# How are 500 MHz standard passive probes tested?

I would like to understand how a standard passive probe is said to have a bandwidth equal to 500 MHz. Actually what is the standard setup to qualify a passive probe?

I found an article on Teledyne LeCroy which explains how to model a passive probe. According to this, the bandwidth of the scope depends on the source resistance and the ground lead inductance, but the source resistance is unknown and the ground lead inductance depends on how you use the probe. I mean the worst case is by using the alligator clip.

When you take a look by doing a simulation, it seems that the bandwidth of its kind of probe can be very low if the source resistance is high.

Does anyone know the standard setup for qualifying a passive probe of 500 MHz?

• In a nutshell, does your question boil down to what source impedance is assumed? Mar 13, 2023 at 16:45
• @Andyaka Yes but it also depends on the ground lead inductance
– Jess
Mar 13, 2023 at 17:30
• When probing fast-edged signals, you connect to the ground ring on the probe with the shortest conductor possible to circuit ground. Do not use the clip-on ground lead. With high impedance circuits the probe capacitance will reduce your bandwidth and may cause instability in the circuit probed. For fast-edged signals or non-trivial impedance circuits, consider using a low capacitance FET probe.
– qrk
Mar 13, 2023 at 17:52
• As said in the earlier comment @jess, the ground lead cannot be used on high frequencies so it's kind of a red-herring to consider it at all. Mar 13, 2023 at 19:01
• Note that the equivalent circuit that you have shown includes the scope. Here is a schematic showing the separate sections. It is from a teardown of a probe, others may vary slightly. electronics-diy.com/schematics/967/Scope-Schematic-1.jpg Mar 13, 2023 at 20:04

A typical x10 probe schematic is shown below, when connected to a typical scope input. The key difference between this schematic, and the schematic of the article mentioned by the OP, is the presence of C1 at the probe tip. C1 compensates for drop-off in frequency response by providing a low-impedance path for high-frequency signals.

simulate this circuit – Schematic created using CircuitLab

Description of the Schematic:
R1 & R2 form a divide-by-10 voltage divider for DC, but the AC response of is determined by the three capacitors:
C1: This is usually a fixed-value high-voltage capacitor with low inductance and low dielectric loss.
C2: This is an adjustable capacitor, the one you tweak to "compensate a probe".
C4: The capacitance within the scope. (sorry, this should have been "C3".)

If C2 is adjusted so that the value of C2 in parallel with C4 is 9x C1, then we have a properly compensated probe with good high-frequency response. For an intuitive idea about how this probe behaves at high frequency: The load that the signal at the probe tip sees will be 9Mohm in parallel with about 3pF, which gives an approximate RC time constant of ~ 30E-6s == 5.3kHz. So you can see that for the majority of the probe bandwidth the characteristics are determined by the capacitors, not the resistors.

This means that for signal frequencies above 5kHz, the loading effect on that signal is due to the probe capacitance rather than its resistance. At 100MHz, the impedance of a 3pF capacitor is just 530 ohms, and at 500MHz it will be just 106 ohms. Hence the reason why good probes tend to have lower values for C1. If the loading effect of ~500 ohms is excessive for the circuit under observation, then consider using a x100 probe (exchange gain for bandwidth and lower capacitance at the tip), or an active probe (the probe head contains an amplifier to reduce loading effect of the signal, and to improve frequency response).

In regards to test conditions for passive probes: most are specified with a sig-gen having an internal impedance of 50ohm, but it is recommended to read the technical specifications of the particular probe you are using.

For an excellent introduction to scope probes including passive probes, active probes, differential voltage probes, & current probes, may I suggest the following document may be of some value:

Here are some other links that may be useful to understand how passive probes trade-off lower gain for higher bandwidth, and practical ideas on implementing passive probes:

Regarding Passive Probes 1:1 and 10:1

Poor man's x100 oscilloscope probe