Development of a Multiresolution Time Domain EMI ... - IEEE Xplore

Development of a Multiresolution Time Domain EMI Measurement System that Fulfills CISPR 16-1 ∗ Institute

Stephan Braun ∗ , Martin Aidam † , Peter Russer

∗

for High-Frequency Engineering, Technische Universit¨at M¨unchen, Arcisstrasse 21, 80333 M¨unchen, Germany [email protected], [email protected] † DaimlerChrysler AG, Hedelfinger Strasse 10 - 14, 73734 Esslingen-Pliensauvorstadt [email protected]

Abstract— Measurement systems, that allow to measure within short measurement time Electromagnetic Interference (EMI), will reduce the costs for compliance tests. A time-domain EMI (TDEMI) measurement system can reduce the measurement time by several orders of magnitude. In order to have the permission to use a time-domain EMI measurement system for compliance measurements it has to fulfill the CISPR 16-1 [2] completely. In the following the requirements given by CISPR 16-1 are applied to the TDEMI measurement system. The characteristics of the TDEMI measurement system are compared with the rules of CISPR 16-1. A multiresolution TDEMI (MRTDEMI) measurement system that uses several analog-to-digital converters (ADCs) is presented. With such a system the signal-to-noise ratio (SNR) is enhanced. Measurements have been carried out in the frequency range 30 MHz - 1 GHz. It is shown, that almost all requirements are fulfillled by the MRTDEMI measurement system.

I. I NTRODUCTION Traditionally EMI measurements are performed with EMIReceivers operating in frequency domain. Measurements in frequency domain take long measurement times, up to several hours, for a single frequency scan. Full compliance EMIReceivers have to fulfill the CISPR 16-1 [2] completely. CISPR 16-1 describes the fundamental characteristics e.g. the masks of the IF-Filter and the response curve of the quasipeak detector. Requirements concerning the dynamic range for transient broadband and stationary narrowband signals are described. For each requirement a measurement procedure is specified. By these measurement procedures the conformity of the system can be validated. By a time-domain EMI (TDEMI) measurement system the measurement time can be reduced by several orders of magnitude. In the following the TDEMI measurement system is shown. The minimum requirements that apply to the system given by the CISPR 16-1 are discussed. A multiresolution time-domain EMI (MRTDEMI) measurement system is presented, which shows an enhanced signal-to-noise ratio (SNR). By this way the dynamic range is enhanced by at least 50 dB. The performance of both systems is investigated and compared using the measurement procedures described in CISPR 16-1. II. T IME - DOMAIN EMI M EASUREMENT S YSTEM The time-domain EMI Measurement System consists of a low noise amplifier, an anti-aliasing low-pass filter, an analog-to-digital converter (ADC) for data acquisition and a PC for digital signal processing [1]. For measurements

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388

Multiresolution TDEMI Measurement System

LP

Floating Point ADC

Digital Signal Processing

Amplitude Spectrum

LISN

Fig. 1.

Measurement Setup

of the conducted emissions a line impedance stabilization network (LISN) is used. Radiated emission measurements are performed with a logarithmic periodic antenna with biconical antenna that works in the frequency range 30 MHz - 3 GHz [3]. An algorithm for measurements in time-domain and evaluation of EMI spectra in the Peak, Average and RMS detector modes has been presented in [4]. A operation of the TDEMI measurement system with implemented quasipeak detector mode has been shown in [1]. The setup for the MRTDEMI measurement system is presented in Fig. 1. The MRTDEMI Measurement System is connected via a switch to a Line Impedance Stabilization Network for conducted emission measurements or to an Antenna for radiated emission measurements. III. A NTENNA For radiated emission measurements in time- and frequencydomain the HL562 ULTRALOG [3] is used. This antenna combines a logarithmic periodic antenna with a biconical antenna. The antenna is characterized with the given antenna factor for full compliance measurements. Because an EMIReceiver is a non coherent system the phase response of the antenna is omitted during the calculation of the field strength after a measurement. Logarithmic periodic antennas exhibit phase distortion. The maximum dispersion caused by the antenna can be estimated by length of the antenna. The used antenna has a length of 1.5 m for the frequency range up to 3 GHz. When we consider that the signals with higher frequencies are received at the top of the antenna and the signals with lower frequencies at the end of the antenna and assuming velocity of c0 we get a maximum dispersion of 1.7 ns for the frequency range up to 1 GHz. For accurate measurements by the time-domain EMI measurement system

Power Splitter -2 dB

A

-22 dB

A

D

Digital Signal Processing

D A

-22 dB

Fig. 2.

