HP 8340A Service Manual page 154

INTRODUCTION
The function of the A38 Phase Lock.Loop 1 is basically to mix the
output of the A39 PLL3 Up Converter
LO output from the A36 PLL l VCO
this assembly is the difference frequency suitably filtered and
amplified to about -10 dBm.
LO AMPLIFIER A
The LO Amplifier consists of common-emitter amplifiers Ql and Q2.
CRl and CR2 provide limiting to prevent overdriving Q2 near 200
MHz where Ql has more gain.
MIXER
B
The double-balanced mixer Ul operates with about +7 dBm LO drive
and with approximately -30 dBm RF signal input. The IF output is
about -36 dBm and covers 30 MHz to 140 MHz. The 185
Filter attenuates the harmonics of the RF signal input. The 10 dB
pad, RlO, Rll, and Rl2 reduces the RF signal input from
approximately -20 dBm to approximately -30 dBm.
IF INPUT AMPLIFIER
The IF INPUT AMPLIFIER has an input filter to partially filter
the RF and LO signals from the mixer. The amplifier Q3 has
emitter degeneration Rl7 to reduce distortion.
IF OUTPUT AMPLIFIER
The IF OUTPUT AMPLIFIER con. s is ts of two common emi tt. e r stages and
an output low-pass filter. The two stages are coupled by C l4 and
Ll7 which provide high frequency peaking�
LOW-PASS FILTER
140 MHZ
The 140 MHz LOW-PASS FILTER is a modified elliptic filter which
must pass 140 MHz while rejecting 160 to 166 MHz by at least 60
dB. The three adjustable coils optimize the stop band by
providing nulls at the frequencies shown on the schematic
8C-18). This filter also filters the LO frequencies
MHz).
Model 8340A - Service
A38 PHASE LOCK LOOP
CIRCUIT DESCRIPTION
C
D
E
Scans by HB9HCA and HB9FSX
(PLLl) IF,
1
(160.15 to 166 MHz) with the
(200 to 300 MHz). The output of
MHz Low-Pass
(Figure
(200-300
8-209/8-210