Why an Oscilloscope Can Display a Lower Amplitude
An oscilloscope may show a signal amplitude that is lower than its real value even when the displayed frequency appears correct. This behavior is closely related to oscilloscope bandwidth. If the signal frequency approaches the instrument’s rated bandwidth, the oscilloscope’s analog front end attenuates the signal instead of reproducing its full amplitude.
For electronics manufacturing, testing, and troubleshooting, this effect matters because an attenuated waveform can lead to incorrect conclusions about signal levels. Bandwidth selection must therefore account for both the main signal frequency and the higher-frequency components contained in complex waveforms.
What Oscilloscope Bandwidth Means
Oscilloscope bandwidth refers to the analog bandwidth of the instrument’s analog front end. It directly affects the range of signals that the oscilloscope can measure accurately.
The rated bandwidth is commonly defined by the -3 dB cutoff point. At this frequency, the measured amplitude of a sine wave is 70.7% of its true amplitude. As signal frequency increases toward this point, the oscilloscope becomes less capable of displaying the correct signal level.
This definition explains why matching a signal frequency directly to the oscilloscope bandwidth does not produce a fully accurate amplitude measurement. The bandwidth rating identifies the attenuation threshold rather than a frequency at which amplitude remains unchanged.
A 100 MHz Measurement Example
Consider a 100 MHz oscilloscope measuring a 100 MHz sine wave with an actual amplitude of 1 Vpp. Under the Gaussian-response condition described for the oscilloscope amplifier, the displayed waveform would remain at 100 MHz, but its measured amplitude would be approximately 0.707 Vpp.
The frequency reading may therefore look reasonable while the amplitude is about 30% below the true value. This is a direct consequence of operating at the oscilloscope’s -3 dB bandwidth point.
If a measurement error of about 3% is required, the measured signal frequency must be much lower than the oscilloscope bandwidth. A commonly used selection guideline is:
Required oscilloscope bandwidth = highest measured signal frequency × 5
Following this five-times guideline provides more bandwidth margin than simply selecting an oscilloscope whose rating equals the signal frequency.
Why Complex Waveforms Need More Bandwidth
The 100 MHz example applies to a sine wave. Many practical electronic signals are more complex because they contain multiple harmonic sine-wave components. These harmonics can extend well beyond the waveform’s fundamental frequency.
If the oscilloscope does not provide enough bandwidth, higher-frequency components may be attenuated or blocked. The displayed result can then suffer from reduced amplitude, slower-looking edges, and lost waveform detail. As a result, the screen may no longer provide a useful representation of the original signal characteristics.
This is why bandwidth should not be selected only from the nominal repetition frequency of a complex waveform. The highest frequency that must be observed is also relevant when applying the five-times guideline.
When to Use Full Bandwidth
Full bandwidth is important when measuring high-frequency signals. For example, when observing a 27 MHz crystal waveform, the oscilloscope should use its full available bandwidth.
If a 20 MHz bandwidth limit is enabled during that measurement, the signal frequency exceeds the selected limit. The crystal waveform will consequently be distorted, making the displayed result unsuitable for meaningful evaluation.
When an unexpectedly low amplitude appears during a high-frequency measurement, checking whether a bandwidth limit is active is therefore an important diagnostic step. The oscilloscope may be intentionally filtering part of the signal.
When a Bandwidth Limit Is Useful
A bandwidth limit is not always undesirable. During low-frequency measurements, enabling it can filter high-frequency interference and produce a clearer waveform. The correct setting depends on the frequency range and the purpose of the measurement.
Use full bandwidth when the signal or its important harmonic content requires the instrument’s complete frequency range. Use a bandwidth limit when measuring lower-frequency signals and when removing unwanted high-frequency interference helps reveal the waveform more clearly.
Bandwidth and Rise Time
Rise time is another important consideration in oscilloscope selection. It is generally defined as the time required for a signal to increase from 10% to 90% of its maximum steady value.
For oscilloscopes below 1 GHz, the relationship between bandwidth and rise time can be estimated with the following formula:
RT = 0.35 / BW
In this expression, RT is rise time and BW is bandwidth. The factor 0.35 represents the relationship between bandwidth and 10%–90% rise time for a first-order Gaussian model.
For example, a 200 MHz oscilloscope has an estimated minimum observable rise time of 1.75 ns. This relationship shows why limited bandwidth affects more than amplitude: it also limits the ability to reproduce rapid signal transitions.
A Practical Bandwidth Selection Process
- Identify the highest frequency to be measured. For complex signals, consider the higher-frequency components needed to preserve waveform detail.
- Apply suitable bandwidth margin. The common five-times guideline calls for oscilloscope bandwidth equal to five times the highest measured signal frequency.
- Check the bandwidth-limit setting. Disable a restrictive limit when measuring high-frequency signals, such as a 27 MHz crystal with a 20 MHz limit.
- Use filtering selectively. For low-frequency signals, a bandwidth limit can reduce high-frequency interference and improve waveform clarity.
- Consider rise time. Use the bandwidth-to-rise-time relationship when fast edges are important to the measurement.
Key Takeaway
A lower-than-expected oscilloscope amplitude does not necessarily mean that the source signal is too weak. The reading may be attenuated because the signal frequency is too close to the oscilloscope bandwidth or because a bandwidth limit is enabled.
Understanding the -3 dB point, allowing adequate bandwidth margin, accounting for harmonic content, and checking rise-time capability can produce a more useful representation of the real waveform. For high-frequency measurements, use the required bandwidth; for lower-frequency work, apply bandwidth limiting only when filtering high-frequency interference is beneficial.
