3-4 Basic Theory of Operation: Analog Mode of Operation
3.2.1.3 Analog To Digital Converter
The ADC IC's front end down converts the first IF to a second IF, a 2.25 MHz signal. The second IF
is sampled at 18 MHz, a signal generated by an integrated clock synthesizer. The sampled signal is
decimated by a factor of 900 to 20 kHz and converted to SSI format at the ADC's output. The Serial
Synchronous Interface (SSI) serial data waveform is composed of a 16 bit in-phase word (I) followed
by a 16 bit Quadrature word (Q). A 20 kHz Frame Synch and a 1.2 MHz clock waveform are used to
synchronize the SSI IQ data transfer to the Digital Signal Processor IC (OMAP) for post-processing
and demodulation.
3.2.2 Transmitting
When the radio is transmitting, microphone audio is digitized and then processed by the DSP and
sent to the Trident IC (see Figure 3-4) via the SSI interface. The Trident IC processes the SSI data
for application to the voltage controlled oscillator as a modulation signal.
Figure 3-4. Transmitter (900 MHz) Block Diagram
3.2.2.1 900 MHz Transmit
Once a 900 MHz frequency for transmit has been selected, the Trident IC and accompanying logic
circuitry enable the correct voltage controlled oscillator which then generates the desired transmit
frequency. This transmit signal is then routed to the TX buffer amplifier which amplifies the signal.
The signal is routed to the 900 MHz Driver amplifier and then to the discrete final power amplifier.
The signal now goes through the antenna switch which routes the power to the harmonic filter which
will filter out the harmonics of the carrier signal and then passes through a directional coupler. The
Log Amp power detector Monitors the output of the directional coupler and adjusts the control
voltages to the driver amplifier and the discrete final power amplifier. Finally, the RF signal is routed
to the main antenna.
Trident IC
Synthesizer
Loop Filter
TX VCO
RX/TX
VCO
RF Switch Matrix
TX Buffer Amp
TX Driver Amplifier
Transmitter Final
FET 900 MHz
Harmonic
LP Filter
Ref. Oscillator
Antenna Connecto
Directional
Coupler
Antenna
Switch
TO RX
GPS
Reverse Power
Detection
Log Amp Power Detector
Digital RF Attenuator
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