Direct digitally tunable microwave oscillators and filters
View Patent ↗A tunable element in the microwave frequency range is described that may include one or more tunable elements that are directly digitally controlled by a digital bus connecting a digital control circuit to each controlled element. In particular, each digital signal is filtered by a digital isolation technique so that the signal reaches the tunable elements with very low noise. The low noise digital signals are then converted to analog control voltages. The direct D/A conversion is accomplished by a special D/A converter which is manufactured as an integral part of a substrate. This D/A converter in accordance with the invention may consist of a resistor ladder or a directly digitally controlled capacitor. The direct digitally controlled capacitor may be a cantilevered type capacitor having multiple separate electrodes or sub-plates representing binary bits that may be used to control the capacitor. A low cost microwave oscillator is disclosed in which some of the filters and oscillators are direct digitally tuned elements.
1. A direct digitally controlled microwave tuning element, comprising:
a microwave circuit that is being tuned to a predetermined microwave frequency;
an electrically tunable element for tuning the microwave circuit to the predetermined microwave frequency, the electrically tunable element being attached to a substrate and further comprising a plurality of tuning signals, means for noise filtering said tuning signals to generate noise filtered tuning signals, a converter, based on the noise filtered tuning signals, that controls the frequency of the microwave circuit to a predetermined microwave frequency and an analog tuning signal that is integrated into the substrate and mounted on said noise filtering means.
2. The element of claim 1 , wherein the converter further comprises a digital to analog converter that changes a predetermined characteristic in response to the noise filtered digital signals so that the microwave circuit tunes itself to a microwave frequency based on the digital signals.
3. The element of claims 2 , wherein said digital to analog converter further comprises one or more resistors in a resistor ladder wherein at least some of the resistors are located inside of a noise shield and some of the resistors are located outside of the noise shield, wherein the resistors inside and outside of the noise shield are connected together by a noise rejection bypass capacitor that is integrated into the noise shield.
4. The element of claim 1 , wherein said tuning signals comprise one or more digital signals for effecting the coarsely tuning of the microwave circuit and at least one analog tuning signal for effecting the fine tuning of the microwave circuit.
5. The element of claim 4 , wherein said converter comprises a capacitor including at least one moving capacitor plate and two or more sub-plates electrically isolated at DC or low frequencies from each other and from the moving plate for electrically controlling the deflection of the moving plate based on the tuning signals.
6. The element of claim 5 , wherein the tuning signals are connected to the sub-plates of the capacitor and wherein the sub-plates of the capacitor further comprises two or more sub-plates, a first sub-plate having predetermined area such that the capacitance change of the capacitor based on the first sub-plate is approximately ½ of the total capacitance of the capacitor and a second sub-plate having a second predetermined area so that the change in the capacitance of the capacitor based on the second sub-plate is approximately ½ the change caused by the first sub-plate to create a binary weighting of the sub-plates so that when one or more of the sub-plates are charged, the control plate deflects a predetermined amount to change the capacitance of the capacitor and tune the microwave device attached to the capacitor.
7. The element of claim 6 , wherein the area of each sub-plate is decreased by decreasing the width of the each sub-plate.
8. The element of claim 7 , wherein the area of a sub-plate having a width too small to manufacture is decreased by changing the length of the sub-plate.
9. The element of claim 5 , wherein the sub-plates of the capacitor further comprises two or more sub-plates, a first sub-plate having an area such that the capacitance of the capacitor is approximately ½ of the total capacitance of the capacitor and a second sub-plate having an area equal to approximately ½ of the area of first sub-plate so that when one or more of the sub-plates are charged, the control plate deflects a predetermined amount to change the capacitance of the capacitor and tune the microwave device attached to the capacitor.
10. The element of claim 5 , wherein the dimensions of each of said sub-plates that is driven by each of said tuning signals are set to represent a predetermined, substantially additive, binary weight of a desired tuning effect effect caused by the moving plate.
11. The element of claim 5 , wherein the moving plate of the capacitor comprises a cantilevered beam and wherein said sub-plates are arranged in parallel to said cantilevered beam attached to the substrate.
12. The element of claim 5 , wherein the moving plate of the capacitor further comprises a cantilevered beam and wherein said sub-plates are arranged perpendicular to said cantilevered beam on the substrate.
13. The element of claim 5 , wherein the moving plate of the capacitor further comprises an interdigital cantilevered beam and wherein said sub-plates are static interdigital beams divided in parallel to said moving cantilevered beam.
14. The element of claim 5 wherein the moving plate of the capacitor further comprises an interdigital cantilevered beam and wherein said sub-plates are static interdigital beams divided perpendicularly to said moving digital beam.