FCC Exam Question: 49G5

The characteristics of a 10:1 attenuation probe for an oscilloscope is as follows:

A. A load of about 10 megohms shunted by 12 pF.
B. A load of about 20 megohms shunted by a 12 mH coil.
C. A load of about 5 megohms shunted by 6 pF.
D. A load of about 5 megohms shunted by a 6 mH coil.
Correct Answer: A

Explanation: 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.

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Includes Elements 1, 3, 6, 7R, 8, and 9.