High temperature superconductor tunable filter

Details High temperature superconductor tunable filter High temperature superconductor tunable filter

1999-03-16

2002-02-12

Lee, Benny T. (Department: 2817)

Superconductor technology: apparatus, material, process

High temperature devices, systems, apparatus, com- ponents,...

High frequency waveguides, resonators, electrical networks,...

C505S700000, C505S705000, C333S09900R, C333S185000

Reexamination Certificate

active

06347237

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a high temperature superconductor (HTS) tunable filter. More particularly, this invention relates to an HTS filter tunable by variation of a microelectromechanical capacitor.
BACKGROUND OF THE INVENTION
The need for a high-quality factor (Q), low insertion loss tunable filter pervades a wide range of microwave and RF applications, in both the military, e.g., RADAR, communications and electronic intelligence (ELINT), and the commercial fields such as in various communications applications, including cellular. Placing a sharply defined bandpass filter directly at the receiver antenna input will often eliminate various adverse effects resulting from strong interfering signals at frequencies near the desired signal frequency in such applications. Because of the location of the filter at the receiver antenna input, the insertion loss must be very low to not degrade the noise figure. In most filter technologies, achieving a low insertion loss requires a corresponding compromise in filter steepness or selectivity. In the present invention, the extremely low loss property of high-temperature superconductor (HTS) filter elements provides an attractive solution, achieving a very low insertion loss yet simultaneously allowing a high selectivity/steepness bandpass definition.
In many applications, particularly where frequency hopping is used, a receiver filter must be tunable to either to select a desired frequency or to trap an interfering signal frequency. Thus, the insertion of a linear tunable filter between the receiver antenna and the first nonlinear element (typically a low-noise amplifier or mixer) in the receiver offers, providing that the insertion loss is very low, substantial advantages in a wide range of RF and microwave systems. For example, in RADAR systems, high amplitude interfering signals, either from “friendly” nearby sources, or from jammers, can desensitize receivers or intermodulate with high-amplitude clutter signal levels to give false target indications. In high-density signal environments, RADAR warning systems frequently become completely unusable.
Both lumped element and distributed element filters suffer from these and other problems. For example, while distributed-element YIG-tuned filters have been used, the high level of insertion loss (usually greater than 10 dB) of suitable YIG filters necessitates their use on a “switch in when absolutely necessary” basis only, as the degradation to noise figure would generally be unacceptable. Lumped element filters also suffer problems. For a lumped element filter to be tunable, the filter requires either a tunable capacitor, or a tunable inductive element. The vast majority of RF tunable lumped element filters have used varactor diodes. Such a design amounts to using a tunable capacitor because varactor diodes, through changing the reverse bias voltage, vary the depletion thickness and hence the PN junction capacitance. While varactors are simple and robust, they have limited Q's, and suffer from the problem that the linear process that tunes them extends all of the way to the signal frequency, so that high-amplitude signals create, through the resulting nonlinearities, undesirable intermodulate products, etc. The same problems of poor Q and high-frequency nonlinearities are anticipated for “tunable materials” such as ferroelectrics.
Consider the case of a conventional varactor diode. In a varactor, the motion of electrons accomplishes the tuning itself. As the reverse bias (V
r
) on the junction of the varactor is changed, then in accordance with Poisson's Equation, the width of the PN junction depletion region changes which alters the capacitance of the junction (C
j
). Because the tuning mechanism of varactors is electronic, the tuning speed is extremely fast. Unfortunately, this also leads to a serious associated disadvantage: limited dynamic range. Because the C
j
(V
r
) relationship is nearly instantaneous in response, extending to changes in V
r
at the signal frequency itself, and because the signal (frequently in a resonantly magnified form) appears as a component of the junction bias voltage, V
r
, the signal itself parametrically modulates the junction capacitance. If the signal amplitude across the varactor is very small in comparison to the dc bias, the effect is not too serious. Unfortunately, for high signal amplitudes, this parametric modulation of the capacitance can produce severe cross-modulation (IM) effects between different signals, as well as harmonic generation and other undesirable effects. While these signal-frequency varactor capacitance variations are the basis of useful devices such as parametric amplifiers, subharmonic oscillators, frequency multipliers, and many other useful microwave circuits, in the signal paths of conventional receivers they are an anathema. This inherent intermodulation or dynamic range problem will presumably extend to “tunable materials”, such as ferroelectrics or other materials in which the change of dielectric constant (∈
r
) with applied electric field (E) is exploited to tune a circuit. As long as the ∈
r
(E) relationship applies out to the signal frequency, then the presence of the signal as a component of the E will lead to the same intermodulation problems that the varactors have.
In addition to the intermodulation/dynamic range problems of varactors, these conventional tuning devices also have serious limitations in Q, or tuning selectivity. Because the varactors operate by varying the depletion region width of a P-N junction, this means that at lower reverse biases (higher capacitances), there is a substantial amount of undepleted moderately-doped semiconductor material between the contacts and the junction that offers significant series resistance (R
ac
) to ac current flow. Since the Q of a varactor of junction capacitance C
j
and series resistance R
ac
at the signal frequency f is given by Q=1/(2 f C
j
R
ac
), this means that the varactor Q values are limited, particularly at higher frequencies. For example, a typical commercial varactor might have C
j
=2.35 pF with R
ac
=1.0 &OHgr; at V
r
=−4V, or C
j
=1.70 pF with R
ac
=0.82 &OHgr; at V
r
=−10V,corresponding to Q values at f=1.0 GHz of Q=68 at V
r
=−4V or Q=114 at V
r
=−10V (or f=10.0 GHz values of Q=6.8 and Q=11.4, respectively). Considering that an interesting X-band (f=10 GHz) RADAR application might want a bandwidth of f=20 MHz (the full width at half-maximum or FWHM), corresponding to a Q=500 quality factor, we see that available varactors have inadequate Q (too much loss) to meet such requirements. While the mechanisms are different, this will very likely apply to the use of ferroelectrics or other “tunable materials”. A general characteristic of materials which exhibit the field-dependent dielectric constant nonlinearities (that makes them tunable) is that they exhibit substantial values of the imaginary part of the dielectric constant (or equivalently, loss tangent). This makes it unlikely that, as in varactors, these “tunable materials” will be capable of achieving high Q's, particularly at high signal frequencies.
An additional problem with both varactors and “tunable materials” for circuits with high values of Q is that these are basically two-terminal devices; that is, the dc tuning voltage must be applied between the same two electrodes to which the signal voltage is applied. The standard technique is to apply the dc tuning bias through a “bias tee”-like circuit designed to represent a high reactive impedance to the signal frequency to prevent loss of signal power out the bias port (as this would effectively reduce the Q). However, while the design of bias circuits that limit the loss of energy to a percent, or a fraction of a percent, even losses of a fraction of a percent are not nearly good enough for very high Q circuits (e.g., Q's in the 10
3
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