Receiver Front-End; Vhf Band; General; Theory Of Operation - Motorola ASTRO Digital Spectra Plus Detailed Service Manual

Theory of Operation: Receiver Front-End
3.6

Receiver Front-End

This section of the theory of operation provides a detailed circuit description of receiver front-end
(RXFE). When reading the Theory of Operation, refer to your appropriate schematic and component
location diagrams located in
Lists". This detailed Theory of Operation will help isolate the problem to a particular component.
However, first use the ASTRO Digital Spectra and Digital Spectra Plus Mobile Radios Basic Service
Manual to troubleshoot the problem to a particular board.
3.6.1

VHF Band

3.6.1.1 General

The Receiver Front-End (RXFE) performs the first conversion of the received signal. The inbound
signal is mixed with the high side injection signal, to produce the 109.65 MHz first IF. The pre-amp/
mixer configuration of the RXFE includes a preamplifier, a factory-tuned, 5-pole L.C. preselector
unique for two ranges, a fixed injection filter, and a double balanced mixer.

3.6.1.2 Theory of Operation

The RF input from the PA first enters the high pass filter consisting of components L3200, L3201,
L3202, C3200, C3201, C3209, and C3210. The high pass filter attenuates signals below the receiver
passband for both RF frequency ranges.
A pair of Schottky diodes (CR3200) located before the high pass filter and after the 5-pole L.C.
preselector, limit the signal amplitude going into the preamplifier. A second pair of Schottky diodes
(CR3201) located after the 5-pole L.C. preselector, further provide signal protection to the mixer.
The RF board supplies DC voltage to the pre-amp. Transistors Q3200 and Q3201 stabilize the bias
for pre-amp device Q3202 through temperature changes. R3206, R3200, R3210, R3208, and R3209
are adjusted to meet radio performance specifications for High or Low sensitivity.
The factory-tuned preselector filter accepts RF input frequencies ranging from 136–162 MHz (Range
1) or 146–174 MHz (Range 2). L3100, L3101, L3102, L3103, L3104 comprise the set of inductors
which are tuned by the factory.
The double-balanced mixer has an injection level of +20 dBm, common for both ranges; at its output,
a diplexer helps terminate the IF port at all frequencies of interest, and forms the bandpass filter.
From the pre-amp input to the IF output, there should be a conversion gain of -1.5 to +3.5 dB for high
sensitivity, and +7.0 to +10 dB for low sensitivity specifications.
3.6.2

UHF Band

3.6.2.1 General
The receiver ceramic filter has a typical insertion loss of about 0.5 to 1.5 dB; it should not have a loss
greater than 2.0 dB. If any soldering must be done on the filter, be very careful not to get any solder
on the filter tuning pads.
The injection filter is a printed pattern on the substrate which is laser-tuned at the factory. The
insertion loss of this filter is about 3 dB.
6881076C25-E
"Chapter 7. Schematics, Component Location Diagrams and Parts
3-53
September 5, 2008