Service Bulletin-17 | Vacuum Capacitors
Measuring of Self Resonance
Frequency
Comet AG Service Bulletin-17 | Vacuum Capacitors
Herrengasse 10 | CH-3137 Flamatt Measuring of Self Resonance Frequency
T +41 31 744 95 00 | pct@comet.tech Issue Date: 01-JUN-2011
pct.comet.tech Page 1 of 4 Replaces: 07-MAY-2004
Grid-dip meter
Rubber band
Aluminum foil
Window in foil
1. Introduction
The self-inductance of a vacuum capacitor depends mainly on its design and dimensions. It is extremely small
and constant for fixed capacitors ranging from approximately 2 to 10 nH, depending on the type. Variable capacitors have a larger internal inductance, ranging from approx. 6 to 50 nH. This is due to the extended structure, such as the bellows, which connect the movable electrode to the external mounting flange.
The self-resonance frequency of a vacuum capacitor is determined by its capacitance and self-inductance. Detailed data for most capacitors are given on the second page of the data sheets.
2. Measurement the Self Resonance Frequency
The self-resonance frequency as a function of capacitance can be measured with a grid-dip oscillator (manufacturer for example Radiometer Copenhagen). The variable vacuum capacitor is wrapped with aluminium foil
to create a resonance cavity. The foil is secured to the capacitor metal-ceramic seal area by means of a tight
wire or with a rubber band.
A window is cut in the foil (area of approx. 6 cm
2
) in the upper part of the ceramic cylinder for RF coupling of the
coil of the grid-dip meter.
The capacitor is adjusted to the maximum capacitance value, and the grip dip meter is placed in a stable position near to the window and the frequency adjusted slowly until the meter indicates a dip in the display.
Please note: As the Q-factor is very high the dip will be very narrow and can be seen as a
small peak only!
The measurement should be repeated for several different capacitance values.
Comet AG Service Bulletin-17 | Vacuum Capacitors
Herrengasse 10 | CH-3137 Flamatt Measuring of Self Resonance Frequency
T +41 31 744 95 00 | pct@comet.tech Issue Date: 01-JUN-2011
pct.comet.tech Page 2 of 4 Replaces: 07-MAY-2004
CLfr⋅⋅⋅
=
π
2
1
3. Calculation of the Self Inductance
With the following formula the self-inductance of the capacitor can be calculated as follows:
Example:
Capacitance = 655 pF
Self-resonance frequency = 39 MHz
Self-inductance 25,4 nH
.
Comet AG Service Bulletin-17 | Vacuum Capacitors
Herrengasse 10 | CH-3137 Flamatt Measuring of Self Resonance Frequency
T +41 31 744 95 00 | pct@comet.tech Issue Date: 01-JUN-2011
pct.comet.tech Page 3 of 4 Replaces: 07-MAY-2004
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Comet AG Service Bulletin-17 | Vacuum Capacitors
Herrengasse 10 | CH-3137 Flamatt Measuring of Self Resonance Frequency
T +41 31 744 95 00 | pct@comet.tech Issue Date: 01-JUN-2011
pct.comet.tech Page 4 of 4 Replaces: 07-MAY-2004
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