-10 dB

D

Floating Point Analog-to-Digital Converter

or the EMI Receiver a dispersion with several ns is negligible. The characterization of transients is performed in the range of several µs. IV. M ULTIRESOLUTION T IME - DOMAIN EMI M EASUREMENT S YSTEM

Fig. 4.

In the following a multiresolution time-domain EMI measurement system is presented. In Fig. 2 the block diagram of the floating point ADC, with three ADCs, is shown. The input signal is distributed by a power splitter into three channels. Each channel consists of a limiter, a low noise amplifier, and an ADC. While the first channel digitizes the amplitude range from 0 to 1.8 mV, the third channel digitizes the amplitude range from 0 to 10 V. The second channel is used to digitize the intermediate amplitude range from 0 to 200 mV. The signal is recorded at all three channels simultaneously. A signal digitized in high resolution is reconstructed by extracting each sampled value from the ADC where the signal shows the maximum non-clipped value.

1) Quantization Noise: The Quantization Noise PN,I of a system consisting of I ADCs is described as PN,I = with

Amplitude r3

a2h a1h a1

a ADC1 a3l 2 ADC2 ADC3

Fig. 3.

r2 r1 -r1 ADC1

l

l

(1)




ai2


H[k] +

k=ai1

H[k]).

(2)

k=ai−1,2

H[k] is the absolute frequency of the digitized values within a predefined time interval. We suppose that the input signal x(t) has no DC-offset. Further we suppose that H(k) ≈ H(−k). We get from (1) and (2) ri I 2
2
(i H(k)) 12N i=1

(3)

r0 = 0.

(4)

k=ri−1

with The quantization step i of the ADC i can be calculated as following: (5) i = 2−bi ri bi is the number of bits of the ADC i. V. H ARDWARE R EALIZATION In Fig. 4 the photo of the analog stages of the MRTDEMI System is shown. The complete analog circuit was implemented as hybrid on a hydrocarbon ceramic substrate. While the amplifiers were conventionally soldered the ultra fast GaAs Schottky diodes for limiting were fixed with a conductive epoxy glue. A. Power Splitter In order to maximize the sensitivity and minimized the noise figure of the system it is important to have a low attenuation in the first channel. Thus the resistive power splitter has been designed asymmetrically. While the first channel shows an attenuation below 2 dB channel 2 and 3 show an attenuation of 22 dB.

-r2 ADC2 ADC3 -r3

Intervals for a configuration of three ADCS

a higher dynamic range.

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ai−1,2

αi = (

PN,I (r1 , ...rI−1 ) ≈

In Fig. 3 the intervals that for a configuration of 3 ADCs are depicted. Each analog value is simultaneously digitized by all ADCs. The lower and upper boundaries of the amplitude interval i are donated with a ali and ahi respectively. If the value is within the range [al1 , ah1 ] the digitized value is extracted from the first ADC. If the value is between [al2 , al1 [∪]ah1 , ah2 ] the value is taken from the second ADC, and so on. Each extracted value is corrected by the known gain of the channel. The result is a signal that is digitized as a floating point with

a3h

a12 i=I

1 (21 H[k] + (2i αi ) 12N i=2 k=a11

A. Floating point A/D conversion

Amplitude

Photo: RF-Circuits of Floating Point ADC

389

IF − Filter Band CD

B. Limiter Circuits

60

C. Amplifiers For all channels monolithic amplifiers with a 3 dB Bandwidth of 11 GHz where used. The noise figure of the amplifiers are 4.9 dB. In channel 1 four amplifiers where cascaded to obtain a gain of 43 dB. In channel 2 two amplifiers where cascaded to obtain a gain of 2 dB.

