Mixed-Signal VLSI Design Course Code: EE719 Department: Electrical - - PowerPoint PPT Presentation

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Mixed-Signal VLSI Design Course Code: EE719 Department: Electrical Engineering Lecture 4: January 19, 2020

Instructor Name: M. Shojaei Baghini E-Mail ID: mshojaei@ee.iitb.ac.in

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IIT-Bombay Lecture 4 M. Shojaei Baghini

Module 3

Coherent Sampling and FFT Simulation

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IIT-Bombay Lecture 4 M. Shojaei Baghini

References

• Coherent Sampling vs. Window Sampling, Tutorial 1040,

Maxim, March 29, 2002

• Exact Signal Measurements using FFT Analysis, Stefan

Scholl, Course Material, 2016

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IIT-Bombay Lecture 4 M. Shojaei Baghini

In general:

l N and K are integers with N > 2K (N: number of samples and

K: number of cycles).

l N and K don't have any common factor (co-prime numbers).

Coherent Sampling

Example: Sampling window = 3 cycles with nonoverlapping samples !" = !"$%&'( × * +

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IIT-Bombay Lecture 4 M. Shojaei Baghini

FFT Simulation

l Practical sampling produces distortion. l Number of samples: N

N-point FFT will provide the power of the discrete time signal at discrete normalized frequencies of 0, 2p /N, 4p /N, 6p /N, ...., 2(N-1)p /N (normalization factor = fsampling).

l FFT bin = 2p /N (bin number L corresponds to w=2pL/N).

• Window functions

l How do we ensure simulation results are accurate?

• Simulation accuracy options
• Coherent sampling with N non-overlapping samples
• Benchmarking simulations

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IIT-Bombay Lecture 4 M. Shojaei Baghini

Coherent Sampling - FFT Simulation Example

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fsampling=82MHz fsignal=25.05444433MHz

• No. of signal cycles=2503
• No. of samples (N points for FFT)=8192

25.05444433/82=2503.00009697/8192 SNR=59.3dB THD=-70.3dBc fsampling-3fsignal fsampling-2fsignal

Example from TUTORIAL 1040, Maxim Coherent Sampling

• vs. Window Sampling

March 29, 2002

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IIT-Bombay Lecture 4 M. Shojaei Baghini fsampling=82MHz fsignal=25.2245MHz

• No. of signal cycles=2520
• No. of samples (N points for

FFT)=8192 25.2245/82=2519.98907/8192 SNR=51.6dB THD=-69.1dBc

Spectral Leakage Noncoherent Sampling - FFT Simulation Example

Example from TUTORIAL 1040, Maxim Coherent Sampling

• vs. Window Sampling

March 29, 2002

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IIT-Bombay Lecture 4 M. Shojaei Baghini

FFT Processing Gain (PG)

Exact Signal Measurements using FFT Analysis, Stefan Scholl, Course Material, 2016

!" = 10log(*/2)

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IIT-Bombay Lecture 4 M. Shojaei Baghini

First Practice: FFT, Matlab Simulation Consider the condition of coherent sampling with the non- repeated samples for this exercise as you choose fs.

• Choose a signal with 1 frequency (single tone) and then 2

frequency components (2-tone signal).

• Start from fs > Nyquist rate but close to it and then increase fs (3

different values of fs). Specify values of N and M.

• For each value of fs, plot dB magnitude of FFT of the sampled signal

(normalized to fs) and label magnitude of main frequency

• components. Make sure highest frequency component is always at

0 dB for the reference.

• Obtain SNR of the sampler in each case.
• You may try one of the windowing methods and repeat the

simulations to compare the results with rectangular windowing.

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IIT-Bombay Lecture 4 M. Shojaei Baghini

Module 4 Introduction to Sampling Switches

Reference Section: Sampling Switches Chapter: Introduction to Switched-Capacitor Circuits Book: Design of Analog CMOS Integrated Circuits Behzad Razavi, 2017

• This part of the lecture is based on class notes only.

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IIT-Bombay Lecture 4 M. Shojaei Baghini

End of Lecture 4