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Author: WisdomAugust

How to select a probe?

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 Author| Post time 2017-5-15 09:00:55 | Show all posts
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.

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 Author| Post time 2017-5-16 16:32:13 | Show all posts
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.


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 Author| Post time 2017-5-17 08:55:07 | Show all posts
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.

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 Author| Post time 2017-6-2 08:12:24 | Show all posts
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.





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 Author| Post time 2017-6-3 08:56:43 | Show all posts
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.

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 Author| Post time 2017-6-5 08:41:49 | Show all posts
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.


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 Author| Post time 2017-6-6 11:45:07 | Show all posts
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.


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 Author| Post time 2017-6-7 08:16:32 | Show all posts
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.


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 Author| Post time 2017-6-10 10:33:53 | Show all posts
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.

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 Author| Post time 2017-6-12 08:27:30 | Show all posts
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.


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