Abstract: In a vector network analyzer with a transmitting and, a receiving side, each including a downward mixing super heterodyne receiver, a high voltage power supply for providing an accelerating voltage is arranged in series with a second power supply with a controllable lower output voltage so that unavoidable voltage variations in the acceleration voltage can be controlled. The second power supply is connected to the output of a phase and frequency discriminator by way of an amplifier and a low-pass filter. The discriminator has two inputs, one receiving the downwardly mixed transmitting side signal and the other the signal provided by a reference oscillator having a constant frequency. As millimeter wave oscillator a backward wave oscillator is used whose frequency drifting by temperature changes or by aging is compensated for by a control signal for the second power supply, which control signal is derived from the phase and frequency discriminator.