Discussion
A signal of one μV is burried under 1V RMS noise. Which of the following devices is suitable to detect it ?
Answer: D
The suitable device is a phase sensitive amplifier.
A signal a million times smaller than the noise cannot be recovered by amplification or ordinary filtering. A phase sensitive (lock-in) amplifier modulates or references the signal at a known frequency, multiplies the input by a reference at that frequency and phase, and averages the product over a long time. The signal gives a steady output while noise at other frequencies and phases averages towards zero, giving an extremely narrow effective bandwidth.
A high-gain linear amplifier amplifies signal and noise alike. A band pass filter cannot be made narrow enough to pull out a microvolt from a volt of noise, and a notch filter removes one frequency, which is the opposite of what is needed.
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