Radio communication services are becoming so numerous they are reaching the 50 GHz millimeter wave spectrum. As the demand for more telecommunications services increases, and the spectrum becomes increasingly crowded, it is foreseeable that applications in the 50-300 GHz millimeter wave spectrums will be utilized for various telecommunications applications.
Circuits for generating and processing signals in the millimeter wave spectrum present significant challenges to component designers. As the frequencies increase, the quality of the components becomes increasingly difficult to maintain. Specifically, for a basic capacitor utilized in circuits operating at these frequencies, the internal equivalent series resistance (ESR) increases significantly using known dielectrics and construction techniques for microwave capacitors. Upper frequency spectrum applications in UHF (300 MHz to 3.0 GHz) to SHF (3 GHz to 300 GHz) are limited because dielectric materials used in the capacitors exhibit a significant change in ESR with frequency. As the frequency increases for a typical high frequency capacitor, the ESR can increase from 0.05 ohm at 200 MHz to significantly higher ESR and higher losses can be expected. Additionally, the dielectric constant .di-elect cons. also changes as frequencies increase. Thus, capacitors in particular have a practical upper limit in UHF to SHF frequency spectrum when they are constructed with conventional dielectric materials.
One of the more advantageous dielectrics is air. Early capacitor designs used in relatively low RF frequency applications (e.g., 100 KHz to 30 MHz) employed air capacitors particularly for high-powered applications. These capacitors were physically large because of the range of the capacitance values (e.g., 20 pF to 800 pF) that are often required to work at lower RF frequencies. However, in order to stand higher working voltages, it is necessary to increase the distance between electrodes. Consequently, the use of air, gas or a vacuum as a dielectric has not seen widespread use outside of the lower RF frequency applications.
Capacitors that utilize air, gas or a vacuum as a dielectric approach the theoretical performance of an ideal capacitor. That is, such capacitors have no losses and a dielectric constant (.di-elect cons.) which remains constant over an extremely wide frequency spectrum up to SHF range (i.e., 3 GHz to 300 GHz). The power factor for low RF frequency gas/vacuum dielectric background art capacitors is low, making them suitable for carrying high current/working voltage levels. In the event of an internal breakdown due to excessive voltage producing a flash over between capacitor electrodes, the dielectric is self-healing. That is, the dielectric is not destroyed or altered as a result of a voltage arc generated between the electrode plates. Further, it is well known with many dielectric materials used in background art capacitor applications, an air, gas or vacuum dielectric will not suffer from aging and degradation in performance over time.
An additional difficulty in using background art capacitor designs at millimeter wavelength frequencies (e.g., Extremely High Frequency (EHF)) is that most of these capacitors have leads with wire length, or an end cap attachment that introduces significant inductance in the circuit, as well as series circuit resistance. In a typical microwave application, the capacitor electrodes are connected by directly bonding or soldering the device to a printed circuit board (PCB) trace. However, even with these connection techniques undesirable series inductance and resistance can be introduced to microwave circuit.
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