Rock Steady VFO

for 40 meters

Initially I was just looking for something to provide a continuous wave at 7.040 Mhz so I could align my RockXceiver and stumbled across this circuit for a VFO. It ended up turning out so stable I've decided to implement it as the VFO for the receiver. I got the circuit from a .pdf at Ocean State Electronic's download page: Designing and Using VFOs.

To begin with I baked a T68-6 toroid in my toaster oven for 15 min. and let it cool slowly. I then wound 33 turns of #24 enameled wire with a tap 9 turns up from the cold end. Next I boiled it in water for 15 min. and let it cool slowly once more. I didn't have any Q-Dope and heard somewhere that Sally Hansen Hard As Nails would work. I painted it with three coats and let dry thoroughly (overnight.) I checked the drift the evening I coated the coil, it was minimal. In the morning it was next to null.

This Radio Shack circuit board worked well. It has the two traces running up the middle both which I used for ground. Notice I mounted one of the 50uH chokes and the 5.6K resistor on the bottom side. The green NPO you see is in series with the variable capacitor to effect the tuning range. I could have changed the value of the 25pf caps and the variable, but adding that series cap was easier.

By my calculations the LC should resonate at 7Mhz being 6.7uH and 77.16 pF. The upper range (~7.150Mhz) would be around 73.95 pF.

I'm looking for a box to shield it in and will probably re-mount the variable capacitor accordingly to accommodate the enclosure.

The 2N2222A provides a 50 Ohm output impedance.

Output drive is around 15-25 mW.

Here we see it oscillating at 7040 Khz. It doesn't even take any warm-up to stabilize, even though it's usually about 38 degrees F. in my shack. I guess I chose the right capacitors for temperature stability. Might have had something to do with annealing that tank coil iron. And we can't forget to thank Sally Hansen.

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