Motorola GM328 Service Manual page 104

1-12
2.2.1 Mic/Data Input Path
The radio supports 2 distinct microphone paths known as internal (from controlhead) and external
mic (from accessory connector J0501-2) and an auxiliary path (FLAT TX AUDIO, from accessory
connector J0501-5). The microphones used for the radio require a DC biasing voltage provided by a
resistive network.
These two microphone audio input paths enter the ASFIC CMP at U0221-48 (external mic) and
U0221-46 (internal mic). Following the internal mic path; the microphone is plugged into the radio
controlhead and is connected to the controller board via J0401-9.
From here the signal is routed via R0409 and line INT MIC to R0205. R0201 and R0202 provide the
9.3VDC bias. Resistive divider R0205 / R0207 divide the input signal by 5.5 and provide input
protection for the CMOS amplifier input. R0202 and C0201 provide a 560 ohm AC path to ground
that sets the input impedance for the microphone and determines the gain based on the emitter
resistor in the microphone's amplifier circuit.
C0204 serves as a DC blocking capacitor. The audio signal at U0221-46 (TP0221) should be
approximately 14mV for 1.5kHz or 3kHz of deviation with 12.5kHz or 25kHz channel spacing.
The external microphone signal enters the radio on accessory connector J0501 pin 2 and is routed
via line EXT MIC to R0206. R0203 and R0204 provide the 9.3VDC bias. Resistive divider R0206 /
R0208 divide the input signal by 5.5 and provide input protection for the CMOS amplifier input.
R0204 and C0202 provide a 560 ohm AC path to ground that sets the input impedance for the
microphone and determines the gain based on the emitter resistor in the microphone's amplifier
circuit.
C0254 serves as a DC blocking capacitor. Multi switch U0251 controlled by ASFIC CMP port GCB4
selects either the external microphone input signal or the voice storage playback signal for entering
the ASFIC CMP at pin 48. The audio signal at U0221-48 (TP0222) should be approximately 14mV
for 1.5kHz or 3kHz of deviation with 12.5kHz or 25kHz channel spacing.
The FLAT TX AUDIO path is used for transmitting data signals and has therefore no limiter or filters
enabled inside the ASFIC CMP. When this path is enabled via CPS and DATA PTT is asserted, any
signal on this path is directly fed to the modulator. Signals applied to this path either via accessory
connector J0501, expansion board connector J0451 or option board connector J0551 must be
filtered and set to the correct level externally or on the option board in order not to exceed the
maximum specified transmit deviation and transmitted power in the adjacent channels. The
attenuator inside the ASFIC CMP changes the FM deviation of the data signal according to the
channel spacing of the active transmit channel.
The FLAT TX AUDIO signal from accessory connector J0501-5 is fed to the ASFIC CMP (U0221)
pin 42 through C0541 and line FLAT TX RTN, switch U0251 and buffer U0211-4. When the radio
switches from receive to transmit mode the µP opens switch U0251 for a short period to prevent that
any applied signal can cause a transmit frequency offset. Buffer U0211-4 sets the correct DC level
and ensures a short settle period when the radio is switched on. Inside the ASFIC CMP the signal is
routed directly to the attenuator, which sets the FM deviation according to the channel spacing of the
active transmit channel and emerges from the ASFIC CMP at U0221-40, at which point it is routed
to the RF section.
Controller Board Audio and Signalling Circuits