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Title: How to select a probe? [Print this page]

Author: WisdomAugust Time: 2016-5-17 11:49
Title: How to select a probe?
Below is a list you need to consider when you choose a probe for a given measurement.

Input resistance

Input impedance is used to describe the loading effects of a probe. At DC and low frequency ranges, the probe's resistive component is the main factor that loads down the circuit under test. But as the frequency goes up, the capacitance of the probe tip, in parallel with the input impedance of the probe, resulting in greater loading and a more adverse effect to the target.

Author: WisdomAugust Time: 2016-6-1 11:34
Input Capacitance
Excessive input capacitance shows down the system's pulse response. Usually the least input capacitance possible is best.

Author: WisdomAugust Time: 2016-6-2 12:25
Maximum input voltage
To ensure user safety, help protect the oscilloscope input from destructive voltage, and avoid damage to the probe, select a probe that is rated for a higher voltage than the signal you intend to test.

Author: WisdomAugust Time: 2017-4-20 09:29
Bandwidth
The rated bandwidth of a probe should match the one of oscilloscope and be adequate for the test signal.But at higher frequencies, grounded lead inductance and input capacitance often influence system performance more than probe bandwidth.

Author: WisdomAugust Time: 2017-5-3 15:56
Attenuation
Choose the attenuation ratio of the probe (X1, X10, X100, X1000) to match the test signal amplitude to the vertical sensitivity range of the oscilloscope.

Author: WisdomAugust Time: 2017-5-4 16:29
Probe compensation range

Most passive probes have a specification that lists the oscilloscope input capacitance
range over which they can be used. When choosing a passive probe, be sure that the
oscilloscope's input capacitance lies within the probe's compensation range or you will
not be able to adjust the probe to chieve a correctly compensated square wave signal.

Author: WisdomAugust Time: 2017-5-5 15:25
Edited by WisdomAugust at 2017-5-5 15:27

Most oscilloscopes have 1 ohm input resistance. This input resistance
is in parallel with the input capacitance. Normally, high-frequency probes
with attenuation factors grater than X1 have adjustable compensation
networks built into them. Adjusting this compensation network provides
the best possible frequency linearity over the oscilloscope's desinged
frequency range. Operating instructions provided with probe explain
how to adjust the compensation network to obtain best signal fidelity.

[attach]1506[/attach]

Author: WisdomAugust Time: 2017-5-8 11:47
To select the appropriate probe, the first thing to do is to understand
the impact of the probe on the test, which includes the meaning of 2 parts:
1) the impact of the probe on the to be tested circuit;
2) The signal distortion caused by the probe.
The ideal probe should have no effect on the to be tested circuit, meanwhile
no distortion on the signal. Unfortunately, there is no such a probe to meet
these two conditions at the same time, usually need to do some trade-off
between the two parameters.

Author: WisdomAugust Time: 2017-5-9 10:41
In order to consider the impact of the measurement of probe,
usually we simply take the probe model as a R, L, C model,
and analysis the model and to be tested circuit together.

[attach]1516[/attach]

Author: WisdomAugust Time: 2017-5-10 13:22
First, the probe itself has an input resistance. The same theory as
testing voltage by multimeter, in order to minimize the impact of
the to be tested circuit, requires the input resistance (Rprobe) of
the probe itself as much as possible. However, because it's impossible
for Rprobe to be infinity, shunting will be generated with the to be
tested circuit, and the actual measured voltage may not be the
the real voltage on probe point, which is ofter encountered on some
power or amplifier circuit test. To avoid the impact of the load resistance
of the probe, there is the level required of Rprobe-10 times more than
Rsource and Rload. Most of the probe's input impedance is between
10K ohms and 10M ohms.

Author: WisdomAugust Time: 2017-5-15 09:00
Second, the probe itself has an input capacitor, which is not deliberately done, but is parasitic.
This parasitic capacitance is also the most important factor affecting the bandwidth of the probe,
because this capacitor will attenuate high frequency, and slow the rising edge of the signal.
Usually high-bandwidth probe parasitic capacitance is relatively small. Ideally the Cprobe should
be 0, but actually it can not. The input capacitance of the common passive probe is between
10pf to several hundreds of pf, and the input capacitor of higher bandwidth active probe
is usually between 0.2pf and several pf.

Author: WisdomAugust Time: 2017-5-16 16:32

Third, the probe input will be affected by the inductance. The input resistance

and capacitance of the probe are better to be understood, but the inductor at

the input of the probe is often overlooked, especially when do high frequency

measurements. Where the inductance comes from? We know where is a wire,

there is a inductor. Between the probe and the measured circuit there must be

a connecting wire, meanwhile the return of the signal also passes through the

ground of the probe.

Author: WisdomAugust Time: 2017-5-17 08:55

Usually 1mm probe ground will own about 1nH inductance. The longer signal and ground,

the greater inductance. The parasitic inductance and parasitic capacitance of the probe

composed a resonant loop. If the inductance value is too big, it is possible to generate

high-frequency resonance under the excitation of the input signal, resulting in distortion

of the signal. So the high-frequency testing requires strict control of the length of the

signal and ground, otherwise it is prone to ringing.

[attach]1518[/attach]

Author: WisdomAugust Time: 2017-6-2 08:12
Edited by WisdomAugust at 2017-6-2 08:13

Before learning the structure of the probe, we need to know the structure

of the oscilloscope input interface first, because it is the place where the

probe is connected. The input interface circuit oscilloscope and the probe

together composed the detection system.

