What are some things to consider when I prepare to program an oscilloscope to get waveform data?
· Initialize the scope settings
The instrument needs to begin activities from a known state. Tektronix oscilloscopes use the ‘FACTory’ command to do that. The default settings are listed in the programmer’s on-line guides or the user manual for your oscilloscope. Then set up the instrument by defining the vertical (volts/division), horizontal (time base, seconds/division), acquisition mode, and trigger settings. A simple way to do this is to use the ‘autoset execute’ command. The scope will attempt to figure out the best settings for the signals sensed by the probe. Depending on the oscilloscope model, the channel used for autoset might be the lowest channel number that is on, the channel with the lowest frequency signal or if no channels are on the autoset command may turn on the first channel it encounters that has a signal. The default for autoset is to select channel #1.
Usually, you will have some idea of the signal you are going to analyze with the oscilloscope. And you know which parts of the signal are of most interest to you. Autoset may not provide the appropriate presentation of the signal of interest. In those cases, (and when autoset simply fails to display a signal) you will need to set the parameters differently than autoset did during it’s experiments. In other words, autoset may get the signal on the screen but be prepared to set vertical, horizontal, triggering and acquisition values that fit your current real world need.
· Set the vertical values
Use the command ‘CH
In setting the vertical values, consider the scope’s dynamic range. As you use more of this range, you can take advantage of more accuracy. Thus, use logical volts/division settings. For example, let’s consider the case where you are using the 5-volt square wave probe compensation signal. There are 8 vertical divisions on the screen. If you set the vertical scale to 1 volt/division then the wave will nicely use most of the screen. It will fill 5 of the 8 vertical divisions.
You could use a smaller volts per division scale. However, then you would show fewer pixels of display information for each waveform. The typical approach is to show as much resolution as practical.
· Set the horizontal values
Use the commands ‘HORizontal:RECOrdlength’ to change the record length or ‘HORizontal:MAIn:SAMPLERate to change the rate at which samples are digitized.
For horizontal values, we suggest you set the record length. This sets how many points you can store in a single acquisition. There is a limit. Once your scope reaches the limit, then it can’t store new points without dropping off old ones. Also, the scope acquires the data on all selected channels synchronously.
Again consider the case where you are using the probe compensation signal. It has a 1k Hz square wave signal. There are 10 horizontal divisions on the screen. If you show two complete waveform cycles that would be 2 milliseconds of data. Since there are 10 divisions across the screen, each division should have
2 milliseconds / 10 divisions = 200 microseconds/division.
You could use a smaller time per division scale, however, then you would show fewer pixels of display information for each waveform. As with the vertical values, the typical approach is to show as much resolution as practical.
· Set the trigger values
Set the trigger type and level. You use the appropriate ‘TRIGger’ command to get the job done the way you want it. Using this command, the trigger can be set to occur on: a rising or falling edge, on a level, on a logic pattern, on a pulse width etc. You would also select video triggering and then choose between fields, lines or a specific line number.
· Set the acquisition mode
Use the ‘ACQuire’ commands to get the acquisition required for your application.
There are different acquisition modes, such as sample, peak-to-peak, and high resolution. In choosing the mode to use in your program, consider the trade-off between effective resolution of your waveform data and the time it takes to transfer that data from your scope to your remote computer.
For example, the sample mode stores each point of data in a single byte and thus is relatively fast to transfer, but with the limitation of 256 levels of resolution. In contrast, the high resolution mode uses multiple bytes and provides up to 14 bits of resolution. You can capture and see more detail, but the cost is that your program will require more time to transfer a high resolution waveform than a sample mode one from the scope to a PC.
Understand the difference between equivalent time and real time acquisition. Equivalent time waveforms are the sum of many triggers. Real time is the result of a single trigger.
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