Varactor tuning for a narrow band filter having shunt capacitors with different capacitance values
An apparatus for providing dynamic and remote tuning of a cryo-cooled bandpass filter is disclosed. The device includes resonator element and a voltage sensitive element as part of the capacitance in a narrow band filter circuit. A varactor such as a GaAs varactor may be used. Alternatively, a capacitor having a voltage-sensitive dielectric such as SrTiO 3 may be used. A computer may be connected to the varactor to provide for automated tuning. The voltage-sensitive capacitor may be integral with the resonator element. The invention provides for remote and dynamic tuning of a narrow bandpass filter while located within its sealed cryo-cooled environment.
1. A tuning circuit for a narrow band filter, comprising
a resonator comprising a half-loop inductor and a first interdigital capacitor connected in parallel between a first end and second end separate from the first end;
a first shunt capacitor having a first terminal coupled to the first end of the resonator and a second terminal coupled to a ground node;
a second shunt capacitor having a first terminal coupled to the second end of the resonator and a second terminal coupled to the ground node, wherein a capacitance of the second shunt capacitor is less than a capacitance of the first shunt capacitor; and
a second interdigital capacitor having a first terminal coupled to the second end of the resonator and a second terminal coupled to a voltage-sensitive device;
wherein a capacitance of the second interdigital capacitor is adjusted by a voltage applied to the voltage-sensitive device therein setting a resonant frequency of the resonator, and wherein a voltage regulator regulates the voltage applied to the voltage-sensitive device.
2. A tuning circuit layout for a bandpass filter, comprising:
a. a resonator element comprising an input node and an output node separate from the input node;
b. a first shunt capacitor coupled to the output node of the resonator element; and
c. a capacitor network exhibiting a variable total capacitance coupled to the input node of the resonator element, the capacitor network comprising a second shunt capacitor having a capacitance less than a capacitance of the first shunt capacitor, wherein a resonant frequency of the resonator element is varied by an adjustment of the variable total capacitance of the capacitor network.
3. The tuning circuit layout of claim 2 , wherein the resonator element comprises a half-loop inductor and a first interdigital capacitor.
4. The tuning circuit layout of claim 2 , wherein the capacitor network further comprises a second interdigital capacitor, wherein a capacitance of the second interdigital capacitor changes the variable total capacitance of the capacitor network.
5. The tuning circuit layout of claim 4 , wherein a first end of the second interdigital capacitor is coupled to the second shunt capacitor.
6. The tuning circuit layout of claim 5 , wherein the adjustment of the variable total capacitance of the capacitor network comprises varying the capacitance of the second interdigital capacitor.
7. The tuning circuit layout of claim 6 , wherein the capacitor network further comprises a voltage-sensitive device coupled to a second end of the second interdigital capacitor.
8. The tuning circuit layout of claim 7 , wherein a voltage applied to the voltage-sensitive device varies the capacitance of the second interdigital capacitor.
9. The tuning circuit layout of claim 8 , wherein the voltage-sensitive device comprises a varactor.
10. The tuning circuit layout of claim 8 , wherein a voltage regulator regulates the voltage applied to the voltage-sensitive device.
11. The tuning circuit layout of claim 10 , wherein the voltage regulator comprises an inductor and a third capacitor.
12. A tuning circuit for a bandpass filter, comprising
a resonator comprising a half-loop inductor and a first interdigital capacitor connected in parallel between a first end and second end separate from the first end;
a first shunt capacitor having a first terminal coupled to the first end of the resonator and a second terminal coupled to a common ground node;
a second shunt capacitor having a first terminal coupled to the second end of the resonator and a second terminal coupled to the common ground node, wherein a capacitance of the second shunt capacitor is less than a capacitance of the first shunt capacitor; and
a second interdigital capacitor having a first terminal coupled to the second end of the resonator and a second terminal coupled to a varactor;
wherein a capacitance of the second interdigital capacitor is adjusted by a voltage applied to the varactor therein setting a resonant frequency of the resonator.