[attach]1528[/attach]

Author: WisdomAugust Time: 2017-6-3 08:56

Most of the oscilloscope input interfaces are in the form

of BNC or BNC compatible. The input of the oscilloscope

has a matching resistance of 1M ohms or 50 ohms. There

are many types of oscilloscope probes, but the oscilloscope's

match is always 2 options: 1M ohm or 50 ohm. Different types

of probes need different matching resistance.

Author: WisdomAugust Time: 2017-6-5 08:41

From the voltage measurement perspective, to minimize the

impact of to be measured circuit, the oscilloscope can use

a high input impedance of 1M ohm. However, the bandwidth

of the high impedance circuit is susceptible to parasitic

capacitance, so 1M ohm input impedance is widely used with

measurements below 500M bandwidth. For higher frequency

measurements, a 50 ohm transmission lead is usually used,

so the 50 ohm match of the oscilloscope is mainly used for

high frequency measurements.

Author: WisdomAugust Time: 2017-6-6 11:45
Edited by WisdomAugust at 2017-6-6 11:54

Traditionally, most of the oscilloscopes below the 100MHz bandwidth
on the market have only 1M ohm inputs because they are not used for
high frequency measurements; most of the 100MHz to 1GHz bandwidth
oscilloscopes have both 1M and 50 ohm switching options, taking into
account both high and low frequency measurements; 2GHz or higher
bandwidth oscilloscopes are mainly used for high-frequency measurement,
so most of them only have 50 ohm input. However, in view of the needs of
the market, some oscilloscopes above 2GHz also provide 1M ohm and 50
ohm input switch.

Author: WisdomAugust Time: 2017-6-7 08:16

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.

Author: WisdomAugust Time: 2017-6-10 10:33
Edited by WisdomAugust at 2017-6-22 13:20

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.

Author: WisdomAugust Time: 2017-6-12 08:27
Edited by WisdomAugust at 2017-6-22 13:21

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.

[attach]1595[/attach]

Author: WisdomAugust Time: 2017-6-15 08:27

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.

Author: WisdomAugust Time: 2017-6-16 08:10

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.

Author: WisdomAugust Time: 2017-6-19 08:24
Edited by WisdomAugust at 2017-6-22 13:21

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.

[attach]1596[/attach]

Author: WisdomAugust Time: 2017-6-20 09:21

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.

Author: WisdomAugust Time: 2017-6-21 08:11
Edited by WisdomAugust at 2017-6-22 13:22

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.

[attach]1597[/attach]

Author: WisdomAugust Time: 2017-6-22 13:17

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.

Author: WisdomAugust Time: 2017-7-12 14:43
Edited by WisdomAugust at 2017-7-12 14:49

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.

[attach]1630[/attach]

Author: WisdomAugust Time: 2017-7-13 09:11

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.

[attach]1636[/attach]

Author: WisdomAugust Time: 2017-7-14 08:24

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.

[attach]1638[/attach]

Author: WisdomAugust Time: 2017-7-17 08:29

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.

Author: WisdomAugust Time: 2017-7-18 16:16

We can see all the excellent features of the active probe are

brought by the front amplifier, but this high bandwidth amplifier

is expensive and need to be placed in a limited space in the

front of the probe, resulting in high cost.
General passive probe prices are hundreds of dollars or so,

while the price of active probes usually are thousands of dollars.
So the active probe is generally used in the need for the

occasion pf high measurement bandwidth.

Author: WisdomAugust Time: 2017-7-19 16:49

However, the active probe is not no shortcomings, limiting the wide

usage of active probes besides price factors, the other reason is its

limited dynamic range. The input voltage range like this high bandwidth

amplifier is not very large, so its measurement range can not be as big

as the passive probe. Commonly used 10: 1 passive probe, its dynamic

range usually at hundreds of volts, and it’s just a few volts for common

active probes. So there will be some limiation for its application.

Author: WisdomAugust Time: 2017-7-20 13:57

There is also a branch of the active probe, that is differential active probe.

The difference is that the front amplifier is a differential amplifier.
The advantage of the differential amplifier is that it can directly test the

high-speed differential signal, while its common mode rejection ratio is higher,

the common mode noise rejection ability is better.

[attach]1648[/attach]

Author: WisdomAugust Time: 2017-7-21 11:06

Current probe is another kind of active probe. There’s a magnetic ring

on the front end of the current probe, when measure, the magnetic ring

cover on the measured power line. Because the generated magnetic

field when the current flowing through the wire, which was collected by

the magnetic ring, the magnetic flux in proportion to the current flowing

through the wire.

Author: WisdomAugust Time: 2017-7-22 09:14

There’s a Hall sensor inside magnetic ring, which can detect

the magnetic flux and its output voltage in proportion to magnetic

flux. Therefore, the current probe output voltage is in proportion

to the measured current flowing through the wire. The conversion

factor for a typical current probe is 0.1V / A or 0.01V / A.

Author: WisdomAugust Time: 2017-8-3 08:47

The main advantage of the current probe is that it can measure current without

disconnecting the power line. Based on the Hall effect, it can measure both DC

and AC. Typical applications of current probes are system power measurement,

power factor measurement, and switching machine impact current waveform

measurements. The main drawback of the current probe is that its small current

measurement capability is limited by the bottom noise of the oscilloscope, so the

small current measurement capability is limited. Generally less than 10mA of

current is difficult to measure.

[attach]1651[/attach]

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