synchronous time / frequency domain measurements using a digital oscilloscope (presented at eelive!...

Synchronous Time and Frequency Domain Analysis of Embedded Systems

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Agenda l Complex Embedded Systems

l The Challenge of Debugging Embedded Systems

l Frequency domain analysis l Time gating l Dynamic range l triggering

l Measurement Example: PLL locking l SPI triggering l Measuring settling time and transient spectrum

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Complex Embedded Systems

D/A

D/A

DSP

Micro controller

IQ modulator

Digital signals (serial/parallel)

Analog signals

RF signals

EMI

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The Challenge of Debugging Embedded Systems

l Baseband digital, RF and analog signals are interdependent l Feedback control of RF by microcontroller l Low speed serial busses l Critical timing relationships l Interference between RF and digital signals l EMI

l Analyzing and debugging in the frequency domain l Frequency domain analysis synchronized with time and digital domains l Frequency analysis speed l Sufficient sensitivity in both time and frequency domains l Triggering ( time, digital and frequency)

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Fourier Transform Concept

Any real waveform can be produced by adding sine waves

Spectrum changes Over time

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Measurement Tools: Spectrum Analyzer

l Spectrum is measured by sweeping the local oscillator across the band of interest l Band pass filter after IF amplifier determines the frequency resolution (RBW) l Very low noise due to IF gain and filtering l Sweep can be fast over narrow span l Real time operation possible over a limited frequency range using FFT after IF

filter

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Spectrum Measurement is a Function of Time Glitches

time

f1 f2 f3 f4 f5 f6 f7

Measurement frequency Center frequency of the RBW filter is swept across the Frequency range to build the signal spectrum

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Frequency Domain Analysis FFT Basics

l NFFT Number of consecutive samples (acquired in time domain), power of 2 (e.g. 1024)

l ∆ fFFT Frequency resolution l tint integration time l fs sample rate

Integration time tint NFFT samples input for FFT

FFT

Total bandwidth fs NFFT filter output of FFT

FFTf∆ts

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f1 f2 f3 f4 f5 f6

Fourier Transform: Instantaneous Spectrum

f1 f2 f3 f4 f5 f6 f7

f1 f2 f3 f4 f5 f6 f7

f1 f2 f3 f4 f5 f6 f7

f1 f2 f3 f4 f5 f6 f7

f7

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FFT Implementation Resolution Bandwidth l Two important FFT rules

l RBW dependent on

l Integration time, e.g. 1 sec => 1 Hz, 100 ms => 10 Hz

l Highest measureable frequency dependent on

l Sample rate (e.g. fs = 2 GHz => fmax = 1 GHz)

l Nyquist theorem: fs > 2 fmax

intmax 22 t

Nsff FFT⋅

==

FFTs Ntf =• int

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FFT Implementation Digital Down Conversion l Conventional oscilloscopes

l Calculate FFT over part or all of acquisition

l Improved method: l Calculate only FFT over span

of interest l fC = center frequency of FFT

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FFT Implementation Time and Frequency Domains

l 10 us time slice = 1e-6 * 10e9 = 10K samples in FFT l Using digital down conversion 100 MHz span @ 100 KHz RBW the

FFT length is 1e-6*2e8 = 200 samples l Multimple FFT's per acquisition e.g. 100 10 us FFT per acquisition

or 200 FFT using 50% overlap

Time line (e.g. 1 ms) 100 KHz RBW = 20 us

FFT 1 FFT 2 FFT 3 FFT 4

FFT 1 FFT 2

FFT 3 FFT 4 50% overlapping

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Tradeoff for Windowing: Missed Signal Events

Original Signal

Signal after Windowing

l All oscilloscope FFT processing uses windowing l Spectral leakage eliminated l However, signal events near window edges are attenuated or lost

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FFT Overlap Processing l Overlap Processing ensures no signal details are missed

Original Signal

…

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Time Gating

•Signal characteristics change over the acquisition interval •Gating allows selection of specific time intervals for analysis

FFT

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Time Gating Tg

gTf 1=∆

FFT

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Frequency domain measurement dynamic range l Analog to digital conversion (ADC) performance sets the

dynamic range l Signal to noise ratio (ENOB) l Frequency domain spurious

l Front-end amplifier gain l Noise figure and sensitivity

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Ideal ADC

Ideal ADC

s(t) s (t )q i

l How can we measure with sufficient range in the frequeny domain?

