The schematics of the AURIGA detector
We summarize in the following figure the lumped elements model of the entire transduction chain, comprehensive of the thermal and electronic noise sources.
of the electric model for the entire transduction chain
It is easy to figure out that the white brownian noise of the bar (Sb) will appear at the output of the chain with a strong amplification at the resonance frequencies of the following three oscillators. For the same reason, the current noise of the SQUID amplifier (SBA) acting back on the transduction chain causes a high voltage drop at the output due to the resonant character of the detector output ammettance. This is called back-action noise. The sum of this two components constitute the narrow-band noise of the amplifier.
On the opposite, the voltage noise of the SQUID (SBB) has a broad-band white noise.
One application of this model is to write down an analytical expression for the sensitivity of the detector, which can be maximized by variation of the detector parameters, e.g. the transducer mass or the coils of the impedance matching superconducting transformer.