E F Johnson Company 2422001-1 Benutzerhandbuch

6-1

August 2000

Part No. 001-2001-200

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 

±

2.5 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 
Figure 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 L101, 
L102 and L103 is a three-pole bandpass filter tuned to 
pass only a narrow band of frequencies within the 
132-178 MHz band.  This filter also attenuates the 
image and other unwanted frequencies.

Impedance matching between the helical filter 

and RF amplifier U103A is provided by C102.  
U103A amplifies the receive signal to recover filter 
losses and increases receiver sensitivity.  Biasing for 
U103A is provided by R105/R106/R107/R108 and 
C105, C106, C107 and C108 provide RF bypass.  
Additional filtering of the receive signal is provided 
by 3-pole helical filter L108-L110.  C103/C104 match 
the output from U103A 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.