Nuts and Volts - September 2013

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 and temperature.

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.