Superconducting Quantum Interference Filters (SQIF) are promising devices for Radio- Frequency (RF) detection combining low noise, high sensitivity, large dynamic range and wide-band capabilities. Impressive progress have been made recently in the fi
eld, with SQIF based antennas and amplifiers showing interesting properties in the GHz range using the well-established Nb/AlOx technology. The possibility to extend these results to High Temperature Superconductors (HTS) is still open, and different techniques to fabricate HTS SQIFs are competing to make RF devices. We report on the DC and RF response of a High Temperature SQIF fabricated by the ion irradiation technique. It is made of 1000 Superconducting QUantum Interference Devices (SQUIDs) in series, with loop areas randomly distributed between 6 micron2 and 60 micron2. The DC transfer factor is around 450 V/T at optimal bias and temperature, and the maximum voltage swing around 2:5 mV . We show that such a SQIF detects RF signals up to 150 MHz. It presents linear characteristics for RF power spanning more than five decades, and non-linearities develop beyond PRF = -35 dBm in our set-up configuration. Second-harmonic generation has been shown to be minimum at the functioning point in the whole range of frequencies. A model has been developed which captures the essential features of the SQIF RF response.
The magneto-optical imaging technique is used to visualize the penetration of the magnetic induction in YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin films during surface resistance measurements. The in-situ surface resistance measurements were performed at 7
GHz using the dielectric resonator method. When only the microwave magnetic field $H_{rf}$ is applied to the superconductor, no $H_{rf}$-induced vortex penetration is observed, even at high rf power. In contrast, in the presence of a constant magnetic field superimposed on $H_{rf}$ we observe a progression of the flux front as $H_{rf}$ is increased. A local thermometry method based on the measurement of the resonant frequency of the dielectric resonator placed on the YBa$_{2}$Cu$_{3}$O$_{7-delta}$ thin film shows that the $H_{rf}$--induced flux penetration is due to the increase of the film temperature.
Using the dielectric resonator method, we have investigated nonlinearities in the surface impedance Zs = Rs + jXs of YBa2Cu3O7 thin films at 10 GHz as function of the incident microwave power level and temperature. The use of a rutile dielectric reso
nator allows us to measure the precise temperature of the films. We conclusively show that the usually observed increase of the surface resistance of YBa2Cu3O7 thin film as function of microwave power is due to local heating.
