would have to be solid-state and
operate from a 12 volt DC power
source. Each would need to have a
frequency range of 25 MHz to 30
MHz, either in a continuous tuning
or in 1 MHz bands, and have either a
digital readout or an output for a
frequency counter. Thank you for
your time and help.
— Bradley Flener
ASince it is not legal to perate a VFO outside the amateur frequencies of 28.000 to 29.700 MHz,
I have limited this design to those
frequencies. The circuit must have
good stability relative to vibration
I recommend mounting the
circuit in a die cast box with a
temperature compensating heater to
maintain the box at 50 degrees C.
Or, alternatively, operate in a
temperature controlled environment.
I simulated the design (Figure 4)
in SwitcherCAD IV — a free SPICE
analysis tool from Linear Technology
— and tweaked the L-C values. I was
designing around a 3. 3 to 23. 3
pico-farad variable capacitor that
was on eBay.
However, realizing that the
capacitor will not be available when
this goes to print, I found values for
the standard 365 pF variable that is
used in AM broadcast radio:
C5 = 18 pF; C4 = 27 pF.
The design is standard Colpitts
and the value of C7 is not critical; it
can vary from 1,000 pF to 3,000 pF.
The voltage on the coil is about 100
volts, so C4, C5, C6, and C8 need
high voltage ratings. C8 provides
isolation of the impedance of Q1
from the tuned circuit to maintain
high Q. I tapped the output down on
the emitter resistor (R3, R4) for
added isolation, and used a two
transistor amplifier which gives a high
degree of isolation of load effects. If
the counter and output signals are
not equal, you may need to adjust
the voltage divider R8, R9.
■ FIGURE 5.
20 September 2013