135 AntiAliasing Filters
Spurious effects in the receiver channel (Figure 13-1) appear as high frequency noise in the baseband signal present at the ADC. The spurious signals (> // 2) must be blocked from getting to the ADC (sampling at Nyquist rate, /s) where they will cause aliasing errors in the ADC output.
A suitable anti-aliasing low-pass analog filter placed immediately before the ADC can block all frequency components capable of causing aliasing from reaching the ADC. The anti-aliasing filter cutoff frequency (fc) is set to the highest baseband signal frequency of interest (fmax) so that fc = fmax. Sampling theorem requires that the ADC minimum Nyquist rate sampling frequency fs = 2fmax. This ensures that the original base band or IF signal can be reconstructed exactly from the ADC's digital outputs. It is important to know that only an anti-aliasing filter having a brickwall-type response could fully satisfy the exacting requirements imposed by the sampling theorem. The rolloff of real filters increases more gradually from cutoff to the stop band, and therefore, in practice, the ADC sampling frequency is usually slightly higher than 2/max.
The anti-aliasing filter must reduce the out-of-band aliasing producing signals to less than 1 LSB of the ADC resolution, without introducing additional distortion of the baseband or IF signal in-band components and without predominating distortion due to the ADC nonli-nearities. The spectrum overlap (aliasing) requirements are determined by:
• Highest frequency of interest
• Sampling rate
• ADC resolution
The highest signal frequency of interest sets the filter cutoff frequency. For example, suppose the input signal is to be sampled to 12-bit accuracy with a sampling frequency of 52 MHz. If the IF signal is 17 MHz an 18 MHz filter -3dB cutoff frequency could be chosen. All frequencies above the Nyquist frequency should be attenuated to < xh LSB, but generally only frequencies above the ADC's limit of resolution will be a problem; i.e., /alias = (52 - 17) MHz = 35 MHz. The frequency rolloff is 18 MHz to 35 MHz (about 1 octave) and the required attenuation is 72 dB (12-bit ADC). A very high-order filter is required to accomplish this task. Practical ant-aliasing filters are limited to fifth-order or sixth-order type because of amplifier bandwidth, phase margin, layout parasitics, supply voltage, and component tolerances. Keep in mind that as the rolloff sharpens, the passband ripple and phase distortion increase.
For communication applications, linear phase characteristic and gain accuracy (low passband ripple) are important. And normally, Chebychev or elliptic (Cauer) filter types are used for the anti-aliasing filter.
For good transient response or to preserve a high degree of phase coherence in complex signals, the filter must be of linear-phase type (Bessel-type filter).
The THS4011 or THS4021 voltage-feedback op amp is a good choice for implementing anti-aliasing filter in this example.
The quality of the capacitors and resistors used to implement the design is critical for performance anti-aliasing filter.
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