Agilent Technologies N2780B Manual de usuario descargar pdf (Pagina 2)

2

How to select a probe

Selecting the correct probe for your

oscilloscope measurement should not

be difficult. This brochure provides

suggestions on how to make the best

decision. Following is a list of probe

parameters you need to consider

when you select a probe for a given

measurement.

Attenuation

Choose the attenuation ratio of the

probe (1:1, 10:1, 100:1, 1000:1) to

match the test signal amplitude to

the oscilloscope’s vertical sensitivity

range.

Bandwidth (BW)

The probe’s rated bandwidth should

match the oscilloscope and be

adequate for the test signal. However,

at higher frequencies, grounded lead

inductance and input capacitance

often influence system performance

more than probe bandwidth does.

Input resistance (Rin)

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.

However, as the frequency goes up,

the capacitance of the probe tip in

parallel with the DC resistance starts

to reduce the input impedance of the

probe, resulting in greater loading and

a more adverse effect to the target.

Input capacitance (Cin)

Excessive input capacitance (some-

times called tip capacitance) will slow

down the system’s pulse response.

Usually the least input capacitance

possible is best.

Maximum input voltage (Vmax)

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.

Probe compensation range

Most passive probes have a specifica-

tion 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 achieve a correctly compensated

square wave signal.

Most oscilloscopes have 1-MΩ input

resistance. This input resistance is in

parallel with the input (shunt) capaci-

tance. Normally, high-frequency probes

with attenuation factors greater than

1:1 have adjustable compensation net-

works built into them. Adjusting this

compensation network provides the

best possible frequency linearity over

the oscilloscope’s designed frequency

range. Operating instructions provided

with the probe explain how to adjust

the compensation network to obtain

best signal fidelity.

Probe Interface

Most Agilent oscilloscope probes offer

either BNC type of probe interface

or the AutoProbe interface. The

AutoProbe interface is an intelligent

communication and power link

between compatible probe and the

Infiniium or InfiniiVision Series oscil-

loscopes. The AutoProbe identifies the

type of probe attached and sets up the

proper input impedance, attenuation

ratio, probe power and offset range as

needed.

Probe tip form factor

Your probe must make a reliable

connection to the test point, and you

may want it to grab the test point.

Generally, this requires a small and

light probe and a tip or grabber that is

compatible with the test point. SMT

and fine-pitch geometries make this

issue especially critical.

Under-compensated

The effects of passive probe compensation:

Over-compensated Properly compensated

1 2 3 4 5 6 7 ... 16 17

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