Motorola ASTRO XTSTM 4000, Detailed Service Manual

Page: 30 / 210

May 28, 2008 6871620L01-C
3-4 Theory of Operation: Transceiver Section
3.1.1.4 Power Conditioning Components
DC power-conditioning components include zener diodes, capacitors, ferrite beads, a power
inductor, and the fuse. Diode VR3 provides over-voltage protection. Ferrite beads (designated
E1104, etc.) and capacitors suppress electromagnetic interference from the transceiver. The
power-line filter consisting of C4, C6, C20 and C24, suppresses digital noise from the VOCON
section switching power supplies that could degrade the transmitter spectral purity.
Pass transistor Q1301 switches the battery voltage to the transceiver when control signal SWB+ or
XSWB+2
from the VOCON section is asserted high. This increases the transceiver’s immunity to
conducted interference that might be present on SWB+ or XSWB+2, such as from switching voltage
regulators on the VOCON section.
The gold plated perimeter of the PC Board surfaces are critical to the GROUNDing of the radio
system. The front flip assembly and radio chassis assembly sandwiches the PC Board to create a
clam-like shielding design for the radio. The arrangement creates a good GROUNDing connection
for the entire radio assembly. The radio GROUNDing is a necessary electrical reference point to
complete the antenna circuit path. Shields SH01 through SH11 appear on the schematic to show
their connection to GROUND.
3.1.2 Receiver
The XTS 4000 transceiver has a dual-conversion superheterodyne receiver. Figure 3-3 illustrates
the major receiver components:
• Receiver Front End
• Receiver Back End
Figure 3-3. Receiver Block Diagram
3.1.2.1 Receiver Front End
NOTE: Refer to Table 8.1, “List of Schematics and Boards Overlays,” on page 8-1 for a listing of
receiver front end schematics.
The receiver front end tunes to the desired channel and down converts the RF signal to the first
intermediate frequency (IF). Channel selection is by way of a tunable local oscillator, RXLO, from the
FGU.
RF Input
RX Front End
Harmonic
Filter
ADC
LO
CKO
RX_SSI_ DATA
to VOCON Section
XTAL
Filter
ABACUS III - RX Back End
Antenna
Switch
LNA
Tuneable
Post-Selector
Filter
Tuneable
Pre-Selector
Filter
1st
Mixer
1st LO
3
MAEPF-27278-B