Agilent Vector Signal Analysis Basics - Agilent Technologies

The final stage of the signal conditioning process is extremely important to a
sampled system and FFT analysis; signal alias protection. The anti-alias filter
performs this function. An analyzer that does not have adequate protection
from aliasing may show frequency components that are not part of the
original signal. The sampling theorem states that if the signal is sampled
at a rate greater than 2 times the highest significant frequency component
present in the signal, the sampled signal can be reconstructed exactly.
The minimum acceptable sample rate is called the Nyquist rate. Thus,
f s > 2 (f max )
where f s = sample rate
f max = highest frequency component
If the sampling theorem is violated, “aliasing” error products can result.
Therefore, to prevent alias products for a given maximum frequency, there
must not be significant signal energy above 1/2 the sample rate. Figure 1-5
shows a set of sample points, which fit two different waveforms. The
higher-frequency waveform violates the sampling theorem. Unless an
anti-alias filter is used, the two frequencies will be indistinguishable when
processed digitally.
To prevent aliasing, two conditions must be satisfied:
1. The input signal to the digitizer/sampler must be band limited. In other
words, there must exist a maximum frequency (f max ) above which no
frequency components are present.
2. The input signal must be sampled at a rate that satisfies the sampling
theorem.
The solution to the aliasing problem seems simple enough. First you select
the maximum frequency (f max ) that the analyzer will measure, then make
sure the sampling frequency (f s ) is twice that frequency. This step satisfies
condition number 2 and makes sure that the analyzer can accurately measure
the frequencies of interest. Next you insert a low-pass filter (an anti-aliasing
filter) to remove all frequencies above f max , thus ensuring that the
measurement will exclude all frequencies except those you are interested
in. This step satisfies condition number 1 and makes sure the signal is
band limited.
Actual
signal
Reconstructed "alias" signal
Unwanted frequency components are
folded onto the spectrum below cutoff.
X(f)
recovered alias
spectrum follows
dashed line.
ADC sample
points
f s ( f s /2)
0 f f f s f
(a) Aliasing in the time-domain
(b) Aliasing in the frequency-domain
Figure 1-5. Aliasing products occur when the signal is undersampled. Undesirable frequencies
appear under the alias of another (baseband) frequency
7