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Sampling Methods and Analog Filtering

Sampling Methods and Analog Filtering. The sampling process is of critical importance in radio receivers using digitization at the RF or IF. Common sampling techniques using a uniform spacing: Nyquist Sampling Oversampling Quadrature Sampling Bandpass Sampling. Nyquist Sampling.

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Sampling Methods and Analog Filtering

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  1. Sampling Methods and Analog Filtering • The sampling process is of critical importance in radio receivers using digitization at the RF or IF. • Common sampling techniques using a uniform spacing: • Nyquist Sampling • Oversampling • Quadrature Sampling • Bandpass Sampling NTU EE LAB 353

  2. Nyquist Sampling • Def: A sampling rate of two times the highest frequency component of the analog signal • Sampling a bandlimited signal at Nyquist rate==>Anti-Aliasing NTU EE LAB 353

  3. Bandlimited Signal in a Practical Sense?? • Solution:The relative amplitude of the undesired signals to the desired signal is important. • Question:What is the relative amplitude of the signals occurring above one-half of the sampling rate? • Solution:Undesired signals appearing in the Nyquist band due to spectrum overlap must be lower in power than the largest spurious response of the ADC due to nonlinearity. NTU EE LAB 353

  4. Bandlimited Signal in a Practical Sense??(Cont.) • To “relax” this requirement • Q:How much distortion of the desired signal is tolerable? • A:Considering Communication System • 1.source information:voice, data, video, etc) • 2.desired signal bandwidth • 3.undesired signal characteristics(BW, power, signal type) NTU EE LAB 353

  5. Realizable Anti-Aliasing Filters • More complicated filters are required to reduce the distortion in the sampled signal due to spectrum overlap for a given sampling rate. • ==>High Order • ==>Phase response tends to become more nonlinear. NTU EE LAB 353

  6. Oversampling • Definition: • Sampling at rates greater than the Nyquist sampling rate is called oversampling. • When sampling at a higher rate, a simpler anti-aliasing filter with less stop band attenuation can be used . • Increasingly faster ADCs are required to digitize low frequency signals. NTU EE LAB 353

  7. Quadrature Sampling • In quadrature sampling the signal to be digitized is split into two signals. • One is in phase component; the other is quadrature phase component. • Be sampled at one-half the sampling rate required for the original signal. • At expense of using two phase-locked ADCs instead of one. NTU EE LAB 353

  8. Bandpass Sampling • For a bandpass signal, the sampling rate be at least two times the BW( ) of the signal. • The sampling frequency must satisfied • Bandpass sampling holds promise for radio receivers that digitize directly at the RF or IF, since the desired input signals to radio receivers are normally bandpass signals. NTU EE LAB 353

  9. Effects of Quantization Noise, Distortion, and Receiver Noise • Relationship among quantization noise, harmonic distortion, and receiver noise (thermal noise) • Quantization: • Uniform quantization • Nonuniform:A-law, u-law, adaptive, and differential quantization NTU EE LAB 353

  10. Uniform Quantization • Statistically, the error signal is assumed to be uniformly distributed within a quantization level. • Mean Squared Quantization Noise Power: NTU EE LAB 353

  11. Noise and Solution • Dithering==>Harmonic Distortion, Thermal Noise(Flat Noise) • Boost Amp Gain • Matching the Input Impedence between the ADC and Receiver Output(50-ohm)==> Quantization Noise • To place a 50-ohm resistive load at the input of the ADC NTU EE LAB 353

  12. Noise and Solution • The Receiver Noise • Higher Resolution==>Smaller Quantization Noise Power==>Less Gain NTU EE LAB 353

  13. Signal-to-Noise Ratio • Theoretical S/N ratio due to quantization noise: NTU EE LAB 353

  14. Signal-to-Noise Ratio • Theoretical S/N ratio due to aperture jitter noise: • Sources: • Externally by jitter in the sampling clock • Internally by sampling switches • SNR=SNRq+SNRaj NTU EE LAB 353

  15. Residual Error • Definition: • Residual error is the combination of quantization noise, random noise, and nonlinear distortion. • Measure: • Output signal - input signal(sin wave) NTU EE LAB 353

  16. Spurious Free Dynamic Range • Definition: • Ratio of the sinusoidal signal power to the peak power of the largest spurious signal in the ADC output spectrum. • Application: • Desired signal BW less than Nyquist BW • SRDR>>SNR NTU EE LAB 353

  17. Noise Power Ratio • Definition: • PSD of noise outside BW of notch filter/PSD of noise inside BW of notch filter • Measure: • By using a noise input signal into the ADC • Application: • Desired signal spectrum with many channels NTU EE LAB 353

  18. Summay of ADC Specifications for Radio Receiver Application NTU EE LAB 353

  19. Critical ADC Specification for Typical Application NTU EE LAB 353

  20. Examples of current high-speed ADC technology NTU EE LAB 353

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