50 Relative input in decibels for constant output

To avoid saturation effects and mismatching of the amplifiers during the overload novel Schottky Limiters with a reflection coefficient below -20 dB up to an amplitude of 6.4 V have been designed. The limiting threshold level is about 400 mV. The used limiters shows an attenuation of 2 dB in the linear state.

40

30

20

10

D. Noise Figure 0

By this way the noise figure of the first channel was minimized to 9 dB. The noise figure as well as the forward transmission of the circuits where measured. The noise figure of the MRTDEMI measurement system can be still decreased by replacing the first amplifier with a low-noise amplifier. By the use of Schottky diodes with a lower resistance in the on state the noise figure can also be decreased. VI. R EQUIREMENTS ACCORDING TO CISPR 16 In the following the requirements given by CISPR 16 are discussed and compared with the Time-Domain EMI Measurement System. The fundamental characteristics are independent of the used analog-to-digital conversion, and are the same for a TDEMI Measurement System and the MRTDEMI Measurement System.

−10 −2.5

−1.5

−1

−0.5

0

0.5

1

1.5

2

Fig. 5.

2.5 5

x 10

Hertz off mid−band

IF-Filter Response Band C,D

The Peak, Average and RMS detector are implemented digitally. The detector modes also fulfill the CISPR 16-1. An example of measurements performed with the RMS detector are shown in Fig. 6. The maximum difference between the ideal CISPR 16 response curve and the measured is within the tolerance. 20

A. Fundamental Characteristics

390

BAND C und D CISPR 16 Band B Relative equivalent level of pulse

The CISPR 16-1 describes the characteristic of the IF-Filter as well as the response to pulse trains, with different pulse repetition frequencies, in the various detector modes. In the following the response of the modelled IF-Filter by the short time fast Fourier transform (STFFT) with a multiplication of a gaussian window function [1] is compared with the requirements given in CISPR 16-1. 1) IF-Filter: The measurement procedure specified in CISPR 16-1 was performed with the TDEMI measurement system to obtain the frequency response of the modelled IFFilter. The modelled IF-filter of the TDEMI measurement system fulfills the critical masks given by the CISPR 16-1 for band A,B,C and D. The result of the measurement in band C and D is shown in Fig. 5 2) Detector Modes: The time-constants of the quasi-peak detector are defined by CISPR 16-1. These time-constants are given as proposed values. For the individual implemented quasi-peak detector the time-constants may be adapted. The TDEMI measurement system has a digitally implemented quasi-peak detector. With the original time-constants given in CISPR-16-1 the weighting curves cannot be fulfillled. By adapting the time-constants the weighting curves according to CISPR 16-1 are fulfillled for Band A, B, C and D.

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−2

15

10

5

0

−5 0 10

Fig. 6.

1

2

10 10 Pulse Repetition Frequency

3

10

Response RMS Detector Band B, C and D

3) Output Signals: A CISPR full compliance EMI Receiver must provide an analogue output of the IF-signal and the output signal of the response of the quasi-peak meter. The TDEMI measurement system does not record the

signal in a continuous way. Various samples of the signal are recorded. The input signals of the detectors are statistical equivalent to the signals in an EMI-Receiver [1]. Thus currently no IF-signal can be provided. The required output signals are only used by disturbance analyzers during the measurement of e.g. washing machines. For typical EMI measurements no disturbance analyzers are used. B. Single Resolution System 1) Overload factor: In the following the requirements of a system according to the dynamic range given by the CISPR 16-1 are discussed. The SNR of a single resolution system is investigated. For a sinusoidal input signal any spurious signals have to be at least 40 dBc. The resulting SNR of a sinusoidal input signal after a digitization with b bit ADC is described by [5](6). SN R = b · 6 dB + 1.76 dB

(6)

By the modelled IF-filter we get at each discrete spectral value a SNR according to (7). SN RIF = SN R

Bs BEN BW IF

IF )dB − 1.76dB 10log(SN RIF BENBBW s

. (8) 6dB In table I an overview of the required number of bits b of the ADC is shown. Ultra broadband systems require only a Band Bs BIF 6dB BIF EN BW SN Rmin b

