Motorola SSETM 5000 - Manual (Page 31)

6881094C12-A November 11, 2004
Theory of Operation: Transceiver Board 2-9
2.4.1.5 Power Conditioning Components
DC power-conditioning components include zener diodes, capacitors, ferrite beads, a power
inductor, and the fuse. Diodes VR1 and VR2 provide over-voltage protection. Ferrite beads
(designated E1, E4, E101) and capacitors suppress electromagnetic interference from the
transceiver. The power-line filter consisting of L1, C13, and C14 suppresses digital noise from the
VOCON board switching power supplies that could degrade the transmitter spectral purity.
Pass transistor Q1 switches the battery voltage to the transceiver when control signal SWB+ or SB+
from the VOCON board is asserted high. This increases the transceiver’s immunity to conducted
interference that might be present on SWB+ or SB+, such as from switching voltage regulators on
the VOCON board.
Ground clip G9 makes contact between the transceiver board ground and the radio chassis. The
chassis connection is a necessary electrical reference point to complete the antenna circuit path.
Shields SH201 through SH702 and the tool hole appear on the schematic to show their connection
to ground.
2.4.2 Receiver
The SSE 5000 transceiver has a dual-conversion superheterodyne receiver. Figure 2-2 illustrates
the major receiver components:
• Receiver front-end
• Receiver back-end
2.4.2.1 Receiver Front-End
NOTE: Refer to Figure 2-2 for the receiver block diagram, Table 2-7 for local oscillator (LO) and first
IF information, and Figure 12-2 for the receiver front-end schematic.
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.
The receiver front-end consists of a preselector filter, an RF amplifier, a second preselector, mixer,
and an IF crystal filter. The SSE 5000 radio also contains a switchable attenuator between the
antenna switch and the first preselector filter. The RF amplifier is a discrete RF transistor with
associated circuitry. The mixer is a double-balanced, active mixer IC, coupled by transformers. The
receiver (RX) local oscillator (LO) is provided by the FGU.
2.4.2.1.1 Preselector Filters
The receiver front-end uses two discrete bandpass filters to achieve its required out-of-band
rejection. The first preselector filter precedes the RF amplifier, while the second preselector filter
follows the RF amplifier.
Table 2-7. Local Oscillator and First IF Frequencies
UHF Range 2
(450–488 MHz)
LO Frequency Range 376.65–414.65 MHz
First IF Frequency 73.35 MHz