Signal flow graph (SFG) representation of small-signal responses of nonlinear microwave circuits around a large-signal operating point is developed using the X-parameters. It is shown that, unlike the SFGs for linear circuits, negative-frequency nodes need to be included explicitly. The development elucidates the circuit-operational meaning of the elusive T-type small-signal X-parameters, which represent interaction between positive- and negative-frequency components. As an example, such an SFG is used to derive a closed-form expression of the output power of an amplifier as a function of the load reflection coefficient. It is then used to plot approximate load-pull power contours. The result is consistent with the expressions of the optimum load reflection coefficient derived by Root et al. (EuMIC 2017) and of power contours derived by Peláez-Pérez et al. (TMTT 2013). SFGs provide an alternative systematic means to derive closed-form expressions in terms of X-parameters and to gain illuminating insights into the workings of weakly nonlinear circuits.

A broadband noise measurement system is newly developed and demonstrated at temperatures between 3 K and 300 K. Using the system, wideband noise spectroscopy (WBNS) from 20 kHz to 500 MHz is carried out for the first time, revealing that shot noise is the dominant white noise down to 3 K. The paper also suggests, by means of WBNS, the possibility of extracting the baseline noise characteristics, which do not include the noise component that varies a great deal from device to device.