Superconductor technology: apparatus – material – process – High temperature devices – systems – apparatus – com- ponents,... – High frequency waveguides – resonators – electrical networks,...
Reexamination Certificate
2000-03-02
2003-02-04
Lee, Benny T. (Department: 2817)
Superconductor technology: apparatus, material, process
High temperature devices, systems, apparatus, com- ponents,...
High frequency waveguides, resonators, electrical networks,...
C333S09900R, C333S205000, C505S211000, C505S700000, C505S705000
Reexamination Certificate
active
06516208
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 actuating a magnetic driver.
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 select a desired frequency or to trap an interfering signal frequency. The vast majority of lumped element tunable filters have used varactor diodes. Such a design amounts to using a tunable capacitor because varactor diodes, by changing the reverse bias voltage, vary the depletion thickness and hence the P-N 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 intermodulation products and other problems.
Consider the case of a conventional varactor diode. In a varactor, the motion of electrons accomplishes the tuning itself. As the reverse bias voltage (V
r
) on the junction of the varactor is changed, then in accordance with Poisson's Equation, the width of the P-N junction depletion region changes, which alters the junction capacitance (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 the input signal (frequently in a resonantly magnified form) appears as a component of the junction bias voltage V
r
, the input 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 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, at lower reverse bias voltages (higher capacitances), there is a substantial amount of undepleted moderately-doped semiconductor material between the contacts and the P-N junction that offers significant series resistance (R
ac
) to ac current flow. Since the Q of a varactor having junction capacitance C
j
and series resistance R
ac
at an input signal frequency f is given by Q=1/(2 f C
j
R
ac
), 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=1.0GHz 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 &Dgr;F=20 MHz (FWHM), corresponding to a Q=f/&Dgr;F=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 loss 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 of the resonator energy is not difficult, 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
to >10
5
range, as achievable with HTS resonators). It would be much easier to design such very high Q circuits using three-terminal, or preferably 4-terminal (two-port) variable capacitors in which the tuning voltage is applied to a completely different pair of electrodes from those across which the input signal voltage is applied (with an inherent high degree of isolation between the signal and bias ports).
One new form of variable capacitor that avoids the intermodulation/dynamic range problems of varactors or “tunable materials” approaches is the microelectromechanical (MEMS) variable capacitor. A number of MEMS variable capacitor device structures have been proposed, including elaborate lateral-motion interdigitated electrode capacitor structures. In the simple vertical motion, parallel plate form of this device, a thin layer of dielectric separating normal metal plates (or a normal metal plate from very heavily doped silicon) is etched out in processing to leave a very narrow gap between the plates. The thin top plate is suspended on four highly compliant thin beams which terminate on posts (regions under which the spacer dielectric has not been removed). The device is ordinarily operated in an evacuated package to allow substantial voltages to be applied across the narrow gap between p
Lee Benny T.
O'Melveny & Myers LLP
Superconductor Technologies Inc.
LandOfFree
High temperature superconductor tunable filter does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with High temperature superconductor tunable filter, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and High temperature superconductor tunable filter will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3162271