zenerdiodes

Zener Diodes

Why Zener Stabilized Voltages Are Not Stable.

Take a look at the data sheet on popular Zener diodes.

Look at the far-right column where it shows temperature coefficient. Scanning down this list, it quickly becomes apparent that the most stable Zener diodes are those for 4.7 and 5.1 volts. Below that voltage the temperature coefficient goes negative, and above the temperature it goes positive. This would seem to say that if you want a stable VFO and are using Zener diodes to regulate the voltage, you should only use 4.7 or 5.1 volt diodes for the purpose. If you wanted around 9 volts of regulation you would use a pair of 4.7 diodes in series in order to get the most stable DC regulation. It might be possible to improve on this situation by selecting a series set of diodes so that the temperature versus voltage drift were complementary, but this chart shows per-cent voltage versus temperature, so just selecting opposing percentage numbers is not adequate. You would have to calculate the drift based on Zener voltage, and then select a lower voltage Zener which has a corresponding amount of voltage drift in the opposite direction.

This could explain why your well planned ultra-stable VFO...isn't.