SEA 157S, Предварительное руководство по обслуживанию

Страница: 34 / 57

DIGITAL SQUELCH PROCESSING: As described above, the digitized
receiver audio is filtered by a 3 KHz high pass filter and fed to
the digital squelch processor. It is similar to a traditional
noise activated squelch. The DSP calculates the magnitude of audio
energy above and below 3 KHz and uses this in conjunction with the
squelch threshold setting to intelligently decide whether to open
or close the digital squelch gate.
5.3.1 CHANNEL 70 WATCH RECEIVER:
GENERAL: The 156.525mHz Channel 70 monitor receiver is a single
channel, dual conversion, crystal controlled stand alone FM
receiver. The first IF frequency is 21.4mHzand and the second IF
is 450kHz.
RF FRONT END: RF from hybrid splitter, T1, on the main receiver
board passes through a pad and then a highpass filter to the
source of the low noise, grounded-gate RF preamplifier Q1. After
being amplified by Q1, the signal passes through the three stage,
top coupled bandpass filter consisting of L3, L4, L5 and their
associated capacitors. The output of the bandpass filter is
applied to the input of the active first mixer, U1. A -7 dBm LO
low-side injection local oscillator signal is also provided to the
first mixer by First Local Oscillator circuit described below.
FIRST LOCAL OSCILLATOR: The first local oscillator is an overtone
crystal controlled oscillator/doubler circuit consisting primarily
of Y1, L6, Q2, L7, L8 and their associated components. The
overtone crystal Y1 operates at 67.5625mHz and frequency trimming
is provided by L6. The crystal frequency is doubled by Q2 and the
resulting 135.125mHz LO signal is filtered by the bandpass filter
consisting of L7, L8 and their associated capacitors.
21.4mHz Filter/First IF AMPLIFIER: The output of the mixer U1 is
directed through the 21.4mHz, 4 pole crystal FL1 to the input of
the first IF amplifier Q3. The output of Q3 is applied to the
input to of the FM IF system U4.
450kHz SECOND IF AMPLIFIER: The input stage of the FM IF system U4
consists of a Gilbert cell mixer that is driven by both the
output of the 21.4mHz first amplifier Q3 and the 21.85mHz master
reference oscillator. The resultant output of the Gilbert cell
mixer is a 450kHz second IF signal which applied to the input of
the 450kHz ceramic filter FL2. The output of FL2 is returned to
the input of the second IF amplifier located within U4. The
output of the second IF amplifier is passed out of the FM IF
system U2 through the 450kHz ceramic filter FL3 and returned to
the limiter-amplifier located within U2.
QUADRATURE DETECTOR: The output of the limiter-amplifier of U4 is
internally fed to the quadrature detector whose phase shifting
network is provided by the ceramic discriminator Disc1. The output
of the discriminator is buffered by an amplifier and the raw
baseband detected audio emerges from U4 on pin 8.
5.4