E.F. Johnson 242-2008-332, Руководство пользователя

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6-1
September 1994
Part No. 001-2008-300
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 and
two 4-pole crystal filters and one 6-pole 450 kHz filter
establish receiver selectivity. A Receiver block dia-
gram is located in Figure 6-1.
6.1.2 REGULATED VOLTAGE SUPPLIES
The +15V DC power source is supplied by the
Universal Station power supply. The +15V supply
enters the receiver on J201, pin 1. U206 provides the
+12V DC receive voltage to the RF and IF amplifi-
ers. U207 su12V DC to the remaining cir-
cuits. U208 su6V DC to the receiver.
6.1.3 HELICAL FILTER (L201-L203), RF AMPLI-
FIER (Q201)
The receive signal enters the receiver on coaxial
connector A201. A helical filter consisting of L201,
L202 and L203 is a three-pole bandpass filter tuned to
pass only a narrow band of frequencies to the receiver.
This filter also attenuates the image and other
unwanted frequencies.
Impedance matching between the helical filter
and RF amplifier Q201 is provided by C201, C202
and a section of microstrip. Q201 amplifies the
receive signal to recover filter losses and increases
receiver sensitivity. Biasing for Q201 is provided by
R201/R203 and C203/C204 provide bypassing. Addi-
tional filtering of the receive signal is provided by
three-pole helical filter L204-L206. A section of
microstrip on the collector of Q201 and C205/C207
match the impedance from Q201 to the filter.
6.1.4 FIRST MIXER (U201), CRYSTAL FILTER
(Z201/Z202)
First mixer U201 mixes the receive frequency
with the first injection frequency to produce the 52.95
MHz first IF. Since low-side injection is used, the
injection frequency is 52.95 MHz below the receive
frequency. Matching between filter L204-L206 and
the mixer is provided by L228 and C208. The output
of U201 is matched to Z201 at 52.95 MHz by L207,
C209 and C267.
Z201 and Z202 form a two-section, four-pole fil-
ter with a center frequency of 52.95 MHz and a -3 dB
bandwidth of 15 kHz. This filter attenuates adjacent
channels and other signals close to the receive fre-
quency. The filter sections are a matched pair and the
dot on the case indicates which leads connect together.
Matching with Q202 is provided by C210, L209 and
C270.
6.1.5 IF AMPLIFIER (Q202), CRYSTAL FILTER
(Z203/Z204)
Q202 amplifies the 52.95 MHz IF signal to
recover filter losses and increase receiver sensitivity.
Biasing for Q202 is provided by R208/R209/R210/
R211/R313 and C212/C213 provide bypassing. The
output of Q202 is matched to crystal filter Z203 at
52.95 MHz by R313, L211, C214 and C293.
Z203 and Z204 form a two-section, four-pole fil-
ter with a center frequency of 52.95 MHz and a -3 dB
bandwidth of 15 kHz. This filter establishes the selec-
tivity 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 U202 is provided by C215, C301,
L225, C216, C338 and R322.