In order to obtain a dB-linear regulation characteristic, the control currents may include for example tagential-hyperbolic characteristic curves.

The local reflection behaviour of the test surface is measured (84) and used (86) to correct the slope of the characteristic control curve.

However, these known amplifiers have not a low noise level or have limited linearity or their regulation characteristic is not dB-linear.

In order to improve the regulation characteristic, the negative feedback network in a differential amplifier (Q21, Q22) is subdivised in individual networks supplied each with its own control current (I1...I3).

By integrating this regulating characteristic into the generator (5) and generator drive (3), synchronization becomes faster and is carried out with as little load shocks as possible.