Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 110

WJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
3.4.5
2ND LO SYNTHESIZER (FE-A1A2A1) (794270)
The Preselector mounts on the Frequency Extender, FE-Al, Assembly (see
paragraph 3.3.2). Figure 3-12 is a detailed block diagram of the 2nd LO Synthesizer
which should be referred to in the following circuit description. Figure 6-8, 2nd LO
Synthesizer Schematic Diagram, may be referred to for greater component level detail,
if desired.
The 2nd LO consists of a VCO and a Synthesizer. Ql is the VCO
oscillator. Frequency of oscillation is controlled by CRl, C15, C19 and L2. Actual
oscillating frequency is set by the tuning voltage from the Synthesizer portion which
drives varactor CRl.
Ql output is amplified by buffer Q2. The drain output is the
2nd LO output which drives the 2nd converters.
The source output is the VCO sample
which drives the Synthesizer portion.
The Synthesizer portion consists of synthesizer chip Ul which is hard
wired for a fixed internal divide ratio. The VCO sample from Q2 is divided by dual-
modulus divider U2 and drives the Ul-Fin input. Ul compares the Fin input with the
1 MHz reference input. The Ul OR and OV outputs consists short spikes (+5 Vdc
peak) when the synthesizer is locked. The Ul output is integrated by U3 to an
average dc level (typically +3 Vdc when locked) and then sends it to CRl as tuning
voltage.
Ql typically oscillates at approximately 189 MHz. The tuning voltage
from U3 adjusts the frequency Ql until it oscillates at exactly 189 MHz.
TUNING
DATA
(FIXED!
1 MHZ
REF
V w
SYNTH.
Ul
VCO
Ql,Q2,CRl
CTRL
U3
1
1
U2
Figure 3-12. 2nd LO Synthesizer, FE-A1A2A1, Block Diagram
2ND
LO OUT
3-27