Specifications
Display Modes
Remember how when using your analog oscilloscope you had to constantly
adjust the brightness? You did this to see the desired level of detail in a
signal, or sometimes to see the signal at all!
The brightness knob on the HP 54645A/D operates much like the brightness
knob on your computer screen. You should set it to a level that makes for
comfortable viewing, given the room lighting, and keep it there.
In the HP 54645A/D, you control the “detail” by selecting a display mode:
Normal, Peak Detect, or Average. While you may want to change the display
mode now and then, rest assured that you will not have to change it nearly as
often as you adjusted the brightness of an analog scope. The waveform will
never dim with low trigger rates or fast sweep speeds.
A discussion of display modes is best begun with a simple sample. Let’s
assume that your digital scope is sampling at 200 MSa/s, yet is storing the
samples at a 1-MSa/s rate. In this case, the scope can only store one out of
every 200 samples.
•
In a traditional digital scope system, HP uses a simple sample rate
decimation algorithm: store the first sample, ignore 199, store a sample,
ignore 199, etc.
•
In dithered systems (HP 54600-family scopes), HP uses a powerful
anti-aliasing display technique: one sample is chosen randomly from the
first 200 and is displayed, then another sample is chosen randomly from
the next 200 samples and is displayed, etc.
•
In peak detect, the traditional scope searches the 200 samples for a
maximum and minimum voltage value. Both the maximum and the
minimum are displayed, and then the next 200 samples are searched.
This method displays signal extremes.
•
In smoothing, the average value of the 200 samples is stored into
memory and displayed. Then the average value of the next 200 samples
is displayed. This technique keeps track of the mean value of the signal
over the sample interval.
With this background information, let’s examine the display modes of the
HP 54645A/D Oscilloscopes.
MegaZoom Concepts
Display Modes
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