IX. FIGURES Figure 1: Schematic description of a typical lock-in type DSFM detection unit. The signal to be analyzed by the DSFM detection is assumed to be centered around some frequency ν$_{0}$ . It enters the detection unit at the input “ in ”, is amplified and usually high-pass filtered (for simplicity the corresponding components are not shown) before being multiplied with two reference signals in quadrature at a frequency ν$_{ref}$ , shifting the signal to the frequencies ν$_{0}$ − ν$_{ref}$ and ν$_{0}$ + ν$_{ref}$ . The resulting signals are then low-pass filtered to remove the higher frequency component ( ν$_{0}$ + ν$_{ref}$ ), resulting in two averaged signals 〈 x ( t ) 〉$_{τ}$ and 〈 y ( t ) 〉$_{τ}$ . For sufficiently small interaction 〈 x ( t ) 〉$_{τ}$ is proportional to the frequency shift and can be used to re-adjust the driving frequency of the VCO (or NCO) by means of an appropriate feedback loop (PI-controller). The output of the PI-controller used to adjust the excitation frequency is then proportional to the frequency shift δν ( t ). 19