This year’s “Atlanticon Kit” was a 10 MHz variable crystal oscillator, or VXO, that can be put to use in many ways around the shack. Once calibrated to WWV, this VXO can serve as an accurate frequency standard for receiver alignment, or as a PLL standard, or even as an LO for a transmitter.  

The “twist” with our Atlanticon Kit this time was not in the construction of the circuit, as we are provided a pc board, but in the accuracy and stability of the oscillator itself. We’ve provided the basic oscillator and buffer circuits, but it was up to the builder to provide any means possible to “bend” the crystal in order to place the oscillator dead-on 10 MHz … and keep it there in the presence of changing ambient temperatures. Especially during the annual construction contest held on Saturday evening at Atlanticon!

Capacitors, crystals and semiconductors have temperature coefficients that represent how the component values change with changing temperature. When the component values change, the oscillator frequency changes. This drift is not good if you want a very stable and unchanging oscillator frequency. 

So the real fun challenge for the builder is to find ways to dynamically adjust, compensate or insulate the circuit against temperature changes. The Adjust signal on connector P1 pin 1 can be a simple potentiometer that presents a nominal 4 volts to varicap D1, but that’s only a static level and if the temperature changes, the pot setting would need to be changed to compensate for that change . One could instead dynamically adjust that control signal in accordance to the ambient temperature. Or perhaps judicious placement of “negative coefficient” capacitors in the circuit could compensate for drift. An insulated enclosure, or a temperature-controlled “oven”  would also be possible solutions!

CRYSTALIZER SCHEMATIC -- Note that we found it better to omit L1 and C5 in order to allow the frequency-adjustability range to definitely reach 10 MHz.  Otherwise, with these parts in place, some units cannot quite reach 10.000 MHz when using the Adjust control input line.  Thus L1 and C5 were not included in  the second round of 200 Crystalizer kit shipments.  The C5 spot on the board should just be left open, and the L1 pads should be shorted together.