l The A/D converter sets the dynamic range l K bit ADC (2K quantization levels) l Effective Number Of Bits (ENOB) = K

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Analog-to-Digital Converter - ENOB l Effective Number of Bits (ENOB): A measure of signal fidelity

l Higher ENOB => lower quantization error and higher SNR => better accuracy

Effective Bits (N)

Quantization Levels

Least Significant Bit ∆V

4 16 62.5 mV

5 32 31.3 mV

6 64 15.6 mV

7 128 7.8 mV

8 256 3.9 mV

Offset Error Gain Error Nonlinearity Error Aperture Uncertainty And Random Noise

+ + +

± ½ LDB Error

Quantatized Digital Level

Sample Points

Analog Waveforms

<

Ideal ADC vertical 8bits = 256 Quantatizing levels

8 Effective Number of Bits !

Ideal

typical

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Signal to Noise and ENOB

l What noise level would be observed in the spectrum measured with 100 KHz resolution bandwidth?

l Assume 2 GHz instrument bandwidth with an ideal 8-bit ADC l SNR = 49.92 dB l SNRspect = 92.93 dB l Processing gain = 43.1 dB

∗+=

+∗=

−≈

519210log10

76.102.602.6

76.1

EESNRSNR

BSNR

SNRB

spect

dB

Displayed noise level is reduced by the ratio of full bandwidth to RBW

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Signal to Noise (6.8 ENOB)

~84 dB Ideal = 85.7

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Effect of interleaving in the frequency domain

harmonics

Interleaving spurious

Interleaved A/D Non-interleaved A/D

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High Gain Amplifier Reduces Noise at 1 mV/div

Noise power in 50 KHz BW = -102 dBm ~ -148 dBm/Hz

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Triggering

l Triggers can be required different “domains” l Time domain (edge, runt, width, etc.) l Digital domain (pattern, serial bus) l Frequency domain (amplitude/frequency mask)

l Sensitivity of time domain triggers l Matching bandwidth with acquisition for all trigger types l Noise reduction (filtering, hysteresis)

l Frequency domain triggers l Processing speed of FFT

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Time Domain Mask l Draw a violation zone or zones l Set for “stop on failure”

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l Most Serial Bus Architectures rely on the concept of Abstraction Layers or a Protocol Stack to transmit information fewer physical lines.

l Since an Oscilloscope captures the analog information (Physical Layer) it often contains the root information for viewing protocol as well.

Protocol or Packet Triggering

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Application Layer

Physical Layer

Data Link Layer

Network Layer

Transport Layer

Application Layer

Bit Stream

Tran

smit

Dat

a

Rec

eive

Dat

a

Physical Link

Framing/Packets

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Example: RS232/UART

Trigger Types: • Start bit • Frame start • Packet start • A specified symbol • Parity errors, and breaks • Frame errors • Stop errors • A serial pattern at any or a specified position

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Frequency Domain Mask

l Mask test on spectrum l Set for “stop on failure”

Frequency mask

Gated FFT

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Digitally Controlled Attenuator

l The 5 ATTEN bits show the digital signal that sources a digitally-controlled attenuator chain that controls the signal strength at the RFOUT port.

l The ATTEN bits form a 5-bit word which is 3dB per LSB.

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Spectrum of Digitally Controlled Signal

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Capture of a Broad band Glitch l Lets view the broadband glitch in the frequency domain. l You will draw a mask on one of the areas you see a violation in. l Set up a “stop on violation”

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Finding the root cause. l Combine digital (MSO) channels with RF and time domain signal l Cause of frequency domain glitch can be easily seen l In this case it is caused by timing skew in the digital channels

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Frequency Hopping Carrier

Microcontroller

SPI Input Control Signal

u1

x2

x1 * / *VCO

CPV

RFOUT

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Time and Frequency Domain Views ı This is the “hop” from 835MHz down to 825MHz. ı The two FFT’s indicate a “safe” settle time for the

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Summary

l FFT based spectrum analysis can be enhanced to enable time-correlated spectrum analysis l Improved throughput using digital down conversion l High dynamic range A/D conversion l High gain amplifier for small signal measurement

l Real time oscilloscope platform is ideal for digital, time and frequency analysis l Synchronized time and frequency domain analysis l Serial protocol trigger and decode l Parallel data channels