To avoid power dissipation that perturbs the gradient temperature shape the preamplifier, voltage gain set to unity, is located behind the detector, not close to the cavity, connected to the detector by a tri-axial cable with an active guard. This
way the lock-in amplifier (Stanford Research Systems SR850 –100 kHz DSP) can measure the signal from the detector transducer buffered by the preamplifier, without placing a significant load on the transducer. Typical detector signals are around 1 mV