How to select a probe?

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

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

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

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

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

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

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

Sorted by  the size of the input impedance is divided into

two types: high impedance passive probe and low resistance

passive probe.

High-impedance passive probe is the one that we usually call

passive probe, which is the most widely used probe, basically

every person using the oscilloscope have used this type of probe.

When the high-impedance passive probe is connected to the

oscilloscope, the input impedance of the oscilloscope is

required to be 1M ohms. The following is a schematic diagram

of a 10: 1 high impedance passive probe.

WisdomAugust

According to the power supply oscilloscope probe can be

divided into passive probe and active probe, sorted by

the measured signal type the probe can be divided into

voltage probe, current probe, optical probe etc. The

so-called passive probe indicates the entire probe by

composed by passive components, including resistors,

capacitors, cables etc; well the active probe inside

has general amplifier, and the amplifier need to be

powered, so named active probe.

WisdomAugust

Broadly speaking, the test leads belongs to probe as well,

like BNC or SMA cable. This type of probe is cheap and high

performance. But the use of test leads connection require the

to be tested circuit has BNC or SMA interface, thus their application

is limited, mainly for radio frequency and microwave signal testing.

For digital or general signal testing, should use a special probe.