Motorola APX 1000 Basic Service Manual page 274

3-4
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
for application to the voltage controlled oscillator as a modulation signal.
TX Buffer Amp
TX Driver Amplifier
TX VCO
RX/TX
RF Switch Matrix
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.
Figure 3-4) via the SSI interface. The Trident IC processes the SSI data
Antenna
Switch
Transmitter Final
FET 900 MHz
TO RX
Loop Filter
Trident IC
Synthesizer
Figure 3-4. Transmitter (900 MHz) Block Diagram
Basic Theory of Operation: Analog Mode of Operation
Log Amp Power Detector
Digital RF Attenuator
Directional
Harmonic
Coupler
LP Filter
Reverse Power
Detection
Ref. Oscillator
Antenna Connector
GPS