General
Theory of Operation 6.3-7
CONTROLLER BOARD AUDIO AND SIGNALLING CIRCUITS
3.0 General
3.1 Audio Filter IC (AFIC)
The AFIC (U0103) used in the controller performs RX/TX audio shaping, i.e. filtering, amplification,
attenuation.
The AFIC is programmable through the SPI BUS (U0103-30/31/33), normally receiving 6 bytes. This
programming sets up various paths within the AFIC to route audio signals through the appropriate
filtering, gain and attenuator blocks. The AFIC also has 4 General Control Bits GCB1,3-5 which are
CMOS level outputs. GCB1 is used to switch the radio on and off under µP control via line B+
CONTROL. GCB3 is used to switch the audio PA on and off (AUDIO PA ENABLE). GCB4 selects
between the UNATTEN RX OUT audio signal and the unfiltered DET AUDIO signal. GCB5 HIGH
LOW BAND can be used to switch between band splits.
3.2 Audio Ground
VAG is the dc bias used as an audio ground for the Op-amps that are external to the Audio Filter IC
(AFIC). U0105-1 forms this bias by dividing 9.3V with resistors R0171, R0172 and buffering the
4.65V result with a voltage follower. VAG emerges at pin 1 of U0105-1. C0172 is a bypass capacitor
for VAG. The AFIC generates its own 2.5 V bias for its internal circuitry. C0153 is the bypass for the
AFIC’s audio ground dc bias. Note that while there are AFIC VAG, and BOARD VAG (U0105-1) each
of these are separate. They do not connect together.
4.0 Transmit Audio Circuits
Refer to Figure 6.3-1 for reference for the following sections.
4.1 Mic Input Path
The radio supports 2 distinct microphone paths known as internal and external mic and an auxiliary
path (FLAT TX AUDIO). The microphones used for the radio require a DC biasing voltage provided
by a resistive network.
These two microphone audio inputs are connected together. Following the internal mic path; the
microphone is plugged into the radio control head and is connected to the controller board via
J0101-16. From here the signal is routed to C0142. R0141 and R0142 provide the 9.3VDC bias.
R0142 and C0141 provide a 1kΩ 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.
The MIC signal is routed to the AFIC´s TX IN input (U0103-10) through R0146 and R0145 (GM950E
radio) or through Op-amp buffer U0106-2 and option board connector J0103-3,1 (GM950i radio).
The audio signal at the output of U0106-2 should be approximately 80mV deviation with 25kHz
channel spacing.
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