How to measure the noise floor of an oscilloscope

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How to measure the noise floor of an oscilloscope [Copy link]

Signal integrity is the main criterion for measuring signal quality with an oscilloscope, especially when we need to observe small signals or slight changes in large signals, signal integrity becomes even more important.

When we design and debug, signal integrity will greatly affect our judgment. There may be noise, signal distortion, or even signal loss in oscilloscope measurements that affect signal integrity. An excellent oscilloscope with proper settings can better maintain signal integrity. An oscilloscope with poor signal integrity is likely to increase our development cycle time, affect production quality, and cause the risk of misselecting components. In order to minimize this risk, we should choose an oscilloscope that can maintain high signal integrity.

Today we will look at one of the factors that affect signal integrity, the oscilloscope's background noise. The oscilloscope's background noise is the oscilloscope's inherent noise, which generally refers to all interference that is irrelevant to the presence or absence of the signal during the oscilloscope's measurement. It is mainly generated by the oscilloscope's attenuator, front-end amplifier, and analog-to-digital converter. When the oscilloscope measures a signal, the signal first passes through the attenuator and front-end amplifier and is then sampled by the ADC analog-to-digital converter. The noise will be superimposed on the measured signal and become part of the signal, so the analog-to-digital converter cannot distinguish it, and in theory there is no way to eliminate it.

Measuring the noise floor of an oscilloscope can be done in just a few simple steps, and anyone with an oscilloscope can quickly make the measurement in a few minutes.

1. Remove all probes connected to the oscilloscope.

2. Ensure that the oscilloscope is operating normally, its coupling mode is set to DC, the sampling mode is normal, and the bandwidth limit is full bandwidth

3. Set the oscilloscope's channel attenuation ratio to 1X

4. Set the vertical range of the oscilloscope to the minimum, as shown below: 1mV/div

5. Move the trigger level to the zero line to make the background noise display stable.

6. Adjust the time base and storage depth of the oscilloscope so that the oscilloscope is at the maximum sampling rate.

7. Open the oscilloscope's measurement item. The peak-to-peak value is the oscilloscope's noise floor, which can be seen to be around 1mV.

We can also turn on the infinite afterglow and color temperature display of the oscilloscope to observe the degree of background noise. The thicker the color of the waveform, the more noise is generated inside the oscilloscope. We can also turn on other channels and observe them separately. Generally, the background noise of each channel is unique and there will be slight differences. This is all normal. If you adjust the vertical gear of the oscilloscope, you will find that the background noise of the oscilloscope will also change. If you have any questions about this, you can read the articles related to the vertical resolution of the oscilloscope to understand the reasons for this phenomenon. Therefore, when measuring the background noise of the oscilloscope, be sure to remember to adjust the vertical gear to the most sensitive gear.

Next, we will look at the impact of the three major indicators of the oscilloscope, namely bandwidth, sampling rate, and storage depth, on the background noise through settings.

We turn on the bandwidth limit of the oscilloscope and set it to 20M. We can see that the noise floor is around 500μV. Does it mean that the lower the bandwidth of the oscilloscope, the lower the noise floor? Since noise signals are basically high-frequency signals, when the bandwidth of the oscilloscope is too low, those high-frequency signals cannot be captured, so the noise floor will appear small. Similarly, the higher the bandwidth of the oscilloscope, the richer the harmonic components of the collected signal, and the greater the noise. In order to avoid the interference introduced by this high bandwidth, some oscilloscopes will automatically limit the bandwidth under high-sensitivity vertical gear, so that the measured waveform is clearer.

We know that the sampling rate of an oscilloscope = storage depth ÷ waveform recording time. The more data points the oscilloscope collects, the more complete the reconstruction of the waveform will be. Therefore, a low sampling rate will definitely make the noise floor display lower. Of course, a low sampling rate may also cause the signal we want to collect to be distorted. For the specific sampling rate that is sufficient, please refer to our previous article: What sampling rate is required for an oscilloscope to measure various types of signals?

We adjusted the oscilloscope's storage depth to 28K. At this time, the sampling rate was 20MSa/s. We can see that the peak-to-peak value of the background noise is 683.8μV.

In summary, the higher the bandwidth and sampling rate of the oscilloscope, the higher the noise floor will be. However, this does not mean that the noise introduced by the low-bandwidth oscilloscope is small, but a large part of the signal is not captured. Therefore, when we compare the noise floors of oscilloscopes, we must ensure that the settings and parameters of the two oscilloscopes are consistent, so that the comparison is meaningful.

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Thanks for sharing! Not bad!

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Noise floor is often difficult to measure

It is related to the bandwidth and sampling rate of the oscilloscope.