Question 49G5

A 10:1 oscilloscope attenuation probe serves two primary purposes: to reduce the signal amplitude by a factor of ten, allowing measurement of higher voltages, and more importantly, to *increase the input impedance* presented to the circuit under test, thereby minimizing loading effects. Standard oscilloscopes typically have an input impedance of 1 megohm (MΩ) shunted by a small capacitance (around 10-30 pF). A 10:1 probe is designed as a voltage divider. It effectively adds a 9 MΩ resistor in series with the oscilloscope's 1 MΩ input, creating a total input resistance of approximately 10 MΩ. To maintain the 10:1 division ratio across a wide range of frequencies, the probe also includes a small, adjustable capacitor that forms a capacitive voltage divider with the oscilloscope's input capacitance. This compensation ensures the frequency response is flat. The resulting *effective* shunt capacitance seen by the circuit under test is typically reduced compared to the oscilloscope's direct input, often to around 10-15 pF. Therefore, a 10:1 probe presents a load of about 10 megohms shunted by a low capacitance (e.g., 12 pF) to the circuit, making option A correct. Options B and D are incorrect because probes use capacitors for frequency compensation, not inductors (mH), which would cause severe frequency distortion. Options C and D are also incorrect because a 10:1 probe with a 1 MΩ scope input should result in a 10 MΩ total resistance, not 5 MΩ.