How to select a probe?

WisdomAugust

To facilitate the measurement, the probe usually

has around 1 meter length. If not adding the matching

circuit, it is hard to imagine that the probe can

provide hundreds of megabytes of bandwidth. The input

parasitic capacitance of the oscilloscope affects the

bandwidth as well.

To improve the high frequency corresponding of the probe,

there will be a corresponding matching circuit on the

probe’s front-end. The most typical is a Rprobe and

Cprobe parallel structure.

WisdomAugust

A condition for the probe to produce a flat gain in

the band is to meet Rprobe * Cprobe = Rscope * Cscope.

Cscope is the parasitic capacitance of the oscilloscope,

so it can only be controlled in a certain range,but

can not be precisely controlled, i.e. The Cscope value

of different oscilloscopes or different channels on

oscilloscope will be different. In order to compensate

for changes in different channels Cscope, at the end of

the probe connected to the oscilloscope, where will be

at least an adjustable capacitor-Ccomp.

WisdomAugust

When the probe is connected to different channel of

the oscilloscope, it can compensate Cscope changes

by adjusting the Ccomp. Almost all oscilloscopes

provide a low frequency square wave output,

which can connect to the probe to adjust

the shape of the saqure wave to adjust

the Cscope.

WisdomAugust

Rprobe improves the frequency response and produces a

divider with oscilloscope input resistance. The so-called

10: 1 divider is the actual measured voltage of the

oscilloscope is 1/10 of the voltage at the front end

of the probe, i.e. the signal through the probe will

have a 10 times attenuation.

WisdomAugust

The simpler probe needs to set the oscilloscope's attenuation manually

to get the correct display. More probes have an auto-detect pin at the

end which connect to the oscilloscope. It can read the attenuation

ratio of the probe through the pin when the probe is plugged in

the oscillsocope, and adjust the display ratio automatically.

WisdomAugust

There are two special types of high impedance passive probes.

One is a high-voltage probe, the attenuation ratio up to 100:

1 or 1000: 1, so the measurement voltage range is large;

another is a type of 1: 1 probe, i.e., there’s no attenuation

of the signal when input the oscilloscope. Not like 10: 1

probe which need the oscilloscope to enlarge the display,

so the noise of the oscilloscope itself is not amplified,

and the measured of noise will be much smaller, which use

a lot in the small signal and power ripple measurement.

WisdomAugust

Another passive probe is a low-resistance passive probe,

which is the least commonly used probe.

The biggest advantage of the low-resistance passive probe

is that it has relatively high test bandwidth (1G-6GHz) with

the price of an ordinary high-impedance passive probe.

The main drawback is that the input impedance is low

(only 500 ohm or 1k ohm), if connect to the circuit, it may

still have a little impact on the measured signal.

The following is its duagrammatic layout.

WisdomAugust

The low-impedance passive probe requires an oscilloscope's input impedance

of 50 ohms and its front-end equivalently series connect a divider resistor.

According to the different series resistance to achieve different partial voltage

ratio. For example, if series a 450 ohm resistor, the partial voltage is 10: 1.

As a result of 50 ohm transmission cable, and the oscilloscope is 50 ohm

as well, so the entire probe bandwidth is relatively high.

WisdomAugust

The high impedance passive probe input impedance is high,

but its bandwidth is not high, low resistance passive probe

bandwidth can be high but the input impedance is not high.

Then is there a probe with both high input impedance and

high bandwidth? Actually yes, it is active probe.

In fact, active probe is a broader understanding, indicates

the probe need to be powered.

The schematic diagram of porpular active voltage probes as

follows.

WisdomAugust

There is  a high-bandwidth amplifier on the front end of the active probe.

The amplifier needs power supply, and the name of active probe attribute

to it. The input impedance of the amplifier is relatively higher, so the

active probe can provide a relatively higher input impedance; while

the amplifier output drive capability is very strong, so it can directly

drive the rear 50 ohm load and transmission line. Because the 50-ohm

transmission line can provide a high transmission bandwidth, plus the

high bandwidth of the amplifier itself, so the entire probe system

compared to passive probes can provide higher bandwidth.