Q.H. Cheng

Design and performance of waveguide E-plane HTSC insert filters

A design theory is described for waveguide E-plane inductive strip filters that takes the high-order mode interaction, and the finite thickness of metal with a dielectric substrate underneath into account. The main idea was to use superconducting material, instead of metal, in the design of inductive strip insert filters. A design method is introduced for this purpose, and high-T/sub c/ superconducting (HTSC) films serve as perfect conductors. An X-band bandpass filter was fabricated with HTSC materials. The measured results of the HTSC bandpass filter are presented and compared with the CAD-predicted filter performance.<<ETX>>

Harmonic frequency mixing at millimeter waveband using high Tc Josephson junction and its applications

Abstract When a signal at the frequency fS and a local oscillator at the frequency fL are both applied, a high TC Josephson junction can be operated as a harmonic mixer to give rise to output at the intermediate frequency fIF=≈ fS-nfL ≈ where n is an integer called harmonic number. Good results have so far been demonstrated for fS up to about 100 GHz and n up to about 120. The laboratory application of such mixing to the phase locking of signal source at millimeter waveband has also been explored. Further attempts to push the operating frequency to submillimeter and far infrared wavebands are being made.

Non-relaxational critical current density measurement of epitaxial thin films by using an improved a.c. magnetization

Non-relaxational magnetic hysteresis loops of (YBCO) thin films have been measured by an improved a.c. magnetization measurement with a.c. external field amplitude up to 1000 Oe and frequency range from 30 Hz - 10 kHz at 77 K. Above a certain frequency , the width of the magnetization hysteresis loop does not change with frequency, indicating that the magnetization can maintain a saturated state, namely the critical state. Experiments show that the critical current density deduced from the non-relaxational magnetic hysteresis loops is quite close to the value obtained by the four-probe measurement.

30 Hz–10 kHz hysteresis loop tracer with field up to 0.1 T for high-Tc superconductors

A hysteresis loop tracer to measure the ac magnetization of high-Tc superconductors in the frequency range of 30 Hz–10 kHz is described, with driving field amplitudes up to 0.1 T. Due to the high frequency of the magnetic field, the hysteresis loops could be displayed on an oscilloscope and their details could be observed. The demagnetization factor, the stability, and the frequency characteristics of the tracer were tested. The ac magnetization of granular high-Tc superconductors was studied as a function of field amplitude and frequency.

A 3 mm waveband Gunn oscillator phase-locked with a high- Josephson harmonic mixer and its applications to the quantum voltage standard

Using a high- superconducting harmonic mixer at 77 K and a phase-locking loop, a signal source (a Gunn oscillator in the 3 mm waveband) was directly phase-locked to a local oscillator for frequencies less than 1 GHz and harmonic numbers up to 120. The phase-locked Gunn oscillator achieved a frequency instability of less than Hz, with the counting time of 1 s, while the phase noise at intermediate frequency 40 MHz was ( kHz) for 120th harmonic mixing and ( kHz) for 42nd harmonic mixing. The output of this phase-locked signal source was applied to a Josephson junction array used in a voltage-standard system against which a solid-state voltage source (a Zener voltage reference) was measured. The experimental results showed that the quality of this phase-locked microwave source was good enough to justify its use in the quantum voltage standard.

The interpretation of the power dependence of the unloaded quality factor of superconductive microstrip resonators

For microstrip resonators fabricated from YBa2Cu3O7- delta (YBCO) thin film on LaAlO3 (001) substrates, this paper reports the experimental observation and theoretical explanation of the power dependence of the unloaded quality factor (Q0). Two kinds of Q0 behaviour were observed. In one case, with the increase of input power, Q0 dropped at first rather quickly, then slowly and then again drastically. In the other case, Q0 remained almost unchanged but, beyond a certain input power level, dropped very rapidly. To interpret these behaviours, two phenomenological expressions for local surface resistance (Rs) were proposed. According to these expressions Rs is a function of the surface rf current density (Js) in the strip which is strongly non-uniform spatially. By choosing proper values for the fitting parameters in these Rs expressions, the dependence of Q0 on the input power was computed, resulting in good agreement between the theoretical calculations and the experimental measurements.

Simulation of the sub-harmonic SIS mixer

We present in this paper a simulation of a Superconductor-Insulator-Superconductor (SIS) sub-harmonic mixer working at a fundamental frequency of 250GHz. With a full five-port model, the mixer performances are studied as functions of principal parameters including embedding admittances, local oscillator (LO) power and voltage bias, and the results are compared with that from a quasi five-port model.

High- T c superconducting thin film/GaAs MESFET hybrid microwave oscillator

A high-Tc superconducting (HTSC) thin film/GaAs MESFET hybrid microwave oscillator operated at 10.6 GHz has been designed, fabricated and characterized. Microstrip line structures were used throughout the circuit with superconducting thin film YBa2Cu3O7-δ(YBCO) as the conductor material. The YBCO thin films were deposited on 15 mm×10 mm×0.5 mm LaAlO3 substrates. The oscillator was common-source, series feedback type using a GaAs-MESFET (NE72084) as the active device and a superconducting microstrip resonator as the frequency stabilizing element. By improving the unloaded quality factorQ0 of the superconducting microstrip resonator and adjusting the coupling coefficient between the resonator and the gate of the MESFET, the phase noise of the oscillator was decreased. At 77 K, the phase noise of the oscillator at 10 kHz offset from carrier was −87 dBc/Hz.

Magnetization of an epitaxial YBa2Cu3O7−δ film at the critical field sweep rate

We have measured the magnetic hysteresis loops of an epitaxial YBa2Cu3O7−δ thin film using our recently developed device which can provide the field magnitude in the range of 0–1000 Oe and the field sweep rate up to 107 Oe/s. The shape of the hysteresis loop measured changes with the field-sweep rate up to the critical sweep rate; and over the critical sweep rate the ac magnetization reaches its real critical state where magnetization does not change even with a further increase in the field-sweep rate. The critical sweep rate is about 106 Oe/s at 77 K. With the hysteresis loops and Bean model, we have calculated the magnetization critical current density (Jcac) which is consistent with that obtained by I–V measurements. We have also studied flux motion and activation energy under the high sweep rate magnetic field. At temperature 77 K, the velocity of the flux motion is of the order 10 m/s and the pinning energy U0/k is about 339 K which is much smaller than the magnetization decay measurement.

A hybrid superconductor/GaAs-MESFET microwave oscillator at 10.6 GHz

A 10.6 GHz hybrid superconducting film/GaAs-MESFET microwave oscillator has been designed, fabricated and characterized on a 10 mm x 15 mm LaAlO3 substrate. The oscillator was a reflection mode type using a GaAs MESFET (NE72084) as the active device and a superconducting microstrip resonator as the frequency stabilizing element. By improving the unloaded quality factor of the superconducting microstrip resonator and adjusting coupling coefficient between the resonator and the MESFET, the phase noise of the oscillator was decreased. At 77 K, the phase noise of the oscillator at 10 KHz offset from carrier was -87 dBc/Hz