How to Choose an Oscilloscope

WisdomAugust

1 Consider the bandwidth range.

Depending on what you use the oscilloscope for, the range of the bandwidth it can process should be the primary consideration.

Remember that the bandwidth specification of an oscilloscope is the frequency of the "-3 dB point" of a sine-wave signal of a particular amplitude, e.g. 1 Vpp. As the frequency of your sinewave goes up (while keeping the amplitude constant), the measured amplitude goes down. The frequency at which this amplitude is -3 dB lower, is the instrument's bandwidth. This means that an oscilloscope of 100MHz would measure a 1Vpp sinewave of 100MHz at only (approx.) 0.7Vpp. That is an error of about 30%! In order to measure more correctly, use this rule of thumb: BW/3 equals about 5% error; BW/5 equals about 3% error.

In other words: if the highest frequency you want to measure is 100 MHz, choose an oscilloscope of at least 300MHz, a better bet would be 500MHz.

WisdomAugust

Understand that today's signals are no longer pure sine waves, but most of the time square waves.
These are built by "adding" the odd harmonics of the fundamental sine wave together. So a 10 MHz square wave is "built" by adding a 10MHz sine wave + a 30MHz sine wave + a 50MHz sine wave and so on.

Rule of thumb: get a scope that has a bandwidth of at least the 9th harmonic. So if you're going for square waves, it's better to get a scope with a bandwidth of at least 10x the frequency of your square wave. For 100MHz square waves, get a 1GHz scope...

WisdomAugust

Bandwidth is defined as the frequency at which a sin wave input signal is attenuated to 70.7% of its true amplitude.
The -3dB or "half power" point, shown here for a 1GHz oscilloscope.

WisdomAugust

Evaluate the rise time. Rapid transitions can potentially be lost if rise time measures are not up to scratch. So, the faster the rise time, the higher the accuracy of measurement for those rapid transitions. The rule of five can apply here too, meaning the oscilloscope should be less than 1/5 times the fastest rise time of the signal being measured.

Oleg10011001

Rule of thumb: get a scope that has a bandwidth of at least the 9th harmonic.  So if you're going for square waves, it's better to get a scope with a bandwidth of at least 10x the frequency of your square wave.

Is this enough? Firstly, I heard that for normal reconstruction of the shape of a rectangular signal, one must have at least 10th harmonics of this signal, and better - 13th. Secondly, these harmonics will also be measured by the oscilloscope and, accordingly, the frequency of the last harmonic we need (9th or 10th or 13th) must not exceed 1/3 of the bandwidth of the oscilloscope, so that the measurement error does not exceed 5% or 1/5 of the passband if the measurement error is not more than 3%. Thus, it turns out that the maximum frequency of a rectangular signal with which the oscilloscope can work normally is much less than the bandwidth specified by you of 1/10. In addition, I think that to restore the waveform is important not so much the bandwidth as the sampling rate ... How much minimum is required samples for the period of the rectangular signal for the normal recovery of its shape, so that for example it was possible to measure its fronts? As a result, it turns out that an oscilloscope with a bandwidth of 200 Mhz at a sampling rate of 1GS / sec will be able to operate normally with a square signal having a frequency of no more than 2-4Mhz (If the measurement error is not more than 3%), and not 20Mhz (1/10 of 200Mhz) ... Am I wrong?

P.S: Sorry my bad english.

WisdomAugust

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

a bigger budget...

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

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

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

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.