FX 146
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94
VCO CIRCUIT SUMMARY:
The control voltage for the D3 and D23 varactor diodes is supplied through
R47 and R25 by the output of U5:A in the PLL synthesizer circuit.
We know already that there must be a 21.4 MHz difference between the
receive and transmit frequencies of the VCO. This swing cannot be
accomplished by PLL programming alone. The VCO must be able to stay "in
range" with the synthesizer. D3 and D23 work in series during transmit,
which reduces their total capacitance to one-half (per the standard formula
for capacitors in series). For example, if a given control voltage runs both
diodes at 5 pf, the actual capacitance is 2.5 pf. In receive, the +8R through
D1 causes D3 to be shunted by C39, which causes D23 alone to control the
VCO L-C circuit, introducing twice as much capacitance and thereby
lowering the frequency.
Transistor Q5 is a common base buffer which affords good isolation, low
input impedance and broadband characteristics. The output from Q7 is fed
into U3, 12017, a Ã64/65 prescaler which divides down the VHF signal to a
lower frequency which the PLL chip can process. The output is further
buffered and amplified by Q16, the VCO buffer which feeds U1 through C35
for receive, and Q10 through C56 for transmit.
Another important role of the VCO is that it is the stage that is modulated by
microphone amplifier U4, which is discussed in Stage M. Notice that the
microphone amplifier output is coupled through C62 directly to the voltage
control line for the varactor diodes. Therefore, any voltage variation imposed
on that line will vary the VCO frequency in step with that variation. If the
frequency is varied in step with speech patterns or other audio signals such
as tone shifts, we are generating 'frequency modulation', hence 'FM'.
D5 and R31 perform an interesting function. Remember that the VCO control
voltage has a range of about 1.0 volts DC (low frequency) to 7.0 volts (high
frequency). Therefore, more modulation voltage is needed at higher VCO
frequencies. As the VCO control voltage increases, D5 turns on and places
R31 in parallel with R33, reducing the resistance in the line to half and
thereby increasing available modulation voltage.
Finally, it should make sense that the VCO should have a very pure source
of well-filtered DC, completely free of AC hum, alternator whine or other
disturbance. R19 and 47 uf C40 form a basic low pass filter. Transistor Q4
serves as an electronic capacitance multiplier. The actual effect of the filter
is that the beta of Q4 multiplies the 47 uf to the equivalent of installing a
huge 4000-5000 uf capacitor in the VCO area.
FX-146
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53
D: DIODES (All Types)
[ ] [ ] C82
10 pf
TX10
[ ] [ ] C83
.001
M2
[ ] [ ] C84
35 pf trimmer
TX32
TX final output tuning
[ ] [ ] C85
.1 uf
G38
PLL loop filter
[ ] [ ] C86
.001
M1
[ ] [ ] C87
39 pf
G4
[ ] [ ] C88
.001
M3
[ ] [ ] C89
.001
M15
[ ] [ ] C90
2.2 uf elec.
G13
PLL loop filter
[ ] [ ] C91
.01
G24
[ ] [ ] C92
2.2 uf elec.
G14
PLL loop filter
[ ] [ ] C93
4.7 or 10 uf elec.
M14
[ ] [ ] C94
3.9pf
F19
[ ] [ ] C95
4.7 or 10 uf elec.
A6
[ ] [ ] C96
4.7 or 10 uf elec.
G12
[ ] [ ] C97
.01
TX14
[ ] [ ] C98
.01
G25
[ ] [ ] C99
.01
E17
[ ] [ ] C100
4.7 or 10 uf elec.
A11
[ ] [ ] C101
4.7 or 10 uf elec.
CR24
[ ] [ ] C104
8.2 pf
DR23
[ ] [ ] D1
1N914 or 1N4148
E15
Shunts D3 with C39 in receive
[ ] [ ] D2
BA482 PIN
M34a
Grounds RX RF input in transmit
[ ] [ ] D3
BB505 varactor
E11
VCO L-C circuit with D23
[ ] [ ] D4
1N914 or 1N4148
DR9
Turns on D6 during receive
[ ] [ ] D5
1N914 or 1N4148
M38
Switches in R31 at high VCO freq
[ ] [ ] D6
BA479 PIN
DR1
See T-R switch theory
