How to Choose an Oscilloscope

WisdomAugust

Since an oscilloscope can store only a limited number of samples,
the waveform duration (time) will be inversely proportional to
the oscilloscope's sample rate.
Time interval=Record Lengh/Sample Rate

So, with a record length of 1Mpoints and sample rate of 250 MSa/s,
the oscilloscope will capture a signal 4 ms in length.

WisdomAugust

Long Record Length
Record lenth is the number of points in a complete waveform record.
An oscilloscope can store only a limited number of samples, so, generally
the greater the record length the better.

To search for infrequent transients such as jitter, runt pulses and glitches,
select at least a mid-end oscilloscope that conbines long record length
with a  high waveform capture rate.

WisdomAugust

Triggering synchronizes the horizontal sweep at the correct point in the signal,
rather than just starting the next trace at the point where the present trace
happens to finish. A single trigger acquires all input channels simultaneously.

WisdomAugust

Versatile Triggering

Triggering gives a stable display and lets you zero in on specific parts of complex waveforms.

To acquire anomalies and make best use of the record length of scope, look for a scope that
provides advanced triggering on more challenging signals. Triggering lets you isolate a group of
waveforms to see what is going wrong. Specialized triggers can respond to specific conditions
in the incoming signal making it easy to detect, for example, a pulse that is narrower than it
should be.

The wider the range of trigger options available the more versatile the scope and the faster you
get to the root cause of a problem.

WisdomAugust

Accurate reconstruction of a signal depends on both the sample rate and the interpolation method used.

Linear interpolation connects sample points with straight edged signals.

Sin x/x interpolation is a mathematical process in which points are calculated to fill in the time between the real samples.
This form of interpolation lends itself to curved and irregular signal shaples, which are far more common in the real world
than pure square waves and pulses.

Consequently, sin x/x interpolation is the preferred method for applications where the sample rate is 3 to 5 times the
system bandwidth.

WisdomAugust

Fast Sample Rate

Sample rate is how often an oscilloscope samples the signal.
A high sample rate increases resolution, ensuring that you'll see intermittent events.
The minimum sample rate may also be important if you need to look at slowly changing signals over longer periods of time.

We recommend to use a sample rate of at least 5 times your circuit's highest frequency component.

WisdomAugust

Choose accurate and enough input channels

How many channels to select depends on your application.
2 or 4 analog channels will allow you to view and compare timings of your waveforms.

Some oscilloscopes share the sampling system between channels to save money.
Thus, the number of channels you turn on can reduce the sample rate.

Isolated channels simplify floating measurements. Unlike ground-referenced oscilloscopes,
the input connector shells can be isolated from each other and from earth ground.

Whatever you select, all channels should have good range, linearity, gain accuracy,
flatness and resistance to static discharge.

WisdomAugust

Matching Probes

Precision measurements start at the probe tip.
The probe's bandwidth must match that of the oscilloscope-the 'five times rule' again,
and must not overload the device under test.

Probes actually become a critical part of the circuit, introducing resistive, capacitive
and inductive loading that alters the measurement.

When selecting a mid range scope choose probes with capacitive loadings of <10pF.

WisdomAugust

Rise Time: Your scope rise time must be fast enough to capture rapid transitions accurately.

Rise time is defned as,  K/Bandwidth  where k is between 0.35 (typically for scopes with bandwidth \<1 GHz) and 0.40 to 0.45 (>1 GHz).
Similar to bandwidth, an oscilloscope's rise time should be < 1/5 x fastest rise time of signal.
E.g. a 4-ns rise time needs a scope with faster than 800 ps rise time. Note: As with bandwidth, achieving this rule of thumb may not always be possible.

Rise times are critical for studing sqaure waves and pulses. Square waves are standard for testing amplifier distortion and timing signals for TVs and computers. Pulses may represent glitches or information bits, too slow a rise time for the circuit being tested could shift the pulse in time and give a wrong value.

WisdomAugust

Yes, you're right.
I just suppose to give

a bigger budget...