Acquisition Controls for Digital Oscilloscopes
Digital oscilloscopes have settings that let you control how the acquisition system processes a signal. Look over the acquisition options on your digital oscilloscope while you read this description. Following figure shows you an example of an acquisition menu.
Example of an Acquisition Menu
Acquisition modes control how waveform points are produced from sample points. Recall from the first section that sample points are the digital values that come directly out of the Analog-to-Digital-Converter (ADC). The time between sample points is called the sample interval. Waveform points are the digital values that are stored in memory and displayed to form the waveform. The time value difference between waveform points is called the waveform interval. The sample interval and the waveform interval may be but need not be the same. This fact leads to the existence of several different acquisition modes in which one waveform point is made up from several sequentially acquired sample points. Additionally, waveform points can be created from a composite of sample points taken from multiple acquisitions, which leads to another set of acquisition modes. A description of the most commonly used acquisition modes follows.
- Sample Mode: This is the simplest acquisition mode. The oscilloscope creates a waveform point by saving one sample point during each waveform interval.
- Peak Detect Mode: The oscilloscope saves the minimum and maximum value sample points taken during two waveform intervals and uses these samples as the two corresponding waveform points. Digital oscilloscopes with peak detect mode run the ADC at a fast sample rate, even at very slow time base settings (long waveform interval), and are able to capture fast signal changes that would occur between the waveform points if in sample mode. Peak detect mode is particularly useful for seeing narrow pulses spaced far apart in time.
- Hi Res Mode: Like peak detect, hi res mode is a way of getting more information in cases when the ADC can sample faster than the time base setting requires. In this case, multiple samples taken within one waveform interval are averaged together to produce one waveform point. The result is a decrease in noise and an improvement in resolution for low speed signals.
- Envelope Mode: Envelope mode is similar to peak detect mode. However, in envelope mode, the minimum and maximum waveform points from multiple acquisitions are combined to form a waveform that shows min/max changes over time. Peak detect mode is usually used to acquire the records that are combined to form the envelope waveform.
- Average Mode: In average mode, the oscilloscope saves one sample point during each waveform interval as in sample mode. However, waveform points from consecutive acquisitions are then averaged together to produce the final displayed waveform. Average mode reduces noise without loss of bandwidth but requires a repeating signal.
Stopping and Starting the Acquisition System
One of the greatest advantages of digital oscilloscopes is their ability to store waveforms for later viewing. To this end, there are usually one or more buttons on the front panel that allow you to stop and start the acquisition system so you can analyze waveforms at your leisure. Additionally, you may want the oscilloscope to automatically stop acquiring after one acquisition is complete or after one set of records has been turned into an envelope or average waveform. This feature is commonly called single sweep or single sequence and its controls are usually found either with the other acquisition controls or with the trigger controls.
In digital oscilloscopes that can use either real-time sampling or
equivalent-time sampling as described earlier, the acquisition controls will
allow you to choose which one to use for acquiring signals. Note that this
choice makes no difference for slow time base settings and only has an effect
when the ADC cannot sample fast enough to fill the record with waveform points
in one pass.