Comparison of Time Domain Scans and Stepped Scans in EMI … · 2016. 8. 10. · EMI Test Receivers...

Comparison of Time Domain Scans and Stepped Scans in EMI Test Receivers

Matthias KellerRohde & Schwarz GmbH & Co.KGMunich

Comparison of Time Domain Scans and Stepped Scans in

EMI Test Receivers | 2

Comparison of Time Domain Scans and Stepped Scans i n EMI Test Receivers

l Conventional EMI measurementsl FFT-based Measuring Receiverl Level Accuracyl Consideration of measurement timesl New insights for EMI diagnosisl Conclusion

Contents



Conventional EMI measurements



l Type of disturbance signal is often unknown(narrowband, broadband)

l Timing of disturbance signal must be considered(CW, intermittent)

l Measurements to commercial standards require quasipeak weighting (and sometimes CISPR-average )

l Measurement procedures are very time-consuming(e.g. radiated emission tests with mast and turntab le)

� Use of time saving methods and procedures� Quasipeak weighting only when necessary

Challenges of EMI testing



Conventional solution for test time reduction(commercial standards)

Preview: Signal detection in the frequency

range with peak-(average-)weighting

Data reduction: (acceptance analysis)

�� frequency list

Final measurement: QP-(CISPR-average-)

weighting only of frequencies in list



FFT-based Measuring Receiver



Receiver with broadband FFT applied at the baseband

Principle of FFT-based measuring receiver (1)

Display

Receiverpreselectionand mixer

widebandIF Filter

IF=90 MHz

ADC FFT Detectors ScreenDisplay

Superheterodyne receiver with broadband FFT of the IF



Bw signal 1 GHzDynamic range > 90 dB ADC

fs > 2 GHz

ADC word length ≥ 24 Bit

ADC word length ≥ 16 Bit

fs > 60 MHz ��

Baseband sampling

IF sampling


requires split of the frequency range into subranges

Mixer

fIF

IF filter

fLO

Localoscillator

Bw signal 26 GHzDynamic range > 90 dB ADC

e.g. 30 MHz




�

�

�

Frequency domain

Split the measured frequency range into

consecutive frequency intervals

Time-domain

Sample the frequency interval with high

sampling rate

F(s) f(t)

Fast-Fourier transformation

Transform the signals from time domain to

frequency domain

Frequency domain

Merge the spectra of all frequency blocks



Modern Preselector Design (1)

with preselection

withoutpreselection

ı Frontend is overloaded without preselectorı No reliable overload indication




E1pk

t

E(t)

t

E(t)

E2pk

t

E(t)

E3pk

voltage ratio E1pk / E2pk = BW pulse / BW preselector

Reduction of Pulse Voltage to prevent overload of Mixer, ADC, IF amps

RF Input Mixer

fIF

RBW

Localoscillator

Preselector Bandwidth

150 kHz to 100 MHzDisplay

e.g. 120 kHz




l Overall design is important for dynamic range

l Preselector is only one part

l ADC length is important

l More bits allow for more bandwidth

l Each extra bit doubles the voltage ratio

l Double voltage range allows double bandwidth



l Disturbance signals are not periodically(mixture of periodic signals , intermittent signals and noise )

l The observation time T meas cannot be set as multiple integer of the period T 0 of the disturbance signal

l In this case signal distortions appearl Leakage effect: signal spectrum becomes wider, additional

spectral components appear

Leakage effect (1)



l A limited observation time partly distributes the spectral energy of the signal over a wide frequency range� The main lobe of the signal spectrum

becomes wider� Additional spectral components

appear (side lobes)� The main lobe amplitude is reduced

l CISPR 16 requires asuppression of the additionalspectral components (side lobes)of at least 40 dB

Leakage effect (2)

Magnitude of the transfer function of a rectangular window function



Sampling

Time f

A "Scan"

Sampling

Time

Time

f

Afs/2fbin

Leakage effect (3)

Main lobe ⇔⇔⇔⇔ Measurement BandwidthRemoval of side lobes!

