Frequency Generation Circuit 2-11
Low Speed Data Filter
This analog circuitry is a 4 pole, 150 Hz cutoff low pass filter comprised of U603-1, U603-2 and
associated passive components. In receive mode, it removes noise and voice band signals leaving
only the low speed data waveform which is applied to the ADC input of the MCU. U608-4 isolates
the receive signal from the filter in transmit mode. When the radio is transmitting PassPort data, the
MCU DAC low speed data waveform is applied to the input of the filter which removes harmonics
that would interfere with voice and applies the resulting sub-audible data to the radio transmitter
modulation point.
Keyboard Circuit
The keyboard consists of a matrix of key switches and resistors as described in section 2.3. U605-2
monitors the column voltage and applies an interrupt signal to the radio microcontroller when any
key is pressed.
BackLight Driver and LED's
The logic level signal from the radio microcontroller is translated via Q611 and applied to Q610
which uses Switched Battery Voltage (SWB+) to operate the keypad backlight LED’s.
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