Set the input sample rate of the object to The DDC object automatically factors the decimation value so that the CIC filter decimates by 64, the CIC compensator decimates by 2, and the third stage filter decimates by 2. Designing decimation filters so that their cascade response meets a given set of passband and stopband attenuation and frequency specifications can be a cumbersome process where you have to choose the correct combination of passband and stopband frequencies for each filter stage.
Choosing stopband frequencies properly ensures lower order filter designs. The DDC object automatically designs the decimation filters based on a set of passband and stopband attenuation and frequency specifications. The DDC object obtains minimum order decimation filter designs with the passband and stopband attenuation and frequency specifications you provide.
Set the MinimumOrderDesign property to true to obtain minimum order filter designs. Set the StopbandFrequencySource property to 'Auto' so that the DDC object sets the cutoff frequency of the cascade response approximately at the output Nyquist rate, i. When you set StopbandFrequencySource to 'Auto' , the DDC object relaxes the stopband frequency as much as possible to obtain the lowest filter orders at the cost of allowing some aliasing energy in the transition band of the cascade response.
This design tradeoff is convenient when your priority is to minimize filter orders. You can analyze the response of the cascade of decimation filters by calling the fvtool method of the DDC object. Specify a fixed-point arithmetic so that the DDC object quantizes the filter coefficients to an optimum number of bits that allow the cascade response to meet the stopband attenuation specifications.
If aliasing in the transition band is not acceptable, set the stopband frequency to an arbitrary value by setting the StopbandFrequencySource property to 'Property'.
Obtain a narrower transition band by setting the stopband frequency to KHz at the expense of a larger third stage filter order. Visualize the response of each individual filter stage and of the overall cascade using the visualizeFilterStages method of the DDC object.
There are cases when filter orders are the main design constraint. You use the DDC object to design decimation filters with a specified order by setting the MinimumOrderDesign property to false. You can still specify the required passband and stopband frequencies of the cascade response. Note however that the stopband attenuation and ripple are now controlled by the order of the filters and not by property values.
The DDC object designs a numerically controlled oscillator based on a small set of parameters. Set the Oscillator property to 'NCO' to choose a numerically controlled oscillator.
Use 32 accumulator bits, and 18 quantized accumulator bits. Set the center frequency to You can set different properties on the DDC object to control the fixed-point data types along the down conversion path.
Cast the word and fraction lengths at the input of each filter to 20 and 19 bits respectively by setting the CustomFiltersInputDataType property to numerictype [],20, Initialize a sine wave generator to simulate a GSM source.
Initialize a buffer to cast the input signal data type to 19 bits word length and 18 bits fraction length. Configure figures for plotting spectral estimates of signals. The DDC object allows you to obtain down converter designs in one simple step. Likewise, Figure shows how we get the filtered continuous quadrature phase portion bottom path of our desired complex signal by mixing xbp t with —sin 2pfct.
From Eq. The minus sign in the term accounts for the down-converted spectra in Xq f being o out of phase with the up-converted spectra. This depiction of quadrature sampling can be enhanced if we look at the situation from a three-dimensional standpoint, as in Figure While the quadrature sampler in Figure a performed complex down-conversion, it's easy to implement complex up-conversion by merely conjugating the xc n sequence, effectively inverting xc n 's spectrum about zero Hz, as shown in Figure Previous page.
Table of content. Next page. Chapter One. Discrete Sequences and Systems Chapter One. Periodic Sampling Chapter Two. Quadrature Signals Chapter Eight. Sample Rate Conversion Chapter Ten. Signal Averaging Chapter Eleven. Decibels dB and dBm Appendix E. Decibels dB and dBm Section E. Digital Filter Terminology Appendix F. Frequency Sampling Filter Design Tables show all menu. Understanding Digital Signal Processing 2nd Edition. Authors: Richard G.
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