Is the Auto function, the most commonly used function on oscilloscopes, really so versatile?

[Introduction] The Auto function is the most commonly used function of the oscilloscope. No matter what the oscilloscope cannot capture the waveform for, it can be captured quickly by using Auto! What exactly does Auto do? Is it really so omnipotent? This article will take you to explore the mystery of Auto!

Auto Mechanism

Before using an oscilloscope, the first thing to do is to calibrate the probe. At this time, you need to connect the probe to the calibration signal of the oscilloscope to perform capacitance calibration. No matter what the state of the oscilloscope is when the probe is connected, after pressing AutoSetup, the perfect square wave shown in the figure below will be displayed, completely saving a series of settings such as adjusting the time base, vertical scale, offset, etc. So what exactly does AutoSetup do?

Figure 1 Result after calibrating the signal Auto

In fact, AutoSetup only does the following three steps:

1. Scan the channels of the oscilloscope. If a channel is open, only the signal of that channel will be scanned, and the final display will only show the waveform of that channel. If no channel is open, all channels of the oscilloscope will be scanned, and any channel with signal input will be opened and the waveform will be displayed.

2. Determine the appropriate vertical observation scale by calculating the amplitude of the waveform. The vertical scale is suitable when the signal is greater than 2.5 grids and less than 7 grids. For example, if the waveform amplitude is 5V, the appropriate vertical scale is 1V/div, and the offset is -2.5V.

3. Determine the appropriate horizontal observation gear by calculating the frequency of the waveform. The appropriate horizontal gear is based on the screen display of 2 to 4 cycles. For example, if the frequency of the waveform is 1K, then the appropriate horizontal gear is 200us/div.

Which waveforms are not suitable for capture using Auto?

After seeing the specific implementation method of AutoSetup, some friends began to worry, only doing these operations, is the Auto function really universally applicable? The answer is of course not.

1. Signals with a high proportion of idle levels or low-frequency signals 

After pressing the AutoSetup button, the oscilloscope will only check the amplitude of the first 40 milliseconds of the signal. That is to say, when the average period of the signal is less than 25Hz, the waveform adjustment may not be appropriate after Auto. Because the protocol signal often has an idle level, it will reduce the average frequency, resulting in the waveform we want to see not appearing after Auto. It seems that Auto is not omnipotent!

2. Unstable trigger point

When the trigger point is not fixed, the waveform on the oscilloscope will keep jumping. At this time, AutoSetup cannot be adjusted automatically, so no matter how Auto is set, it may not be able to trigger stably. At this time, it is necessary to adjust the trigger holdoff to achieve stable display of complex waveforms. It is the minimum time interval between the current trigger and the next trigger. It is usually used to isolate burst pulse trains and fix the trigger point at the same position of each frame of data. As shown in the following figure:

Figure 2 Trigger holdoff setting principle

3. System integration

When you are unsure of the appropriate waveform scale, it is recommended to use the AutoSetup SCPI command. In this case, it is not recommended to send the vertical/horizontal scale command after setting Auto.

Clear the clutter of your oscilloscope settings

Oscilloscopes have more and more functions, and as their functions are enhanced, their complexity of use is gradually increasing. When we use an oscilloscope, after turning on many functions, the interface of the oscilloscope is usually messy, and we may even not be able to find the waveform. At this time, many engineers hope to use AutoSetup to restore the original clean interface with one click. Will this be a problem?

The answer is: there may really be a problem. AutoSetup will indeed clear some parameter settings, but it will also reset some settings and retain some settings. This function varies depending on the oscilloscopes of different manufacturers. For example: The version of the ZDS series oscilloscope as of the date this article was published, retains the following settings after Auto: DC/AC (GND coupling will be changed to DC coupling), 50 ohms, bandwidth limit, inversion, capture mode, storage depth, probe type, probe ratio, channel delay, waveform brightness, and scale line brightness.

However! This function can be optimized greatly. ZLG Zhiyuan Electronics is committed to providing users with better and more convenient services. It is also very likely that AutoSetup will add other retention or reset parameters in the future. You can communicate with our engineers for specific details!

Figure 3 ZDS4054Plus Oscilloscope