Antialiasing And Sync-Compensation Filter

Description: Two dual-biquad filter chips and some external components form a multipurpose filter to reconstruct D/A converter signals. Connected to a converter's output, the filter provides antialiasing, reduces the D/A converter's quantization noise, and compensates for the "sync" function (attenuation). The circuit incorporates an inverse-sync function that operates at one-third of the converter's sample rate. Beyond one-third, the filter's response shifts to a stopband filter, which provides -70 dB attenuation. This attenuation conforms to the converter's inherent signal-to-noise ratio and quantization error. To prevent aliasing, the stopband edge must be no higher than the Nyquist frequency (fs/2 + 2). To achieve 70-dB stopband rejection with this eighth-order filter requires a transition ratio (f_stopband - f_passband) of 1.5, which sets the passband's upper limit at fs + 3. Notice also that a simple divide-by-64 circuit can be applied to the 192-kHz clock frequency to set the necessary 3:1 ratio between the converter's sample rate and the filter's 1-kHz corner frequency.

The schematic for this multipurpose filter circuit utilizes two dual-biquad filter chips, which are essential for implementing the required filtering characteristics. These chips are typically configured in a cascaded arrangement to achieve an eighth-order filter response, allowing for precise control over the frequency response characteristics.

The primary function of this filter circuit is to reconstruct D/A converter signals while providing antialiasing to mitigate the effects of high-frequency noise. The filter is designed to reduce quantization noise inherent in the D/A conversion process, thereby improving the overall signal integrity. The compensation for the "sync" function ensures that the output signal remains within the desired amplitude range, preventing distortion during signal reconstruction.

The inverse-sync function is a critical feature of this design, operating effectively at one-third of the converter's sample rate. This operation allows the filter to maintain its effectiveness at lower frequencies while transitioning to a stopband filter response as the frequency exceeds this threshold. The stopband filter is designed to provide a significant attenuation of -70 dB, which is essential for maintaining the fidelity of the output signal in the presence of high-frequency components.

To ensure compliance with the Nyquist criterion and prevent aliasing, the design stipulates that the stopband edge must not exceed the Nyquist frequency, which is defined as half of the sampling frequency (fs/2). The transition ratio of 1.5 between the stopband and passband frequencies is a critical parameter that influences the filter's performance. This ratio establishes the upper limit of the passband frequency, set at fs + 3, which is crucial for maintaining the desired filtering characteristics.

The implementation of a divide-by-64 circuit from the 192-kHz clock frequency is a practical approach to achieve the necessary frequency scaling. This division sets the appropriate 3:1 ratio between the converter's sample rate and the filter's corner frequency, ensuring optimal performance of the filter circuit in conjunction with the D/A converter.

Overall, the design of this multipurpose filter circuit leverages advanced filtering techniques to enhance the quality of D/A conversion, ensuring minimal distortion and effective noise reduction across the intended frequency range. Two dual-biquad filter chips and some external components form a multipurpose filter to reconstru ct D/A converter signals. Connected to a converter"s output, the filter provides antialiasing, reduces the D/A converter"s quantization noise, and compensates for sin(7rx)()—the "sync" function (attenuation). The circuit incorporates an inverse-sync function that operates to one-third of the converter"s sample rate.

Beyond one-third, the filter"s response shifts to a stopband filter, which provides -70 dB attenuation. This attenuation conforms to the converter"s inherent signal-to-noise ratio and quantization error. To prevent aliasing, the stopband edge must be no higher than the Nyquist frequency (/„ + 2). To achieve 70-dB stopband rejection with this eighth-order filter requires a transition ratio (/stopband -K/passband) of 1.5, which sets the passband"s upper limit at fs +3.

Notice also that you can apply a simple divide-by-64 circuit to the 192-kHz clock frequency to set the necessary 3 ratio between the converter"s sample rate and the filter"s 1-kHz corner frequency.