Information Technology Reference
In-Depth Information
Figure 3.40:
An example of a filtering search. Pairs of samples are subtracted and multiplied by sampled sine and
cosine waves. The products are added to give the sine and cosine components of the search frequency.
It is possible to combine the calculation of the DC component which requires the sum of samples and the
calculation of the fundamental which requires sample differences by combining stages shown in
Figure 3.41
(
a)
which take a pair of samples and add and subtract them. Such a stage is called a butterfly because of the shape of
the schematic.
Figure 3.41
(b) shows how the first two components are calculated. The phase rotation boxes
attribute the input to the sine or cosine component outputs according to the phase angle. As shown, the box
labelled 90° attributes nothing to the sine output, but unity gain to the cosine output. The 45° box attributes the
input equally to both components.
