Johnson DM3474, Руководство по обслуживанию

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CIRCUIT DESCRIPTION
4-8
Part No. 001-3474-002
4.3 RECEIVER CIRCUIT DESCRIPTION
4.3.1 HELICAL FILTER (Z201), RF AMPLIFIER (Q201)
Capacitor C201 couples the receive signal from the antenna switch to helical filter Z201. (The antenna switch
is described in Section 4.4.5.) Z201 is a bandpass filter tuned to pass only a narrow band of frequencies to the
receiver. This attenuates the image and other unwanted frequencies. The helicals are factory set and should not be
tuned.
Impedance matching between the helical filter and RF amplifier Q201 is provided by C203, C204 and L201.
Q201 amplifies the receive signal to recover filter losses and also to increase receiver sensitivity. Biasing for Q201
is provided by R201, R202 and R203; and C208/C209 provide RF bypass. CR201 protects the base-emitter
junction of Q201 from excessive negative voltages that may occur during high signal conditions. Additional
filtering of the receive signal is provided by Z202. L202, and C205 provide impedance matching between Q201
and Z202. Resistor R204 is used to lower the Q of L202 to make it less frequency selective.
4.3.2 MIXER (U221), FIRST LO AMPLIFIER (Q301)
First mixer U221 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. The
RF signal is coupled to the mixer through C206.
The first injection frequency from the VCO is coupled to the first local oscillator amplifier Q301 through
C301. L301 and C302 match Q301 to the VCO. Bias for Q301 is provided by R301, R302 and R303, and C303
decouples RF signals. Impedance matching to the mixer is provided by L302, R304 and C304.
4.3.3 AMPLIFIER (Q222), CRYSTAL FILTER (Z221/Z222), IF AMP (Q221)
The output of U221 is coupled to buffer Q222. C222, R229 and Q222 match the 50 ohm output of U221. Bias
for Q222 is provided by R228 and R229. The output of Q222 is matched to crystal filter Z221 via L222, C223 and
R230. This filter presents a low impedance to 52.95 MHz and attenuates the receive, injection, and other
frequencies outside the 52.95 MHz passband.
Z221 and Z222 form a 2-section, 4-pole crystal filter with a center frequency of 52.95 MHz and a -3 dB
passband of 8 kHz (12.5 kHz BW) or 15 kHz (20/25 kHz BW). This filter establishes the receiver selectivity by
attenuating the adjacent channel and other signals close to the receive frequency. C232, C224, and L223 adjust the
coupling of the filter. L224, C225 and C227 provide impedance matching between the filter and Q221.
IF amplifier Q221 amplifies the 52.95 MHz IF signal to recover filter losses and improves receiver sensitivity.
Biasing for Q221 is provided by R222, R223, R225 and R226 and C228, C229 decouple RF signals. The output of
Q221 is coupled to the detector by C230.