Main lobe must match filter characteristic requirem ents(CISPR, MIL-STD)

Time

?



Leakage effect (4)Windowing

Flattop

HanningRectangular

Gaussian

A

1

Gaussian40 dB

The width of the main lobe and the amplitude of the side lobes depend on the type of the window function



CISPR 16-1-1 tolerance mask forIF selectivity

Gaussian-type window (1)

FFT-based scan:

Frequencybin

Stepped frequency

scan:IF

bandwidth



l Gaussian window function�� Effective reduction of the leakage effect

l The Fourier transformation of a Gaussian function in the time-domain results in a Gaussian function in the frequency domain

l The characteristic of the measurement bandwidth (select ivity)defined by CISPR 16-1-1 or MIL-STD-461 matches with a Gaussian filter function

l Measurement bandwidth requirements are fulfilled

Gaussian-type window (2)



t

A

t

A

Rectangular window

Gaussian window

t

AGuassian window with overlapping

Continuous overlapping:Short-time FFT (STFFT)

T1 T2 T3 T4

FFT of intermittent signals (1)



FFT of intermittent signals (2)

Short-time FFT (STFFT) of a pulse spectrum using Ga ussian-type windowing with different overlapping factors

25%

75% 90%Reference:

0%



Detector weighting functions

FFT n f

f

f

f

Bins

FFT n+1

FFT n+2

FFT n+3

FFT n+4

FFT n+5

FFT n+6

FFT n+7

f

f

f

f

Continuous overlapping of windows in time domaint

A

t

A

Continuous overlapping of w

indows in tim

e domain

parallel for all 'bins'e.g. 16384 x Quasipeak

Result per 'bin' after dwell time

Weighting per 'bin' (e.g. QP)

tD

well tim

e

Dw

ell time

Video Voltage per 'bin'



Technical realization



Leakage effectSide lobe suppression of R&S ESR (CISPR bandwidth 9 kHz)



Measurement Bandwidth 9 kHz

Selectivity for CISPR band B (measurement bandwidth 9 kHz)

Frequency in MHz

Am

plitu

de in

dB

µV

Tolerance mask



Level Accuracy



Level Accuracy R&S ESR (1)Time Domain Scan versus Stepped Scan

Pulse with 4.00 µs Pulse period, 0.10 µs Pulse width Detector Quasi Peak

Yellow Trace: Time-Domain-ScanGreen Trace: Stepped Scan



Level Accuracy R&S ESR (2)Time Domain Scan versus Stepped Scan

y-axis: level difference, x-axis: frequency

-0,50

-0,40

-0,30

-0,20

-0,10

0,00

0,10

0,20

0,30

0,40

0,50249000

1000500

1749750

2499000

3250500

3999750

4749000

5500500

6249750

6999000

7750500

8499750

9249000

10749750

11499000

12250500

12999750

13749000

14500500

15249750

15999000

16750500

17499750

18249000

19000500

21750000

22499250

23250750

24000000

24749250

25500750

26250000

26999250

27750750

TD-Scan vs. Stepped ScandB

Hz



Consideration of measurement times



l Annex B of CISPR 16-2-1 to 16-2-3 specifies minimum sweep times or the fastest scan rates

l Quasi-peak detection results in unacceptably long m easurement times

Frequency range Peak detector Quasipeak detector

A 9 to 150 kHz 100 ms/kHz: 14.1 s

20 s/kHz: 2 820 s = 47 min

B 0,15 to 30 MHz 100 ms/kHz: 2.985 s

200 s/MHz: 5 970 s = 1 h 39 min

C/D 30 to 1 000 MHz 1 ms/MHz: 0.97 s

20 s/MHz: 19 400 s = 5h 23 min

Minimum scan times for commercial standards (CISPR)

… If the level of disturbance is not steady, the reading on the measuring receiver is observed for at least 15 s for each measurement.



… For equipment that operates such that potential emissions are produced at only infrequent intervals, times for frequency scanning shall be increased as necessary to capture any emissions.

