Oscilloscope Probe Basics Series 5 - Oscilloscope Probe Usage Guide (Part 2)

For many applications that require high impedance and frequency (such as above 2GHz), active probes become a powerful tool. Active probes use field effect tube amplifiers and buffer circuits with high input impedance. Typical active probes provide 1:1 voltage gain and 1MOhm input resistance and 2.2pF input capacitance, 1G bandwidth. Its circuit schematic is shown in Figure 14:

Figure 14 Schematic diagram of active probe

Key components include a source made of a field effect transistor and a transmitter made of a complementary bipolar transistor. FETs provide very high input resistance, greater than 1011 times the probe input resistance. Active probes require power and have a more stringent dynamic range. In fact, the biggest disadvantage of high-bandwidth active probes is that they are easily damaged by high voltage. Because active probes are much more expensive than passive probes, users should be careful to avoid this. In fact, active probes fill in the gaps between high-impedance probes and low-capacitance probes. They have a bandwidth of up to 2GHz and a relatively high input resistance, and can drive relatively long cables. In addition, active probes

The head can adjust bias and coupling, so it is often used in ATE automatic test systems where the test instrument needs to be a certain distance away from the circuit under test.

Figure 15 Simplified model of probe ground lead inductance

Probe Grounding and Signal Integrity

As the frequency increases, the second-order effects of the probe, such as the probe ground lead inductance, will affect the measured waveform. The ground lead inductance is determined by the ground lead length and the signal frequency. Considering the simplified model in Figure 15, the probe ground lead provides a signal loop with an inductance of

The ground plane geometry is a function of the ground plane geometry. For example, the traditional winding inductance is 25nH/inch, 10:1, and the inductance of the high resistance probe is 100-150nH. These inductors and the probe capacitance form a resonant loop. For example, the typical probe capacitance is 15pF, and a 7-inch ground wire is 175nH, which has a resonant frequency of 98Mhz. If the signal frequency is around this value, "ringing" will occur.

Figure 16 10:1 probe, 7-inch ground wire measurement results

Summarize

For modern electronics that use fast analog and digital circuits, the use of probes becomes extremely complicated. As signal frequencies get higher, the loading effects on the circuits become more complex. Engineers need to consider the appropriate probe and connection techniques to reduce these effects.

Useful formulas for probe users

Measurement System for Calculating Bandwidth and Rise Time

Cs source capacitance (unit: Farads)

Co Input capacitance (in Farads)

Rs source resistance (in Ohm)

f source frequency (in Herts)

txxx Rise time (in seconds)

BW Bandwidth (-3dB, unit: Herts)

Rise time, Tr:

Bandwidth, BW:

Rise time as a function of bandwidth:

System Rise Time:

Signal rise time: