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.

2.4.2Receiver

The SSE 5000 transceiver has a dual-conversion superheterodyne receiver. Figure 2-2illustrates the major receiver components:

Receiver front-end

Receiver back-end

2.4.2.1Receiver Front-End

NOTE: Refer to Figure 2-2for the receiver block diagram, Table 2-7for local oscillator (LO) and first IF information, and Figure 12-2for 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.

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

 

 

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.

6881094C12-A

November 11, 2004

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Motorola SSETM 5000 service manual Power Conditioning Components, Receiver Front-End, UHF Range, 450-488 MHz