TX audio mute gate
circuit. Muting is controlled by microcontroller U401 via
U402-39, an Audio Filter IC expanded output port. Q406 is
saturated and RX audio unmuted by programming U402-39
to a logic "LO" state. Q406 is placed well into cut-off and
RX audio muted by programming U402-39 to a logic "high"
state.
Audio Power Amplifier
Variable resistor R460 and resistor R461 provide RX audio
volume adjustment. R461 sets the minimum volume level.
R466 and R464 form a resistor divider to set the audio input
amplitude into the amplifier (U409-10), which is ac-coupled
by C518. Fixed level Alert Tone audio is generated by micro-
controller U401-56 and coupled through capacitor C437 and
resistor R465 into the audio path. The audio amplifier
(U409) has three amplifiers designed to differentially drive
its load. Two of the three amplifiers simultaneously drive the
16- ohm speaker. All the amplifiers are enabled with "high"
(5 volts) on U409-23, which is activated by the AFIC (U402-
3). The common amplifier (U409-31 and U409-32) is always
on, and either the external amplifier (U409-4 and U409-5) or
the internal amplifier (U409-27 and U409-28) is on, depend-
ing on the bias of the logic circuits (U409-24). If U409-24 is
"high," the internal amplifier (U409-27 and U409-28) is on;
if U409-24 is low, the external amplifier (U409-4 and U409-
5) is on. This is how the audio is switched between the inter-
n a l s p e a ke r a n d

Noise Squelch Attenuator

The Audio Filter IC U402 contains a 16 step programmable
digital squelch attenuator between U402-16 and U402-18.
Noise squelch is set using the Radio Service Software, with
open squelch at step 0, and tight squelch at step 15.

Vox Circuit Operation

As mentioned above, with VOX option enabled, a VOX
(non-PTT) accessory can be plugged into the adaptor for
voice-activated transmit operation. The external MIC ele-
ment is always supplied with operating bias through resistor
R451 and external PTT sense transistor Q408. The external
PTT sense at microcontroller U401-14 is therefore, always
"enabled." A second output circuit of MIC amplifier U409-
11 couples MIC audio through capacitor C445 to U406B, the
VOX detector circuit. Resistors R492 and R493, and capaci-
tor C451 form a syllabic filter which reduces VOX circuit
4-6
t h e a c c e s s o r i e s .
6880904Z07-O
GP350 Portable Radios Service Manual
triggering by "high" frequency ambient noise. Resistors
R442, R443, R444, R445, and R491, capacitor C423, recti-
fier diode CR404 and U406B form a linear peak detector cir-
cuit. MIC audio causes capacitor C423 to char g e to a
potential related to the relative amplitude of ambient noise.
Microcontroller U401-19 monitors the potential of C423 and
establishes a threshold for non-voiced ambient noise. When
a positive rise in potential above threshold or voice is
detected by an algorithm in the microcontroller ROM, the
radio is configured to transmit mode.
August, 1996