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Why the measured amplitude less than real value
- 2017/6/25 10:00:00 -

Try a small test. Use your 100 MHz oscilloscope to measure a 100MHz, 3.3V amplitude waveform. The measured amplitude isn’t accurate. This issue refers to the bandwidth of oscilloscope.


1. What is bandwidth?

Bandwidth is an essential parameter for oscilloscope. But, what is bandwidth? Bandwidth refers to the analog bandwidth of the analog front end of the oscilloscope. It directly determines the oscilloscope on the signal measurement capabilities. Specifically, the oscilloscope bandwidth is the highest frequency when the amplitude of the sine wave measured by the oscilloscope is not lower than the 3dB amplitude of the true sine wave signal (ie, 70.7% of the true signal amplitude), also known as -3dB cutoff frequency point. As the signal frequency increases, the oscilloscope's ability to accurately display the signal will drop down.


When the measured sine wave frequency is equal to the bandwidth of the oscilloscope (oscilloscope amplifier is for the Gaussian response), we can see that the measurement results error of about 30%. If the measurement error is reduced to 3%, the frequency of the measured signal should be much lower than the bandwidth. For example, with a bandwidth of 100MHz oscilloscope to measure the frequency of 100MHz, amplitude 1Vpp sine wave signal, the final result shows the signal is 100MHz, the amplitude of 0.707Vpp sine wave waveform, which is only the case of sine wave. Since most of the signal is much more complex than the sine wave, in order to achieve a certain measurement accuracy, we use the oscilloscope common law that is commonly referred to as 5 times the standard: The required bandwidth of the oscilloscope = the highest frequency of the measured signal * 5


2. Select the bandwidth correctly


Complex signal of the waveform is formed by a variety of different harmonic sine wave signal. The bandwidth of these harmonics may be very wide. When the bandwidth is not enough, the harmonic components will not be effectively amplified (blocked or attenuated), which may cause amplitude distortion, edge loss, loss of detail data, etc. The signal characteristics such as bells and tones, etc. will have no reference value.

So for different frequency signal measurement, the correct bandwidth is very important. When measuring high frequency signals, such as measuring 27MHz crystal, you should use the full bandwidth measurement.

If the bandwidth limit is enabled, that is, the bandwidth limit is set to 20MHz, the crystal waveform will be distorted and the measurement will be of no value. When measuring low-frequency signals, you need to set the bandwidth limit, the high-frequency signal interference filter, so that the valid signal shows more clearly.


3. Bandwidth and rise time


When mentioning the bandwidth, the rise time cannot be missed. The rise time is usually defined as the time at which the signal amplitude changes from 10% of the maximum steady value to 90%.


The bandwidth of the oscilloscope can directly show the minimum rise time of the signal. The rise time of the oscilloscope system can be evaluated from the specified bandwidth. You can use formulate: RT (rising time) = 0.35 / BW (bandwidth) (oscilloscope below 1GHz) to calculate.


Where 0.35 is the scale factor between the oscilloscope bandwidth and the rise time (10% -90% rise time in the first order Gaussian model). According to the above formula, if the bandwidth of the oscilloscope is 200MHz, can calculate RT = 1.75ns, that is, the minimum observable rise time.