Service manual

6-1
Month 2000
Part No. 001-2004-601
CIRCUIT DESCRIPTION
SECTION 6 CIRCUIT DESCRIPTION
6.1 RECEIVER
6.1.1 INTRODUCTION
The receiver is a double conversion type with
intermediate frequencies of 52.95 MHz and 450 kHz.
The first injection frequency is phase locked to a tem
-
perature compensated crystal oscillator (TCXO) with
a frequency stability of ±1.0 PPM from -30° to +60°C
(-22° to +140°F). Two 3-pole bandpass filters in the
front-end reject signals outside the receive band. Two
4-pole crystal filters and two 4-pole ceramic filter
establish receiver selectivity (see block diagram Fig
-
ure 6-1).
6.1.2 REGULATED VOLTAGE SUPPLIES
The +15V DC power source is supplied by the
repeater power supply. The +15V supply enters the
receiver on J201, pin 1. U302 provides the +12V DC
receive voltage to the RF and IF amplifiers. U303
supplies +12V DC to the first and second injection
amplifiers. U304 supplies +12V DC to the remaining
RF circuits. U301 supplies +6V DC to the remaining
circuits.
6.1.3 HELICAL FILTERS, RF AMPLIFIER
The receive signal enters the receiver on coaxial
connector A201. A helical filter consisting of L102,
L103 and L104 is a three-pole bandpass filter tuned to
pass only a narrow band of frequencies within the
380-520 MHz band. This filter also attenuates the
image and other unwanted frequencies.
Impedance matching between the helical filter
and RF amplifier Q101 is provided by C103, L106,
and C104. Q101 amplifies the receive signal to
recover filter losses and increases receiver sensitiv
-
ity. Biasing for Q101 is provided by R102/Q102/
R107/R108 and C112 provides RF bypass. Additional
filtering of the receive signal is provided by 3-pole
helical filter L108-L110. L107/C113/C114 match the
output from Q101 to 3-pole helical filter L108-L110.
6.1.4 12.5 KHZ IF
First Mixer and Crystal Filter
First mixer U101 mixes the receive frequency
with the first injection frequency to produce the 52.95
MHz first IF. Since high-side injection is used, the
injection frequency is 52.95 MHz above the receive
frequency. Jumper J203 selects between a 12.5 kHz
IF and a 25 kHz IF. Install jumper plug P203 on J203,
pins 2-3 to select the 12.5 kHz IF. The output of U101
is matched to Z211 at 52.95 MHz by L211, C236 and
C237.
Z211A and Z211B form a two-section, four-pole
filter with a center frequency of 52.95 MHz and a
-3 dB bandwidth of 8 kHz. This filter attenuates adja-
cent channels and other signals close to the receive
frequency. The filter sections are a matched pair and
the dot on the case indicates which leads connect
together. Matching with Q202 is provided by C241,
L213 and C240.
IF Amplifier, Crystal Filter
Q202 amplifies the 52.95 MHz IF signal to
recover filter and mixer losses and improve receiver
sensitivity. Biasing for Q202 is provided by R236/
R233/R234/R235 and C242/C243/C246 provide RF
bypass. The output of Q202 is matched to crystal fil
-
ter Z212 at 52.95 MHz by C245, C247 and L214.
Z212A and Z212B form a two-section, four-pole
filter with a center frequency of 52.95 MHz and a
-3 dB bandwidth of 8 kHz. This filter establishes the
selectivity of the receiver by further filtering the 52.95
MHz IF. The filter sections are a matched pair and the
dot on the case indicates which leads connect together.
Matching with U203 is provided by C250, C251,
C252, L216 and R237.