Minimum scan and sweep times for military standards (MIL-STD-461E/F)Bandwidth and measurement time requirements



Comparison of measurement times (CISPR)R&S ESR

Measurement TimesFrequency Range Detector,

Dwell Time,Measurement BW(Number of Points)

Stepped Scan Time Domain Scan

CISPR Band B150 kHz – 30 MHz

Pk, 100 ms, 9 kHz (13.267)

22 min 117 ms

CISPR Band B150 kHz – 30 MHz

QP, 1 s, 9 kHz (13.267)

3.6 h 2 s *

CISPR Band C/D30 MHz – 1 GHz

Pk, 10 ms, 120 kHz (32.334)

5 min, 23 s 630 ms

CISPR Band C/D30 MHz – 1 GHz

Pk, 10 ms, 9 kHz (431.000)

71 min, 50 s 850 ms

CISPR Band C/D30 MHz - 1 GHz

QP, 1 s, 120 kHz (32.334)

~ 9 h 80 s *

* incl. 1 s settling time per FFT segment



How to select the right Measurement Time (1)Pulse modulated carrier, 12 ms pulse period – Stepped Scan

Even 10 ms

measurement time

yields a closed trace



How to select the right Measurement Time (1)Pulse modulated carrier, 12 ms pulse period – Stepped Scan

Even 10 ms

measurment time

yields a closed trace

Zooming in reveals

gaps in the trace



How to select the right Measurement Time (2)Pulse modulated carrier, 12 ms pulse period – Time Domain Scan

Closed trace with 12 ms

measurement time



How to select the right Measurement Time (2)Pulse modulated carrier, 12 ms pulse period – Time Domain Scan

Closed trace with 12 ms

measurement time

Gaps in trace with 10 ms

measurement time

Important:

Measurement time ≥

signal period



How to select the right Measurement Time (3)Pulse modulated carrier, 12 ms pulse period – Spectrum Analyzer Zero Span

Zero span display in

spectrum analyzer

measures signal

period



New insights for EMI diagnostics



Scan Spectrogram

l Spectrogram Gapless Spectrogram of Quasi peak trace.Quasi peak values of whole CISPR Band B is processed in real time.

EUT is a laptop power supply. Different load conditions change the spectrum over time.



Persistance Spectrum (1)

l The trace color shows how often a signal occurs at a specific frequency and level

l ⇒⇒⇒⇒ Spectral histogram

Virtual table and result display



Persistance Spectrum (2)

Clear write display (yellow trace) Max hold display (blue trace) of a broadband interferer (windshield wiper motor) with conventional spectrum analysis

The same disturbance signal in persistence spectrum mode : A second pulsed disturbance signal is hidden by the broadband noise and not detectable by conventional spectrum analysis



Frequency Mask Trigger (FMT)R&S ESR EMI Test Receiver

Frequency mask editor:

The spectrum display is continuously updated.

Draw the mask by touch screen or enter points numerically.

Mask



l Commercial standardsl DUTs with rapidly changing disturbance characteristic (e.g. lighting equipment)or drifting interferersl Intermittent disturbances requiring long dwell timesl Fieldstrength measurements with EUT positioning (mast/turntable)

l Automotivel DUTs with very short duty cycle (e.g. starter, power windows)l Use of small IF bandwidths due to low limit lines

l Military/Aerospace (A&D) l EUT cycle time in emissions testing is made practicall Probability of intercept for transient emissions increased

Applications and User Benefit



l The time needed for an EMI measurement (diagnosis/ precompliance/certification) is significantly reduced with the FFT-based time-domain scan

l The measurement uncertainty of the time-domain scan is equivalent to the uncertainty of the stepped frequency scan

l The method is applicable for EMI compliance measurements to CISPR 16-1-1 ed. 3

l In EMI diagnosis realtime functions offer new and powerful measurement and analysis capabilities

Conclusion



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Comparison of Time Domain Scans and Stepped Scans in EMI … · 2016. 8. 10. · EMI Test Receivers...

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