Frequency transformation
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Transcript of Frequency transformation
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Frequency Transformationi.e. Low Pass to High Pass Filter etc.
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Frequency Transformations• We need to apply a suitable frequency transformation, if we wish to
designbandpass, bandstop and high-pass filters, using the low-pass approximatingfunction analysis
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Filter Transformations• We can use the concept of filter transformations to determine the
new filter designs from a lowpass design. As a result, we can construct a 3rd-order Butterworth high-pass filter or a 5th-order Chebychev bandpass filter!
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Filter Transformations
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Normalized Lowpass Filter• When designing a filter, it is common practice to first design a
normalized low-pass filter, and then use a spectral transform to transform that low-pass filter into a different type of filter (high-pass, band-pass, band-stop).
• The reason for this is because the necessary values for designing lowpass filters are extensively described and tabulated. From this, filter design can be reduced to the task of looking up the appropriate values in a table, and then transforming the filter to meet the specific needs
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Lowpass to Lowpass Transformation
• Having a normalized transfer function, with cutoff frequency of 1 Hz, one can modify it in order to move the cutoff frequency to a specified value
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Transfer Function
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Analog Element Values
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Lowpass to Highpass
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Lowpass to Highpass
Normalised HPF response
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Lowpass to Highpass
Equivalent LPF response
we define values for the transformed frequency Ω as
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Lowpass to Highpass
but with frequency transformation, we substitute for ω/ωp with
In the Chebychev case, we apply the substitution
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Conversion of Low-pass and High-pass Filter transfer functions from continuous time to discrete time difference equations.
• The following converts two filter transfer function that are represented in the Laplace Space
• (Continuous time) into their discrete time equivalents in the Z-space using the Bilinear Transform
• (AKA Tustin’s Method), then converts them to difference equations expressing the current output as a combination of previous inputs and outputs.
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Definition of Values
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Definition of Values
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Low Pass Filter
Conversion
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Low Pass Filter
Conversion
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High Pass Filter
Conversion
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High Pass Filter
Conversion
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High Pass Filter
Conversion