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2.6.2 Random Interleaved Sampling
This mode is a little more complex and is suited only to observing
certain types of waveform.
The USBscope50 includes special circuitry that allows it to trigger
off the input waveform and also to be able to measure the time
between the trigger instant and the first ADC sample that is taken.
In this way, several single shot acquisitions are taken and the data
points for each “sweep” are ordered based on the relative time as
measured. For this to work of course, the relationship between the
trigger instant and the waveform must remain constant i.e. a
repetitive waveform is required like a continuous sine wave or
square wave for example. It also needs the scope to be in normal
trigger mode…the software automatically switches to this mode on
entering RIS mode.
The software automatically sweeps the input several times before
updating the display. Because each sweep happens at a random time
with respect to the input waveform, the relative times measured for
each sweep are also random. So it can take a few seconds for the
“picture” of the waveform to update fully. While this acquisition
process is happening, you may see some waveform displays that are
not related to the actual input waveform…this is because the random
nature of the sweeps means that the software may not have
completely built up the picture of the signal, leaving some data from
previous sweeps on the screen. This is normal. Wait a while and the
waveform will update completely.
Another side-effect of the RIS mode is that because the relative
sweep time measurement is “quantized” i.e. the time value is
rounded up or down to place it into one of 20 possible values, or
“bins”, a fast changing input signal can show small anomalies
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on
the screen as a result…the software forces the sweep data into one or
other of the bins even though the data may have been at one end of
the “bin” or the other.
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This may show up as small spikes that come and go, particularly near to fast changing edges. In
essence the data for a particular sweep is forced to a particular place on the time axis and these data
points may therefore appear to be in slightly the “wrong” place in time…showing as an error in voltage
i.e. a spike. This is normal.
Elan Digital Systems Ltd. 19 USBscope50 USER’S GUIDE Iss5