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Oscilloscope probe voltage derating vs frequency

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schmitt trigger

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What is the technical reason which the scope probes are derated with frequency?

This is a generic probe from TPI, but have seen the same behavior on other brands, too
 

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Current handling of the compensation caps?
 

Frequency limit is unusually low. Many passive probes have full voltage rating up to 1 MHz.

I'd expect that voltage divider HV resistor as well as HV capacitor have high frequency derating.
 

yes, current at 600Vrms at 1GHz in the probe's capacitance would be high - also almost all caps have to be derated with freq - due to losses in the dielectric and loss of volt withstand with freq
 

From Tektronix application note Oscilloscope Probe Circuits by Joe Weber:
The maximum input signal amplitude for any probe changes with frequency. (…) Since probe input impedance decreases as applied frequency increases, probe current increases with frequency. The maximum input voltage rating is decreased so that the power rating of the probe components is not exceeded. In probes that have a reverse termination resistor in the probe head, this 1/2 watt resistor will determine the derating curve. In probes that have the termination circuitry at the oscilloscope end of the cable, the I²R characteristic of the cable determines the derating curve.

Reverse termination is referring to series resistor R2 in the below schematic, cable I²R is significant because passive probes are using a lossy cable with resistance wire inner conductor.

probe.PNG
 
Makes sense now!

I was wondering how come a 600VDC probe would only withstand about 50V @ 10 Mhz, which is not that high in frequency.

Fortunately, logic circuits running that fast are all low voltage.

But it could be a problem in some modern SMPS powertrains.
 

The more typical derating curves of modern 10:1 and 100:1 probes should be sufficient for most SMPS applications.

derating.PNG derating_100_1.PNG
 

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