Bandwidth and sampling rate for oscilloscope Measurement

During the test, how much bandwidth does the oscilloscope choose?

First, check the following instance.

 

It can be seen that the larger the bandwidth, the richer the signal frequency component that can be displayed, the closer the real signal waveform.

1. Precise bandwidth calculation of an oscilloscope

Perform the following steps:

A. Determine the fastest increase/decrease time of the tested Signal

B. Determine the highest signal frequency f

F = 0.5/RT (10% ~ 90%)

F = 0.4/RT (20% ~ 80%)

C. determine the required measurement accuracy

required accuracy	Gaussian Frequency Response	maximum flat frequency
20%	BW = 1.0 * F	BW = 1.0 * F
10%	BW = 1.3 * F	BW = 1.2 * F
3%	BW = 1.9 * F	BW = 1.4 * F

D. Calculate the required bandwidth.

Example:

Determine the minimum bandwidth required by a Gaussian response oscilloscope to measure the tested digital signal. The fastest increase time of the tested signal is 1ns (10% ~ 90% ):

F = 0.5/1ns = 500 MHz

If 3% measurement error is required: the required oscilloscope bandwidth = 1.9*500 MHz = 950 MHz

If 20% measurement error is required: the required oscilloscope bandwidth = 1.0*500 MHz = 500 MHz

Therefore,The most important factor that determines the bandwidth of the oscilloscope is the fastest increase time of the tested signal.

 

OscilloscopeThe system bandwidth is composed of the oscilloscope bandwidth and the probe bandwidth.Jointly decided:

A. Gaussian Frequency Response: system bandwidth =

B. Maximum flat frequency: system bandwidth = min {oscilloscope bandwidth, probe bandwidth}

For example, for an oscilloscope with 1 GHz bandwidth, a passive probe with 1 GHz bandwidth is configured. If their frequency response is Gaussian frequency response, the system bandwidth is about 700mhz.

 

2. Factors affecting the bandwidth of the oscilloscope

Generally, these factors include:Sampling rate and frequency response curve.

A. Frequency Curve

Shows the frequency response curve.

 

 

B. Sampling Rate

The sampling frequency must be 2 times the highest signal frequency, that is:

FS> 2 * fmax

In order to ensure that the signal can be reconstructed without mixing.

(1) For the frequency response of ideal brick walls,Sampling Rate=Oscilloscope bandwidth * 2To reconstruct the signal. However, this situation does not exist in the real world. The frequency response of most oscilloscope is between the frequency response of the ideal brick wall and the Gaussian frequency response.

(2) Gaussian frequency response,Sampling Rate=Oscilloscope bandwidth * 4To reconstruct most of the frequency components in the tested signal without mixing. Generally, the frequency of the actual oscilloscope is a little longer than that of Gaussian.

(3) For the maximum flat frequency,Sampling Rate=Bandwidth of the oscilloscope * 2.5To restore most of the frequency components in the tested signal. At present, some high-end oscilloscope can be used2.5Times the bandwidth Sampling RateTo reconstruct the signal.

 

Is the higher the sampling rate, the higher the measurement accuracy?

Take 1 GHz sine wave observation as an example. See.

 

 

Take an oscilloscope (20gsa/s and 40gsa/s) with a maximum flat frequency of 6 GHz bandwidth as an example,

Tested signal: GHz clock, rising at around ps. The test result is as follows:

 

It can be seen that waveform reconstruction is not much improved when the sampling rate is 6.6 times the bandwidth. Therefore, the sampling rate is sufficient.

 

On the contrary, higher sampling rates do not necessarily lead to higher measurement accuracy, for the following reasons:

(1) A higher sampling rate may be spliced by multiple ADC, resulting in waveform distortion.

(2) If the sampling rate is too high, the effective digits of the ADC will be reduced (may only reach 4 ~ 5-bit resolution ).

Therefore,The measurement accuracy is determined by multiple factors. When the sampling rate is sufficient, the higher the sampling rate, the better. In some cases, the higher the sampling rate, the worse the measurement accuracy..