We analyze the performance of a microwave chip mount that uses wirebonds toconnect the chip and mount grounds. A simple impedance ladder model predictsthat transmission crosstalk between two feedlines falls off exponentially withdistance at low frequencies, but rises to near unity above a resonancefrequency set by the chip to ground capacitance. Using SPICE simulations andexperimental measurements of a scale model, the basic predictions of the laddermodel were verified. In particular, by decreasing the capacitance between thechip and box grounds, the resonance frequency increased and transmissiondecreased. This model then influenced the design of a new mount that improvedthe isolation to -65 dB at 6 GHz, even though the chip dimensions wereincreased to 1 cm by 1 cm, 3 times as large as our previous devices. Wemeasured a coplanar resonator in this mount as preparation for larger qubitchips, and were able to identify cavity, slotline, and resonator modes.
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