Vacuum CapacitorCharacteristics
The Tech Note has been developed toexplain the primary characteristics that are considered when developing vacuumcapacitors. Additionally this document will provide a clear description ofterms used in the capacitor industry.
Current/Voltage
Maximum operating current for vacuum capacitors is limited by temperature riseand working voltage. At low capacitive reactance (high capacitance) values itmay be impossible to apply rated voltage without exceeding rated current.Therefore, the vacuum capacitor will be current limited. At high capacitivereactance (low capacitance) values, it may be impossible to apply rated currentwithout exceeding rated voltage and the capacitor will be voltage limited.These operational characteristics are indicated for each capacitor model.
Temperature
vacuum capacitors are designed to meetMIL-C-23183 specifications. Based upon actual current tests, most ceramiccapacitors are rated for a maximum operating temperature of 120°C (250°F) andglass capacitors for 87°C (188°F) with normal convection cooling at an ambienttemperature of 25°C (72°F). Derating curves for elevated ambient temperatureoperation are available upon request.
Fixed capacitors with a nominalcapacitance above 50 pF shall be within ±5 %. Capacitors with a nominalcapacitance of 50 pF or less shall be within ±10%, or .5 pF, whichever isgreater. For variable capacitors, the low end will be equal to or less thanminimum rating. The capacitance change is substantially uniform with rotation,and there are no capacitance reversals. Capacitance is within ±10% of thenominal value of the curves shown (Capacity vs. Turns), when the turns settingfor reference purposes (defines point) is established near the low capacity endof the linear portion of this curve.
Automatic Shorting Feature
A number of variable capacitors have been designed with an internal shortingdevice that shorts out the capacitor when it has been turned beyond maximumrated capacitance. This feature is useful for tuning antenna couplers withoutthe vacuum capacitor in the circuit and also serves as a reference point foradjusting the capacitor to a previously measured capacitance value.
Tracking
Variable Capacitors will track within 10% if set together near the low capacityend of the linear portion of the curve. On special order, units may be obtainedto closer tracking tolerances.
Torque/Direct Pull
In variable capacitors, the linear sliding motion of the moving electrodeassembly is converted to rotary tuning via a threaded shaft. The torque valuesgiven in the tables are the maximum torque needed to reach minimum capacitancewhen rotated with a standard lead screw; the torque required to tune away fromminimum may be less than half this value.
For most variable capacitors, direct pulltuning is a possible alternative to rotary tuning. Maximum required pull forcevalues are also given in the tables.
Capacitance range end-stops are built intoevery variable capacitor. It is recommended that the user install his own stopsto prevent damage from gear-reduction drives.
In addition, also offers several"Adjustable" capacitors which are designed to be operated as a fixedcapacitor, but can be hand adjusted to any value within their range and thenlocked in position with a locking nut.
Quality Factor (Q)
Extremely low losses occur in vacuum capacitors because of the vacuumdielectric, compact construction, and the use of low loss glass or ceramicenvelopes as well as copper and precious metal solder construction.Consequently, vacuum capacitors are able to handle large RF currents at high RFfrequencies that would destroy other types of capacitors. The "Q"factor, or ratio of stored energy to dissipated energy, is typically in theorder of 1000 or 5000 or higher.
Because Q is a function of frequency,capacity and E.S.R. (Equivalent Series Resistance), it is perhaps moremeaningful to consider the value of E.S.R. In modern high power capacitorapplications, E.S.R. is significant for determining cooling requirements. Theslight loss results from the RF resistance in the copper. Based upon actualtests, the E.S.R. value in not effected by change in capacity, other parametersbeing fixed. The value of E.S.R. varies over a range of 5 to 20 milliohms from2.5 to 30MHz.
Thermal Stability
vacuum capacitors are designed to meetMIL-C-23183 specifications which state that the absolute value of thecapacitance change with temperature shall not exceed 1.1% over the applicableoperating temperature range. In typical tests, values for ceramic capacitorsshow stability within 50 ppm/°C and for glass capacitors, 100ppm/°C. Specifictests can be performed upon individual capacitors on special request.
Salt Spray and Humidity
capacitors are designed to withstand theharmful effects of salt spray and humidity, without degradation in performance.
Inductance
The self-inductance of vacuum variable capacitors is typically in the order of6 to 20 nanohenries while that of a fixed capacitor is significantly lower, inthe range of 2 to 6 nanohenries.
For most applications, the self-inductancecan be ignored. It becomes a factor only when the ratio of capacitive reactanceto inductive reactance is small. Graphs of inductance or resonant frequency vs.capacity are available (see Figure 2).
Mechanical Life
The mechanical life of variable capacitors is related to length of stroke,speed of operation, bellows material, and total number of cycles. Extensivemechanical life tests have been run, operating units for complete cycles frommaximum to minimum and back to maximum capacity covering 95% of the full strokeof the movable plates. Capacitors with a large bellows and a short stroke willhave the greatest life expectancy under cycling operation. Our most recentmodels are rated for 1 million cycles, ideal for the semiconductor processingindustry.
Application engineers can check your specificapplication to assure that the optimum capacitor is selected for yourrequirements.