Watkins-Johnson Company WJ-8718-19/FE Instruction Manual page 108

WJ-8718-19/FE HF RECEIVER
CIRCUIT DESCRIPTION
3.4.3
VCO FREQUENCY (MHZ)
E2
E3
E4
190 to 200
0
• 0
0
200 to 210
1 0
0
210 to 220
0
1
0
220 to 230
1
1 0
230 to 240
0
0
1
240 to 250
1 0
1
250 to 260
0
1
1
NOTE: 0 = 0 Vdc: 1 = +5 Vdc
VCO (FE-A1A1A2) (370960)
The VCO mounts on the Frequency Extender, FE-Al, Assembly (see
paragraph 3.3.2) Figure 3-10 is a detailed block diagram of the VCO which should be
J?L?r11 nl? inihf f°llowinS circuit description. Figure 6-5, VCO Schematic Diagram
may be referred to for greater component level detail, if desired. ocnemauc Ula?ram,
The VCO consists of oscillator transistor Ql. The oscillation frequency
of Ql is controlled by C6/C7/CR4, T3 and L2/CR1, L3/CR2, L4/CR3. Inductors L2 13 14
are tapped across T3 and are controlled by bandswitch data from the 1st LO through
CR1-CR3. By switching CR1-CR3 high or low, L2-L4 may be switch in or out of the
tuning circuit allowing the tuning range of Ql to increment in 10 MHz steps between
190 and 260 MHz.
Diode CR4 is a varactor that swings the frequency of Ql over a 10 sweep
as the tuning voltage goes from +1 to +4 Vdc. The tuning data going into the 1st LO
Synthesizer sets the tuning voltage at the correct level to lock the Ql oscillation
frequency at the desired frequency.
Ql output is amplified by buffers Q2 and Q3. Each buffer has broad band
tapped down transformers in the collectors for stability and impedance matching The
output from Q2 is the VCO sample sent back to the 1st LO. The output from Q3 is
the 1st output which is sent to Input Converter/2nd LO, FE-A2.
BAND
N
BAND
\
SWITCH
CR1-3.L2-4
l
'
TUNE
*•
VCO
BUFFER AMP
02,03,T3
01 ,CR
4.T3 *- 1st LO OUT
Figure 3-10. VCO, FE-A1A1A2, Block Diagram
3-25