  We propose a new approach to detailed balance violation in electrical
circuits by relying on the scattering matrix formalism commonly used in
microwave electronics. This allows to include retardation effects which are
paramount at high frequencies. We define the spectral densities of phase space
angular momentum, heat transfer and cross power, which can serve as criteria
for detailed balance violation. We confirm our theory with measurements in the
4-8 GHz frequency range on several two port circuits of varying symmetries, in
space and time. This validates our approach, which will allow to treat quantum
circuits at ultra-low temperature.
