Specifications

ManualsBrandsB&K Precision ManualsPower Tools2125A

1M

(9 + 1)M

= 0.1

This is why the probe is called a 10X probe as it attenuates the signal by 10 times. You'll also see them

called X10 probes. The cable's distributed capacitance and the scope's input capacitance will shunt the

signal around R

at higher frequencies -- this leads to the basic fact that a passive probe's impedance

at its rated bandwidth can be more than four orders of magnitude smaller than its DC

resistance. Keep this in mind when measuring signals with high frequencies as your probe will likely be

loading your circuit significantly. A common technique to see if a probe is loading a circuit is to connect

another identical probe to the scope, then connect this second probe to where the first is probing. If the

signal from the first probe changes, then circuit loading is a significant issue.

For example, a 150 MHz probe was measured to have a distributed cable capacitance of 58 pF. With a

20 pF typical scope input capacitance, this means a capacitance of about 80 pF. At 150 MHz, the

capacitive reactance of this capacitor is 13 Ω, so you can see that this effectively shorts out the 1 MΩ

input resistance of the scope. Though the capacitive reactances are low, the capacitances still behave

as a capacitive divider and attenuate the signals appropriately. To see this is approximately true, the

adjustable capacitor is on the order of 10 pF and C

+ C

is 80 pF -- almost 90 pF. If it was 90 pF, the

reactances as impedances would have a voltage divider ratio exactly the same as the resistances (the

2 terms for the reactances are factored out and we're ignoring the much larger resistances which are

effectively open circuits).

= 0.1

You may wonder what the adjustable capacitor C

is for. This capacitance is used to compensate the

probe. Compensation means to adjust the capacitance so that the probe has the correct amplitude

response for different signal frequencies. The compensation capacitor C

is adjusted to make the time

constant R

equal to the time constant of the distributed cable capacitance and scope's capacitance

of R

+ C

). This yields a circuit with minimal distortion of the signal. A typical passive probe has an

adjustment between 10 to 30 pF.

For higher frequency passive probes, the equivalent circuit may be more complex -- and compensation

may be more complicated than simply adjusting a single capacitor. In fact, probes and their circuits

need to be analyzed as transmission lines -- and such analysis will show that probe design is not a

trivial task.

You can measure your probe's DC attenuation with a digital multimeter, but you'll have to calculate the

probe's voltage drop from knowing the digital multimeter's input resistance. For example, with a 10X

probe and a digital multimeter with a measured 10.05 MΩ input resistance, an 11.44 volt DC signal was

measured with a probe. The digital multimeter read 6.05 volts using the probe. The relevant voltage

divider is the 10 MΩ input resistance of the digital multimeter in series with the 9 MΩ of the probe.

Thus, the calculated voltage that the digital multimeter should have measured is:

10.05

9 + 10.05

11.44 = 6.035 volts

This probe's measured attenuation was within 0.24% of its expected value.

A probe's rise time 



can be measured with a fast-rising pulse. Since the oscilloscope also has a rise

time 



, the measured rise time t needs to be corrected:

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