FX 146
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54
FL: FILTERS
[ ] [ ] D7
BA482 PIN
M34b
See T-R switch theory
[ ] [ ] D8
1N914 or 1N4148
G34
works with D10,C85: see PLL theory
[ ] [ ] D9
1N914 or 1N4148
A18
[ ] [ ] D10
1N914 or 1N4148
G35
see D8
[ ] [ ] D11
1N914 or 1N4148
M31
assures positive switching by U4D
[ ] [ ] D12
1N914 or 1N4148
M32
" " " U4C
[ ] [ ] D13
1N914 or 1N4148
H8a
disables -RPT during receive
[ ] [ ] D14
1N914 or 1N4148
H8b
turns on Q15 during receive
[ ] [ ] D15
1N914 or 1N4148
H7
diRPT during receive
[ ] [ ] D16
1N914 or 1N4148
H9
turns off offsets during Simplex
[ ] [ ] D17
LED
M42
transmit indicator
[ ] [ ] D18
1N4002
A7
[ ] [ ] D19
1N914 or 1N4148
H6
disables AUX offset for receive
[ ] [ ] D20
6.2 volt Zener diode
CR32
[ ] [ ] D21
not used
[ ] [ ] D22
1N914 or 1N4148
M36
[ ] [ ] D23
BB505 varactor
E10
VCO L-C tuning
[ ] [ ] D24
1N914 or 1N4148
G36
[ ] [ ] D26
1N914 or 1N4148
TX36
[ ] [ ] D25-DX
1N914 or 1N4148
Stage H
Synthesizer Programming
[ ] [ ] FL1
21.4 MHz. crystal
CR30
Receiver 1st IF
[ ] [ ] FL2
455 KHz. ceramic
CR18
Receiver 2nd IF
[ ] [ ] J1
5-pin DIN
A12
Packet I/O
[ ] [ ] J2
miniature 3.5mm.
B15
Speaker connection
[ ] [ ] J3
SO-239
TX45
Antenna
[ ] [ ] J4
subminiature 2.5mm.
M43
Microphone
J: JACKS
FX-146
•
93
Stage E-F: The FX Transceiver VCO
The VCO (Voltage Controlled Oscillator) provides basic frequency control for
both transmit and receive modes. It is essential to understand its function in
the transceiver circuit. Q7 is the oscillator transistor and the key VCO
components are L7, D3 and D23.
Think of the VCO for now as just a simple 'VFO' type RF oscillator whose
output frequency can be changed by adjusting the slug in L7. Notice that
there are no capacitors, either fixed or variable, to form a 'tuned-circuit' with
L7. Instead, varactor diodes D3 and D23 perform this function. (A varactor
diode changes capacitance in step with the amount of voltage applied
across it.) If the output of this simple oscillator reached U1 through C35, you
would certainly be able to receive any signals on a frequency determined by
that oscillator frequency plus or minus 21.4 MHz, the Receiver 1st IF.
Similarly, if the oscillator output could reach Q9 and Q8 in the transmit
section, or even the transmit buffer, Q10, some sort signal could be put on
the air.
After the VCO is assembled on the PC board, the interested builder has the
option of experimenting with it in receive mode before proceeding. The VCO
must be capable of considerable frequency range. To receive a signal at 140
MHz, the VCO must tune to 140 MHz minus 21.4 MHz or 118.60 MHz. To
transmit at 160 MHz, it must tune to 160 MHz. This indicates a tuning range
of over 40 MHz!
For now, consider Q5 and Q16 to be buffer stages for oscillator Q7 and that
transistor Q4 does some nifty voltage filtering that we'll explain later on.
What we have so far is a master oscillator for the transceiver that can be
tuned by varying the voltage applied to the varactor diodes. All we really
need now is some precision way of controlling the voltage applied to those
varactor diodes so that we can put that oscillator on any VHF channel we
want with 5 KHz precision. And this control obviously must be extremely
stable, since a change of only a few picofarads can tune through many MHz
of VHF spectrum. We know we can get stability by using a crystal oscillator,
but that would not allow us to operate on a multitude of channels without
changing crystals for each channel. How can we get such precision
performance from small tuning diodes, a very ordinary shielded coil, and a
transistor with its supporting capacitors and resistors?
We guess already that the answer must lie in the functions of the Frequency
Synthesizer IC. However, the key to grasping what this IC really does lies in
understanding TWO concepts: frequency synthesis and the "Phase Locked
Loop" or PLL. The more of a handle that you can get on these two
interconnected ideas, the less mysterious will be your FX transceiver as well
most other modern radio gear - from ham transceivers to car stereos to
cellular phones. The VCO is an integral part of the phase locked loop. You
can read ahead about PLL and synthesis in the next section, or you can
proceed with building and testing the VCO alone.
