  We study current fluctuations in tunnel junctions driven by a voltage source.
The voltage is applied to the tunneling element via an impedance providing an
electromagnetic environment of the junction. We use circuit theory to relate
the fluctuations of the current flowing in the leads of the junction with the
voltage fluctuations generated by the environmental impedance and the
fluctuations of the tunneling current. The spectrum of current fluctuations is
found to consist of three parts: a term arising from the environmental
Johnson-Nyquist noise, a term due to the shot noise of the tunneling current
and a third term describing the cross-correlation between these two noise
sources. Our phenomenological theory reproduces previous results based on the
Hamiltonian model for the dynamical Coulomb blockade and provides a simple
understanding of the current fluctuation spectrum in terms of circuit theory
and properties of the average current. Specific results are given for a tunnel
junction driven through a resonator.
