Main Board; Table 3-1. Local Oscillator And First If Frequencies; Figure 3-3. Transceiver (Vhf) Block Diagram (Power And Control Omitted) - Motorola Astro XTS 4000 Service Manual

Basic Theory of Operation: Main Board
3.2.2 Transmitting
When the radio is transmitting, microphone audio is passed through gain stages to the CODEC,
where the signal is digitized. The CODEC passes digital data to the DSP, where pre-emphasis and
low-pass (splatter) filtering are done. The DSP passes this signal to a digital/analog converter (DAC),
where it is reconverted into an analog signal and scaled for application to the voltage-controlled
oscillator as a modulation signal
Reference
Oscillator
EPIC
FracN
3
MOD
IN
LPF
DAC
ABACUS III
3
Sample
Clk
Figure 3–3. Transceiver (VHF) Block Diagram (Power and Control Omitted)
Transmitted signaling information is applied to the DSP from the microcontrol unit, where it is coded,
and passed to the DAC, which handles it the same as a voice signal. The DAC output connects to
the synthesizer modulation input. A modulated carrier is provided to the transmitter power amplifier,
which transmits the signal under dynamic power control.
3.3

Main Board

3.3.1 Transceiver Operation
Refer to Figure 3–3, on page
amplifier, a post-selector filter, and a mixer. Both the pre-selector filter and pro-selector filter are
varactor-tuned bandpass filters, controlled by the microcontroller. See
(LO) and first IF information.
(See Figure
VCOBIC
Loop
Filter
VCO
VCO
Crystal
Filter
2ND
LO
3-3. The receiver front end consists of a pre-selector filter, low-noise
Table 3–1. Local Oscillator and First IF Frequencies
LO Frequency Range
First IF Frequency
3–3).
Power
TX TX Driver
Buffer Amplifier Module
Current
Sensing
PCIC
Post-Selector RX LNA
Mixer
Filter
VHF
180.85–218.85 MHz 306.65–396.65 MHz
44.85 MHz
Antenna
Switch
Pre-Selector
Filter
MAEPF-27322-O
Table 3–1 for local oscillator
UHF1
73.35 MHz
3-3
To
Antenna
Harmonic
Filter