calibratedcapacitor

Calibrated Capacitor

You do have a "Calibrated Capacitor", don't you?

Having a Calibrated Capacitor as part of your test equipment allows you to:

• Determine the inductance of junkbox inductors (just put the cap across them and tune for resonance on your Dip Meter), then calculate the inductance from frequency and the capacitor value.
• Find the amount of capacitance needed for peaking an LC network to a specific frequency. Use the Calibrated Capacitor to bring things to resonance, read the C value from the dial, and place a fixed capacitor of that value into the circuit.
• Measure impedance of input stages, or antennas, or coaxial cables. Wind up a small center-tapped balun (Trifilar winding method), connect your unknown to one side of the secondary and the Calibrated Capacitor to the other side of the secondary, and an RF source to the primary. Use an LED between center-tap of the secondary and ground to indicate match/un-match condition. When the impedances match, you will be able to calculate the impedance of the unknown from frequency and capacitance.

My particular "Calibrated Capacitor" is a fairly wide spaced 3-section unit with 15 pf to 500 pf per section. There are 4 connectors (Ground, C1, C2, & C3) along one side of the plastic housing. This lets me select capacitance range by which combination of the three C connectors I use. If additional capacitance is needed a fixed capacitor can be paralleled with the variable sections. For less-than-15pf requirements I usually put a 22 pf cap in series with one of the variable sections, then calculate the series capacitance of this combination.

When I first built this Calibrated Capacitor I was not convinced that it would be valuable, but after 8 years of use I could not do without it on my workbench. It is simple to use, does not require external power, and fits with my experimental approach to circuit design.