Test probe: Difference between revisions

To minimize loading effect, attenuator probes (e.g., 10× probes) are used. A typical probe uses a 9&nbsp;megohm series resistor shunted by a low-value capacitor to make an RC compensated divider with the cable capacitance and scope input. The RC time constants are adjusted to match. For example, the 9&nbsp;megohm series resistor is shunted by a 12.2&nbsp;pF capacitor for a time constant of 110 microseconds. The cable capacitance of 90&nbsp;pF in parallel with the scope input of 20&nbsp;pF (total capacitance 110&nbsp;pF) and 1&nbsp;megohm also gives a time constant of 110 microseconds. In practice, there will be an adjustment so the operator can precisely match the low frequency time constant (called compensating the probe). Matching the time constants makes the attenuation independent of frequency. At low frequencies (where the resistance of ''R'' is much less than the reactance of ''C''), the circuit looks like a resistive divider; at high frequencies (resistance much greater than reactance), the circuit looks like a capacitive divider.<ref>{{Harvnb|Wedlock|Roberge|1969}}</ref>