Probe Types I: Passive Probes

There are a number of different types of oscilloscope probes to choose from and each has an area where it best performs. There are two major categories that voltage probes are classified, passive and active probes.

The passive probe category includes low-impedance resistor divider probes and compensated high-resistance, passive divider probes. A subset of the compensated high-resistance passive divider probes is the high-voltage probe. This probe is required when looking at very large signals.

There are two basic types in the active probe category: the FET probe and the non-FET (or bipolar) input active probe. There is also the differential probe which is a special case of the active probe.

Figure 6

Figure 6 shows a diagram of the low-impedance resistive divider probe. The tip contains a resistor, either 450 or 950-. They give 10:1 and 20:1 attenuation, respectively. The probe cable is a 50- cable that is terminated with the 50- input of the scope. The benefits of using this probe are the low capacitive loading and a very wide, or very high, bandwidth. As a result, you get very accurate timing measurements. In addition, this probe has a relatively low cost compared to the active probe. Resistive divider probes are useful for probing low-voltage signals (less than 50 Volts), such as ECL circuits and 50- transmission lines. The one tradeoff is that this probe has relatively heavy resistive loading. So, you must have a good understanding of the circuit being measured.

Figure 7

Figure 7 shows a diagram of the compensated high resistance passive divider probe. It is used to measure low to intermediate signals less than 500 Volts. This is a commonly used probe, and is shipped with the majority of oscilloscopes today. The probe tip is typically 900-k which results in a 10:1 attenuation. The cable is a high-impedance cable in this case. A termination box at the end of the probe cable connects to the input of the oscilloscope. In the termination box is a 111-k resistor that works in parallel with the input resistance of the oscilloscope. This ensures that there is a 100-k input impedance to the oscilloscope to give a 10:1 division ratio. The probe has a very high resistance, has the adjustable compensating capacitor for matching to the input capacitance of the oscilloscope, and has a very high dynamic range. This probe is the most rugged of all the probes. The applications for which it is best suited are general probing and troubleshooting. The probe's high input impedance is usually 1-M or greater, and is ideal for measuring summing junctions of operational amplifiers. The tradeoffs are that it has the lowest bandwidth and has the heaviest capacitive loading as compared to the low-impedance resistive divider probe. On the other hand, this probe provides excellent pictures, or a look at circuit activity. If more precise measurements are needed, different probes could be used for characterization.

Figure 8

Figure 8 shows how to properly adjust the compensating capacitor that is in the termination box at the end of the probe cable. You can have either overshoot or undershoot on the measurement of the square wave if the low frequency adjustment is not made properly. This will result in inaccuracies in high frequency measurements due to damping or peaking of high frequencies in the pulse edges. When the capacitor is adjusted properly, the scope will display a square wave with no overshoot or undershoot.

Figure 9

Figure 9 shows the circuit diagram of the HP1137A high-voltage probe. This probe is used for measuring signals over 500 Volts. In this case the probe tip resistance is 500-Mohm. The cable used is very similar to the one that is in the standard, compensated high- resistance passive divider probe. Again, there is the adjustable compensation capacitor at the connection to the oscilloscope, so that the capacitances can be matched. The divider ratio of the probe can be selected, and it has a very high dynamic range. The benefit is that very high voltages can be measured with this probe, and it is the safest way to look at voltages over 500 Volts. Typical oscilloscope probes could break down if they were used to look at such high voltages. The applications for this probe are high-voltage video signals, switching power supplies, and large power transmission signals. The tradeoffs in this case are a physically larger probe, and a lower bandwidth as compared to standard oscilloscope probes.

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