Considerations of the external circuit for the three-wave nondegenerate case. (a) This figure depicts the physical, general circuit considered in this section, containing a flux-pumped SQUID. This circuit accounts for an external source current, , as well as external () and local shunt () admittances. (b) It is possible to represent the general circuit in an equivalent way that separates the external loading effects of the circuit at the “signal” () and “idler” () frequencies. This is done by introducing hypothetical, ideal bandpass filters at and . These filters act open-circuited at their respective frequency, but short-circuited for all other frequencies. In this representation, the external admittance, , is represented at different frequencies by and . The input admittance () is then based on the signal voltage response to current , which depends upon the idler in a way that may allow for gain.