Motorola APX 5000 Service Manual page 186

Sec 2: 3-6
3.2.1.4 UHF2 Front-End
From the RX/TX select switch, a UHF2 signal is routed to the first pre-selector filter followed by an
LNA and a second pre-selector filter. Both filters are discrete and tunable designs and are used to
band limit the incoming energy and suppress known spurious responses such as Image spur. The
output of the second pre-selector 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 and IF amplifier which drives the input of the Abacus 3 Analog to Digital Converter IC (AD9864).
3.2.1.5 700/800 Front-End
From the RX/TX select switch, the signal is routed to an RF switch which selects the 700 or 800 Mhz
band signal and routes it through a filter, an LNA, another filter, and another LNA. All filters are
Surface Acoustic Wave (SAW) designs used to band limit the received energy and suppress known
spurious responses. The output of the second 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 and an amplifier which drives the input of the Abacus 3 Analog to Digital Converter IC
(AD9864).
3.2.1.6 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.
Basic Theory of Operation: Analog Mode of Operation