A spectrum analyser shows the output noise density of a photodiode transimpedance…

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Feedback, operational amplifiers and analogue signal processing

Electronics for Measurement, Ch Noise and the signal-to-noise ratio

Problem

A spectrum analyser shows the output noise density of a photodiode transimpedance amplifier. Above about

100\,\mathrm{Hz}

the trace is flat at

12\,\mathrm{nV/\sqrt{Hz}}

; below that it rises, reaching

120\,\mathrm{nV/\sqrt{Hz}}

1.0\,\mathrm{Hz}

. ((a)) Estimate the

1/f

corner frequency

f_{\rm knee}

of this amplifier. ((b)) The signal is modulated at

1.0\,\mathrm{kHz}

, a decade above the corner, and a single measurement of it, limited by the flat floor there, takes

20\,\mathrm{ms}

. Estimate the total measuring time needed, using averaging of independent records, to improve the signal-to-noise amplitude ratio by a factor of

20

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