Model 8340A - Service
The 160 MHz bandpass filter selects the 160 MHz signal that is
amplified by Q5. Ll8, C49 and C50 form a tank circuit that is
tuned to 160 MHz which also helps to reject unwanted frequency
components from the output of U3.
R46, R47 and R48 form a 5 dB pad keeping the distortion down in
the mixer by providing a better 50 ohm match to the mixer input.
MIXER
B
U7 is a double-balanced mixer which operates with about +7 dBm of
LO power and approximately -20 dBm of RF signal power at .160 MHz.
The desired output is the difference frequency and lies between
150 kHz and 6 MHz.
IF AMPLIFIER
C
The output of the mixer U7 is filtered by an 8-section elliptic
10 MHz Low Pass filter which must reject 14 to 20 MHz, and other
higher frequencies. The filter output is amplified by Q3.
160
TO
166 MHZ
VCO
D
The 160 to 166 MHz VCO uses Ql in a Colpitts configuration. The
tank circuit is formed by Lll, C33, C36, C37 and CR3 (a varactor
diode). C33 and C36 provide a capacitive divider for the signal
that is fed back into the emitter of Ql. Lll is adjustable
allowing the center of the
vco
tuning range to be varied.
Q2 is connected in a common base configuration and is used by
PLLl as a buffer amplifier for the 160-166 MHz output. Q4 is
connected in a common emitter configuration and is used as a
buffer amplifier for the 160-166 MHz signal that drives the LO
port of the mixer.
PHASE/FREQUENCY DETECTOR
E
The PHASE/FREQUENCY DETECTOR generates a differential output
signal that i}s· used by the LOOP AMPLIFIER (Block F) as well as
the PHASE LOCK INDICATOR (Block
G).
The main components of the
PHASE/FREQUENCY DETECTOR consist of two ECL flip-flops and three
ECL NOR gates.
U6B and U6C generate ECL level inputs for the flip flops from the
two frequencies to be compared. R25 and R26 center the input
frequency signals to the midpoint of the ECL logic level
thresholds so the PHASE/FREQUENCY DETECTOR will be less sensitive
to amplitudes of the two comparison frequencies.
8-218
Scans by HB9HCA and HB9FSX
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