Chapter 3 Theory of Operation
This chapter provides a detailed circuit description of the ASTRO APX 2000/ APX 4000/ APX
4000Li/ APX 1000 (900 MHz) mainboard and keypad boards. When reading the theory of operation,
refer to the appropriate schematic and component location diagrams located in the back of this
manual. This detailed theory of operation can help isolate the problem to a particular component.
The ASTRO APX 2000/ APX 4000/ APX 1000 radio, which is a single-band synthesized radio, is
available in the following frequency bands.
• VHF (136–174 MHz)
• UHF1 (380–470 MHz)
• UHF2 (450–520 MHz)
• 700/800 MHz (764–870 MHz)
• 900 MHz (896–941 MHz).
And the ASTRO APX 2000 M1.5, APX 4000Li M1.5 and APX 4000Li M2, APX 2000/ APX 4000 (Two
Knobs) is available in the following frequency bands.
• VHF (136–174 MHz)
• UHF1 (380–470 MHz)
• UHF2 (450–520 MHz)
• 700/800 MHz (764–870 MHz)
All ASTRO APX 2000/ APX 4000/ APX4000 Li radios besides the radios with 900 MHz are capable
of analog operation (12.5 kHz or 25 kHz bandwidths), ASTRO mode (digital) operation (12.5 kHz
only), X2-TDMA mode (25 kHz only) and Phase 2 TDMA mode (12.5 kHz only).
For radios with 900 MHz, they support analog operation (12.5 kHz only), ASTRO mode (digital)
operation (12.5 kHz only), and Phase 2 TDMA mode (12.5 kHz only).
NOTE: The APX 2000 M1.5, APX 4000Li M1.5 and APX 4000Li M2 radio do not support any Global
Positioning System (GPS), Bluetooth, MACE and Accelerometer functions. As such,
disregard all references to the functions mentioned above in “Chapter 3 Theory of Operation”.
The ASTRO APX 2000/ APX 4000/ APX 4000Li/ APX 1000 (900 MHz) radio (Figure 3-1) consists of
the following:
• Main Board – Contains all transmit, receive, and frequency generation circuitry, including the
digital receiver back-end IC and the reference oscillator. The mainboard also contains a dual
core processor, which includes both the microcontroller unit (MCU) and a digital signal
processor (DSP) core, the processor’s memory devices, an audio and power supply support
integrated circuit (IC), a digital support IC, external power amplifier, as well as combination
Global Positioning System (GPS) and Bluetooth 2.1 IC and front-end circuitry.
• Top Control
- APX 2000/ APX 4000/ APX 4000Li/ APX 1000 (900 MHz): Contains a Multi-Function knob,
a push button switch used for Emergency call and a light bar. The top control also includes
TX/RX LED that is solid amber upon receive, red on PTT, and blinks amber on secure TX/
RX.
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