Uncovering the secrets of testing and measurement - Test every Monday [Week 3]

2. How to reduce the waveform capture rate? For example, from 1 million times per second to 1000 times per second?
 

3. Which settings will affect the waveform update rate of the oscilloscope (usually the speed of the oscilloscope) 

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1. How to measure the actual waveform capture rate of an oscilloscope?

(The answer is provided by Agilent's Du Jiwei) For a real-time oscilloscope, there are two situations. One is that the oscilloscope itself has a trigger output, and the other is that the oscilloscope itself does not have a trigger output.

Case 1: The oscilloscope itself has a trigger output. Most oscilloscopes have a trigger output. Every time the oscilloscope is triggered, that is, a waveform is captured, there is a pulse output at the trigger output position. Therefore, the frequency of the signal at this location is equal to the actual trigger rate or waveform capture rate of the oscilloscope. You can use a frequency meter to directly measure the signal frequency at this location, or you can use another oscilloscope to measure the details of the waveform at this location and its frequency. If you use Agilent's lower-priced InfiniiVision oscilloscope (DSOX2000, DSOX3000, DSO5000, DSO6000, DSO7000) series oscilloscope, it has a built-in frequency counter. You can freely use the built-in counter or directly observe the waveform. The advantage of observing the waveform is that you may find that the time intervals between two consecutive waveforms are likely not the same.

Case 2: The oscilloscope itself does not have a trigger output, which is only the case with a very small number of oscilloscopes. In this case, you need to use a separate pulse signal source, such as Agilent's 81150A, to generate a double pulse signal. The amplitudes of the two pulses are obviously different, and the time interval is adjustable. At a certain moment when the time interval is adjusted, two different pulses can be seen on the oscilloscope, and then they can no longer be seen. This critical point can be regarded as the dead zone of the oscilloscope, and its reciprocal is the waveform capture rate.

2. How to reduce the waveform capture rate? For example, from 1 million times per second to 1000 times per second?

(The answer is provided by Agilent's Du Jiwei) To change the waveform capture rate, to put it simply, is to change the trigger rate. A parameter directly related to the trigger is the trigger holdoff, which means waiting for a period of time before checking whether the trigger conditions are met. For example, the waveform capture rate of the Agilent DSOX3000A oscilloscope is 1 million times per second. If you set the trigger holdoff time of the oscilloscope to 1 millisecond, the trigger rate or waveform capture rate will be reduced to 1000 times per second.

Of course, you can also reduce the waveform capture rate by changing the horizontal time base settings, sampling rate, memory depth, interpolation, turning on more software processing related functions or functions that require the use of the oscilloscope's internal CPU. However, to reduce the oscilloscope's waveform capture rate to a known rate without affecting the oscilloscope's other settings, using the trigger holdoff parameter is a common method.

3. Which settings will affect the waveform update rate of the oscilloscope (usually the speed of the oscilloscope)

(The answer is provided by Agilent's Tian Zhidong) The waveform update rate of an oscilloscope is a very important indicator of the oscilloscope. Because digital oscilloscopes have dead time, the oscilloscope will lose part of the signal (within the dead time). The higher the waveform update rate, the shorter the dead time, and the less information is lost.

The oscilloscope settings will affect the waveform update rate. The following setting parameters will affect the waveform update rate: sampling rate, storage depth, acquisition time, sin(x)/x interpolation, and trigger suppression time.

The first three parameters are interrelated. Usually, acquisition time = storage depth/sampling rate. The longer the acquisition time, the lower the waveform update rate. Under the same acquisition time, the higher the sampling rate of the oscilloscope, the greater the required storage depth, and the dead time will be correspondingly longer, and the waveform update rate will be reduced. These three parameters are usually more intuitive, because when using the oscilloscope, these three parameters are always observable by the user.

Sin(x)/x interpolation is a function that all mainstream oscilloscopes on the market have. In short, Sin(x)/x interpolation is to insert a certain number of points between sampling points to make the waveform closer to the real signal. If Sin(x)/x interpolation is turned on, the number of points that the oscilloscope needs to process (sampling points + interpolation points) will increase exponentially, so the dead time will also increase, and the waveform update rate will be reduced. However, if the oscilloscope does not turn on Sin(x)/x interpolation, the tested signal will be greatly distorted in many cases, and the error will be even greater when testing the jitter of high-speed signals.

The trigger holdoff time is the minimum delay between two trigger events of the trigger circuit, which is determined by the hardware and data processing capabilities of the oscilloscope. Each oscilloscope has a minimum value, and the user can manually change the trigger holdoff time. When this value increases to a certain value (related to other settings of the oscilloscope), the waveform update rate of the oscilloscope will be reduced. However, when testing burst signals, trigger holdoff is a necessary setting parameter to help the oscilloscope display the waveform stably.