Motorola APX 1000 Basic Service Manual page 169

Basic Theory of Operation: Analog Mode of Operation
3.2.1.4 700/800 MHz Front-End
From the 15 dB Step Attenuator, a 700/800 MHz band signal is routed to the first band SPST switch
which selects the 700 or the 800 band signal and routes it to the appropriate first pre-selector filter. A
second band select switch selects the output of the appropriate filter and applies it to an LNA
followed by a similar pre-selector filter/ band-select switch circuit. The signal is then routed to a
second LNA whose output is applied to a discrete image filter. Both preselector filters are Surface
Acoustic Wave designs used to band limit the received energy and suppress known spurious
responses such as Image and the ½ IF spur. The output of the discrete image filter is applied to the
RF port of the Mixer IC. The Mixer IC is also excited by a Local Oscillator (LO) signal at the LO port
to down-convert the RF signal to a 109.65 MHz intermediate frequency (IF). The down converted IF
signal is passed through a crystal filter which drives the input of the Abacus 3 Analog to Digital
Converter IC (AD9864).
3.2.1.5 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.
DC_RAW_B+
Current
Detection
TX
Buffer Amp
ALC
RF Switch Matrix
Figure 3-6) via the SSI interface. The Trident IC processes the SSI data
TX Driver
Amplifier
PA
Loop Filter
TX VCO
RX/TX
Figure 3-6. Transmitter Block Diagram
50
Antenna
Ohm
Switch
Harmonic
Filter
TO RX
Trident IC
Synthesizer
To Antenna
Directional
Coupler
RX
PDADI
Ref. Oscillator
3-5