Oscilloscope Passive Voltage Probes

Keywords: Passive Voltage Probe, Attenuation Ratio, Input Impedance, Equivalent Circuit.

10:1 Passive voltage probes are standard accessories for oscilloscopes. Compared to active voltage probes, they offer advantages such as a wider dynamic range, no need for an external power supply, and lower cost.

Common attenuation ratios include 10:1, 1:1, and switchable 10:1 / 1:1. Input impedance options are available to match oscilloscope inputs, typically 1 MΩ or 50 Ω.

This article explains the structure of passive voltage probes and important usage considerations.

1. If it is a 10:1 and 1:1 switchable passive voltage probe, it is essential to understand that the bandwidths of the two are different, because the capacitance of the 1:1 probe is as high as 10 pF. Taking the GW Instek GTP-200B-4 as an example, the bandwidth at 10:1 is 200 MHz, while the bandwidth at 1:1 is 10 MHz. The most intuitive phenomenon is that the high-frequency square wave becomes a sine wave when switched to 1:1.
   
   
2. A 10:1 probe with a resistance of 9 MΩ, paired with an oscilloscope input impedance of 1 MΩ, forms a 10:1 attenuation circuit. If your oscilloscope's input impedance can be set to 50 Ω in addition to 1 MΩ, you will immediately see the waveform disappear because the
   attenuation ratio =

   50 9M + 50
   , and the signal is attenuated.
   
   
3. The input capacitance of a 10:1 passive probe is approximately 5 pF to 30 pF, while that of a 1:1 passive probe is approximately 65 pF to 105 pF. Although they have better dynamic range (≤600V DC+Peak AC for 10:1 probe and ≤200V DC+Peak AC for 1:1 probe), they are not suitable for measuring high-frequency wideband signals with fast rise times.
   
   

1:1 Passive Probe Equivalent Circuit

10:1 Passive Probe Equivalent Circuit

Figure 1: equivalent circuits of 1:1 and 10:1 probes

The input impedance of a spectrum analyzer is 50 Ω, and oscilloscopes with bandwidths above 350 MHz also have a 50 Ω input impedance. A special passive voltage probe can be used to match these applications with a 50 Ω characteristic impedance.

For example, if the input withstand voltage of a spectrum analyzer is insufficient, and the oscilloscope requires a low-cost probe solution with high bandwidth, a passive voltage probe named after its characteristic impedance Z0 can be used. Its input resistance is 450 Ω. After matching the 50 Ω input of the spectrum analyzer or oscilloscope, a voltage divider attenuation circuit with an attenuation ratio of 10:1 = 50 450 + 50 is formed. Figure 2 shows its equivalent circuit, and Figure 3 shows the actual circuit. It should be noted that the dynamic range is only ≤ 8.5V DC+Peak AC.

Figure 2：Equivalent circuit of a passive voltage probe named after its characteristic impedance

Figure 3：GW Instek's GTP-1501R 1.5 GHz 10:1 Z0 Passive Voltage Probe

Conclusion:
Although passive probes are the simplest in structure and the most widely used, many measurement problems can still arise in practical operation if the working principle and the load effect of capacitance are not understood.

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Diana

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E-mail: diana@goodwill.com.tw