Appendix A
553
Example: If we use a sampling frequency F
s
of 120 kHz and the
signal contains a 100 kHz component, this component will appear
as a 20 kHz frequency.
Undersampling Example in Frequency Domain
Aliasing is not always bad. It is frequently used in radio tuners
and communication products and is also called heterodyning.
NOTE Aliasing can be used for testing devices in cases where the capturing
instrument cannot cope with the signal frequency.
But we often don’t like alias products, because they appear to be
part of the original signal.
To eliminate aliasing, we need to separate the original signals from
their alias components. This is done by
• limiting the signal spectrum (lowpass or bandpass filtering),
• choosing a suitable sampling frequency.
If the original and the alias frequencies are properly separated, we
can be sure that our samples represent the original signal.
Averaging
In the real world, the received signal often must be measured in
the presence of significant noise. The standard technique to
improve the estimates of a value is to average.
Averaging is a built-in feature of the mixed-signal test functions.
SmartTest supports two ways of averaging:
• Point repetition
• Waveform repetition
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