17
5 THEORY OF OPERATION
Refer to block diagram 597–8104–2 in the SCHEMATIC AND ASSEMBLY DRAWINGS section as required for
the following discussion.
5.1 Converter, 913–2132–150/–240/–330/–450/–950
The received RF signal is applied to the converter circuit board. This circuit board is: 1) assembled for specific
bands of operation and 2) can accept a 50 MHz (30 MHz for 950 MHz units) wide group of frequencies
within the specific band. The received signal passes through pre–selector filter FL1. Following the filter, the
signal is routed to a second filter network consisting of FL2, FL3, FL4, FL5, FL6, FL7, FL8, and FL9. This
network consists of filter sections divided into 12.5 MHz segments. As determined by the operating
frequency, the RF signal is routed to the appropriate filter section. Once filtered, the signal is routed to 2
frequency synthesized PLL down converter circuits. The first PLL circuit converts the signal to 70.7 MHz The
second PLL circuit converts the signal to a 10.7 MHz output. The output of the second PLL circuit is then
routed to the IF bandpass filter.
Control of the converter is provided by microprocessor U1. This microprocessor is also used to control all of
the operating functions of the unit.
5.2 IF Bandpass Filter, 800–207
The IF signal output of the converter is impedance matched to the IF bandpass filter which provides the
selectivity or channel bandwidth of the receiver. Several IF filters are available for various bandwidth
requirements. The output of the bandpass filter is impedance matched to the following stage.
5.3 IF Amplifier/Detector, 800–208
The IF amplifier chain consists of two discrete transistor stages coupled by ceramic filters to an integrated
circuit. IC–1 contains high–gain FM limiters, quadrature detection, audio preamplification and wide range
signal level metering. IF symmetry at the IC input is compensated by L1 and C19.
5.4 Audio Amplifier, 800–228
Audio from the FM detector first passes through notch filter IC1B, to remove the subaudible encoding tone.
This signal is then fed into the IC1A de–emphasis circuit. The feedback circuit of this op–amp has a fixed 75
micro- second loop with an additional 75 microseconds selected by Q1 to provide noise–reduction on weak
signals of 1.5 microvolts or less. Q1 is switched on by the microprocessor on the converter circuit board.
Following the de– emphasis circuit the signal passes through two stages of Butterworth low–pass filtering
(IC2A and IC2B), then to a dual op–amp for the audio output (IC3A and IC3B). Diodes D1 and D2 rectify the
output audio for level metering. A
600/600 ohm transformer isolates the output op–amp from the line. One op–amp of IC3 drives the line
output, the other drives the headphone receptacle.
5.5 Tone Decoder, 800–229
Pre–emphasized audio from the FM detector at connector pin 5 is fed into the inverting input of IC1A, which
functions as a low–pass filter/amplifier. Since subaudible tones in the band of 25–29 Hz are to be selected
from the wide–band audio, the low–pass filter/amplifier attenuates the higher frequencies at a slope of about
To Next Page
Loading...