Basic Theory of Operation
7
The resulting intermediate frequency (IF) signal is fed to the IF
circuitry, where it is again filtered and passed to the Abacus III digital
back-end IC. In the digital back-end IC, the IF signal is mixed with the
second local oscillator to create the second IF at 2.25 MHz. In the back-
end IC, a bandpass, sigma-delta, analog-to-digital converter then
decodes the second IF signal, and outputs, on the radio’s serial
synchronous interface (SSI) bus, digital audio to the VOCON board.
On the VOCON board, the dual-core processor’s digital-signal
processor (DSP) digitally filters the PCM audio. The DSP decodes the
information in the signal and identifies the appropriate destination
for it.
• For a voice signal, the DSP will route the digital voice data to the
CODEC inside the audio and power supply support IC, for
conversion to an analog signal. The CODEC will then present the
signal to the receive audio pre-amplifier, then to the audio power
amplifier, which drives the speaker.
• For signaling information, the DSP will decode the message and
pass it internally to the microcontrol unit of the dual-core
processor.
Transmitting When the radio is transmitting (see Figure 3.), microphone audio is
passed through gain stages to the CODEC, where the signal is
digitized. The CODEC passes digital data to the DSP, where pre-
emphasis and low-pass (splatter) filtering are done. The DSP passes this
signal to a digital/analog converter (DAC), where it is reconverted into
an analog signal and scaled for application to the voltage-controlled
oscillator as a modulation signal.
Figure 3. Transceiver Block Diagram (Power and Control Omitted)
Transmitted signaling information is applied to the DSP from the
microcontrol unit, where it is coded, and passed to the DAC, which
handles it the same as a voice signal. The DAC output connects to the
synthesizer modulation input. A modulated carrier is provided to the
Reference
Oscillator
FracN
U203
MOD
IN
Loop
Filter
LPF
FL200
EPIC
Y200
DAC
U203
VCO
VCOBIC
U202
VCO
Crystal
Filter
FL403
Mixer
U401
2ND
LO
Sample
Clk
RX_SSI to
VOCON Board
TX_SSI from
VOCON Board
Preselector
Filter
FL401
PCIC
U104
Power
Module
TX
Buffer
Q304
TX Driver
Amplifier
U102
RF Power
Detector
D103
Directional
Coupler
U101
Antenna
Switch
Preselector
Filter
FL402
RX LNA
Q401
To
Antenna
Harmonic
Filter
Serial EE
PROM
U4
ABACUS III U500
VCO
3
3
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