Radio frequency SQUIDs operating at 77 K with 1 GHz lumped-element tank circuits

We have developed a matching scheme for the operation of a rf superconducting quantum interference device (SQUID) with a lumped-element tank circuit at pumping frequencies up to 1 GHz. This conventional LC resonant circuit is inductively coupled to a matching coil, which is connected to the 50 Ω transmission line. We characterized the flux noise of high-temperature rf SQUIDs at resonant frequencies between 221 and 950 MHz. At about 900 MHz, we obtained the white flux noise of about 14.6 μφ0/Hz with a SQUID inductance LS=150 pH and 20 μφ0/Hz with LS=260 pH. Several of such tank circuits can be driven in parallel or in series with only one cable.