DFP2 Option
923134 Rev A ISSUED: June 2013 11
9. Touch the Frequencies tab.
10. Depending on the class (FIR/IIR) and kind of filter you selected, and whether or not Auto
Length is enabled, you can change the cutoff frequencies, transition width (edge width), stop
band attenuation, and pass band ripple.
MULTIRATE FILTERS
Description
In many of today's development environments, digital filter design has become most
c
hallenging. Specifications typically require higher order filters, implying increased storage
capacity for filter
coefficients and higher processing power. Moreover, high-order filters can be
difficult, if not impossible, to design. In applications such as 3G wireless systems, for example,
at the receiver end data must be filtered very tightly in order to be processed.
Although the Teledyne LeCroy DFP option provides many filter types, the correlation between
edge
frequencies and sample rate may be a limiting factor: edge frequencies are limited from
1% to 49.5% of the sample rate, while the minimum transition width region is 1% of the sample
rate.
Multirate, multistage filters are a practical solution for the design and implementation of FIR
filters with narrow spectral constraints. Multirate filters change the input data rate at one or more
intermediate
points within the filter itself, while maintaining an output rate that is identical to the
input rate. This approach provides a solution with greatly reduced filter lengths, as compared to
standard single-rate filters.
This can be achieved in two or more simple steps. First, a filter (with a relatively limited edge
frequency)
is applied and the results are decimated. Then, a second filter is applied to the
decimated waveform, substantially reducing the lower edge frequency limit.
Example
A sine wave with a frequency of 3 MHz has a higher frequency noise component. A low-pass
filter is required to remove the noise component. The sample rate of the scope is 2 GS/s. The
minimum edge frequency of the low-pass filter for this sample rate is 20 MHz. While this filter is
sufficient for removing part of the noise, it cannot remove the high frequency component
c
ompletely. In such a case, the problem can be solved in two stages.
(11 Seiten)
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