Someone argues: "A probe has of input resistance

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Circuit foundations and passive networks · Circuit foundations and passive networks

Electronics for Measurement, Ch The bench: probes, cables, grounds and what the instrument shows

Problem

Someone argues: "A

\times1

probe has

1\,\mathrm{M}\Omega

of input resistance, a thousand times a

1.0\,\mathrm{k}\Omega

source resistance, so it can never load that source appreciably, no matter what frequency the signal is." The probe's cable and the oscilloscope input together present

1\,\mathrm{M}\Omega

in parallel with

120\,\mathrm{pF}

. The probe is attached to a

1.0\,\mathrm{k}\Omega

Thevenin source carrying a sinusoid at

2.0\,\mathrm{MHz}

. ((a)) Find the magnitude of the probe's input impedance at

2.0\,\mathrm{MHz}

. ((b)) Find the fraction of the source's open-circuit voltage that actually reaches the oscilloscope, and say whether the claim of negligible loading is correct.

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