  We present a technique to measure the transfer function of a control line
using a qubit as a vector network analyzer. Our method requires coupling the
line under test to the the longitudinal component of the Hamiltonian of the
qubit and the ability to induce Rabi oscillations through simultaneous driving
of the transversal component. We used this technique to characterize the 'flux'
control of a superconducting Transmon qubit in the range of 8 to 400\,MHz. Our
method can be used for the qubit 'flux' line calibration to increase the
fidelity of entangling gates for the quantum processor. The qubit can be also
used as a microscopic probe of the electro-magnetic fields on a chip.
