Frequency Limitation of an Oscilloscope

Oscilloscopes are an incredibly useful tool of the electronics world after a multimeter. Without a scope, it’s quite difficult to know what is happening in a circuit. But this type of test equipment has its own limitation. To overcome this limitation, one must understand the weakest links in the system fully and compensate for that in the best way possible.
The important feature of the oscilloscope is the bandwidth. How fast the number of analog sample per second that it can read is the key factor for an oscilloscope. Let’s understand first, what is bandwidth? Most of us believe that the maximum allowed frequency by a scope is bandwidth. Actually, the bandwidth of an oscilloscope is the frequency at which a sinusoidal input signal is attenuated by 3dB, which is 29.3% low of the signal’s true amplitude.

It means that at the maximum rated frequency point, the amplitude shown by the instrument is 70.7% of the actual amplitude of the signal. Suppose at maximum frequency, the actual amplitude is 5V but it will display on the screen as ~3.5V.

Oscilloscope with the specification of 1 GHz bandwidth or below that shows a Gaussian response or low-pass frequency response which is one-third of the -3 dB frequency in the beginning and slowly rolls off at higher frequencies.
Scopes with specification greater than 1 GHz shows a maximally flat response with a sharper roll-off near the -3dB frequency. The lowest frequency of oscilloscope at which the input signal is attenuated by 3 dB is considered as the bandwidth of the scope. The oscilloscope with a maximally flat response can attenuate in-band signals which are less as comparative to the oscilloscope with the Gaussian response and do more accurate measurements on in-band signals.

On the other hand, the scope with Gaussian response attenuated out-bands signals which are less comparative to the scope with the maximally flat response. It means that such scope has a faster rise time comparative to other scopes with same bandwidth specification. Rise time specification of a scope is closely related to its bandwidth.
A Gaussian response type oscilloscope will have a rise time of 0.35/f BW approximately based on a 10% to 90% criterion. A maximally flat response type scope has a rise time of 0.4/f BW approximately based on the sharpness of the frequency roll-off characteristic.

You must understand that rise time is the fastest edge speed that could be produced by the scope if the input signal has a theoretically infinitely fast rise time. But to measure theoretical value is impossible so its batter to calculate the practical value.

Precaution Required for Precise Measurements in Oscilloscope

1. The foremost thing that users must know is the bandwidth limitation of the scope. The bandwidth of the oscilloscope should be wide enough to accommodate the frequencies within the signal and display the waveform properly.

2. The probe used with the scope played an important role in the performance of the equipment. The bandwidth of the oscilloscope, as well as probe, should be in proper combination. Using an improper oscilloscope probe can spoil the performance of the entire test equipment.

3. To measure frequency as well as amplitude accurately, the bandwidth of both the scope and the probe attached to it are well above the signal you want to capture precisely. For example, if the required accuracy of amplitude is to ~1%, then berate factor of scope by 0.1x, that means 100MHz scope can capture 10MHz with a 1% error in amplitude.

4. One must take into consideration the correct triggering of the scope so that the resultant view of the waveform is much clearer.

5. Users should be aware of ground clips while taking high-speed measurements. The wire of the clip produces inductance and ringing into the circuit which affects the measurements.

6. The summary of the whole article is that for analog scope, the bandwidth of the scope is at least, three times higher than the highest analog frequency of the system. For the digital application, the bandwidth of the scope is at least, five times higher than the fastest clock rate of the system.

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