Variable capacitor with very fine resolution
First Claim
1. A variable capacitor, comprising:
- a binary capacitance ladder comprising a plurality N of parallel connected branches and having a predetermined resolution Δ
C, each of said branches comprising a fixed capacitor of capacitance Cn coupled in series with a switch having a parasitic capacitance Cn '"'"' when open, adjacent branches being numbered sequentially, n being the number of a respective branch, said binary capacitance ladder having a maximum total capacitance CTotal given by;
space="preserve" listing-type="equation">C.sub.Total =C.sub.1 +C.sub.2 +. . .+C.sub.n,each respective branch having a variation in capacitance Δ
Cn due to opening and closing of the respective switch Sn therein given by;
##EQU3## such that said binary capacitance ladder has a total capacitance variation between values of CTotal and CTotal -(2N -1)Δ
C in steps of Δ
C, said variable capacitor further comprising an additional fixed capacitor coupled in parallel with at least one said switch, such that the capacitance Cn of said switch includes the capacitance of said additional fixed capacitor.
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Abstract
The resolution ΔC of a binary capacitance ladder having a plurality of N branches is precisely determined by appropriately selecting the value of a fixed capacitor in each branch relative to the parasitic capacitance of a corresponding series-connected switch. The capacitance of each respective branch varies by 2n-1 ΔC, where n is the number of the branch, adjacent branches being numbered sequentially. The total capacitance is variable between CTotal and CTotal -(2N -1)ΔC in steps of ΔC, where CTotal is the sum of the capacitances in the ladder with all switches closed, and CTotal -(2N -1)ΔC is the capacitance of the ladder with all switches open. For any particular branch of the ladder, the switch capacitance may be further controlled by adding a fixed capacitance in parallel with the respective switch. Such a variable capacitance is useful as a tuning capacitor in an electrodeless HID lamp ballast.
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Citations
7 Claims
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1. A variable capacitor, comprising:
a binary capacitance ladder comprising a plurality N of parallel connected branches and having a predetermined resolution Δ
C, each of said branches comprising a fixed capacitor of capacitance Cn coupled in series with a switch having a parasitic capacitance Cn '"'"' when open, adjacent branches being numbered sequentially, n being the number of a respective branch, said binary capacitance ladder having a maximum total capacitance CTotal given by;
space="preserve" listing-type="equation">C.sub.Total =C.sub.1 +C.sub.2 +. . .+C.sub.n,each respective branch having a variation in capacitance Δ
Cn due to opening and closing of the respective switch Sn therein given by;
##EQU3## such that said binary capacitance ladder has a total capacitance variation between values of CTotal and CTotal -(2N -1)Δ
C in steps of Δ
C, said variable capacitor further comprising an additional fixed capacitor coupled in parallel with at least one said switch, such that the capacitance Cn of said switch includes the capacitance of said additional fixed capacitor.- View Dependent Claims (2, 3, 6, 7)
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4. A ballast for an electrodeless high intensity discharge lamp of the type having an excitation coil situated about an arc tube for exciting an arc discharge therein upon application of a radio frequency signal to said excitation coil, said ballast comprising:
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at least one switching device; a ballast driver coupled to said switching device for driving said switching device; and a resonant circuit coupled to said switching device, said resonant circuit comprising said excitation coil and at least one variable capacitor, said variable capacitor comprising a binary capacitance ladder comprising a plurality N of parallel connected branches and having a predetermined resolution Δ
C, each of said branches comprising a fixed capacitor of capacitance Cn coupled in series with a switch having a parasitic capacitance Cn '"'"' when open, adjacent branches being numbered sequentially, n being the number of a respective branch, said binary capacitance ladder having a maximum total capacitance CTotal given by;
space="preserve" listing-type="equation">C.sub.Total =C.sub.1 +C.sub.2 +. . .+C.sub.n,each respective branch having a variation in capacitance Δ
Cn due to opening and closing of the respective switch Sn therein given by;
##EQU4## such that said binary capacitance ladder has a total capacitance variation between values of CTotal and CTotal -(2N -1)Δ
C in steps of Δ
C. - View Dependent Claims (5)
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Specification