Effect of rf Pumping Frequency and rf Input Power on the Flux to Voltage Transfer Function of rf-SQUIDs

We present the results on the correlation between the flux to voltage transfer function, V_spp, of the rf-SQUID and the rf-bias frequency as well as rf-bias power. Measurements were performed for different SQUID gradiometer samples chosen from the same batch or different batches. In order to have full control on the electronics parameters, an experimental rf-SQUID circuit was designed and implemented with an operation frequency of 600 MHz to 900 MHz. According to our findings, it has been observed that at any particular rf-bias power, V_spp vs. rf-bias frequency shows Sinc-like behavior. We observed that the main lobe maxima exist close to the resonance frequency of the LC tank circuit and by changing only the power, amplitude of the main lobe and side lobes can be controlled. The V_spp vs. rf-bias power analysis shows that maximum of V_spp, strongly depends on the bias frequency. This can be correlated with the S11 parameter of LC tank circuit. We also observed that the devices from the same batch show main lobe maxima at different frequencies and/or power. Our SQUIDs with high working frequency gave their maxima at lower rf-bias powers leading to the need of having high frequency electronics with low bias power handling capabilities. It has also been observed that the SQUIDs from the same chip show similar characteristics regarding V_ssp vs. frequency and power while the SQUIDs from different batches show completely different behavior for a fixed LC tank circuit configuration.