A 150 kHz 0.22 kHz 0.17 kHz 16.5 dB 4 bit

B 30 MHz 9 kHz 6.8 kHz 9.5 dB 3 bit

C,D 1000 MHz 120 kHz 90.3 kHz 5.5 dB 2 bit

TABLE I R EQUIREMENTS FOR A SINUSOIDAL INPUT SIGNAL

low number of bits to digitize narrowband signals. It is also shown that a TDEMI Measurement System with a single ADC with moderate resolution of 8 bit fulfills the CISPR 16 for a sinusoidal input signal. The additional input range is 36 dB without any additional switchable attenuators. In CISPR 16-1 the overload factor OCISP R is defined by the dynamic range of the IF-Stage. This corresponds directly to the display range of receivers with a quasi-peak detector when a variation of the pulse repetition frequency is performed. For Band C and D the overload factor is 43.5 dB. The quantization noise PN is given by PN =

2 12

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Vp . (10) 2b The noise power is distributed over the whole frequency band according to (11). =

BEN BW IF (11) Bs The overload factor OCISP R is described as the difference between the noise floor PN IF and the maximum displayed response Pmax . Pmax (12) OCISP R = PN IF The corresponding voltage level at 50Ω in dBµV is named Amax . An ADC must have the number of bits according to:   Vp b = log2 . (13)  PN IF = PN

With (9),(11) and (12) we obtain:  b = log2  

(7)

Bs is the total bandwidth of the system and BEN BW IF is the equivalent noise bandwidth [6] of the modelled IF-Filter. We consider the peak detector mode with a noise floor up to 11 dB [7] higher than the average noise floor. Thus the SN RIF has to be at least 51 dB. With (7) and (6) we obtain: b=

with

(9)

391

 Vp

Bs 12 OCISPPRmax BEN BW IF



(14)

In Table II an overview of the requirements are shown. Band OCISP R Amax tp Vp b

A 24 dB 72.1 dBµV 2000 ns 6.75 V 12 bit

B 30 dB 72.6 dBµV 10 ns 31.6 V 13 bit

C,D 43.5 dB 78 dBµV 0.3 ns 147 V 16 bit

TABLE II R EQUIREMENTS GIVEN BY THE OVERLOAD FACTOR

2) Spurious free dynamic range: In CISPR 16 the minimum requirements to the linearity and the SNR of the system is given in section 4.1.6 [2]. A pulse generator is applied to the system. The pulse has a flat frequency response within the selected band. After the insertion of a notch filter an attenuation of not less than 36 dB has to be shown. The pulse repetition frequency is set to 1000 in Band B,C and D and 100 in Band A. In the average detector mode a SNR of 36 dB is sufficient. The noise floor is increased by the peak detector mode up to 11 dB [7] in comparison to the average noise floor. In the RMS detector mode the noise floor is increased by 1 dB. In the quasi-peak detector mode the noise floor is increased by 6 dB. Thus we obtain a minimum SNR for the RMS detector mode of 37 dB. In the quasi-peak detector mode the minimum SNR is 42 dB. In the peak detector mode the minimum SNR is 47 dB. For Band A and B an ADC with at least 14 bit are required for the average detector mode. For band C and D an ADC with 16 bit are already required for the average detector mode. Today are ADCs with 14 bit available for Band A and B. But due to their imperfection they have an effective number of bits (ENOB) that is lower than 13 bit. For Band C and D no ADCs with 16 bit are available. ADCs working in that frequency range have typically 8 bit.

A single ADC system with currently available ADCs does not fulfilll the requirements for the dynamic range given in CISPR 16-1. C. Multiresolution System In the following the requirements according to the multiresolution time-domain EMI measurement system are discussed. 1) Overload factor: A multiresolution time-domain EMI measurement system digitizes the signal between pulses by the ADC that has a highest sensitivity. The pulses that have a high amplitude are digitized by the ADC that digitizes the complete amplitude scale. The quantization noise is decreased by the most sensitive ADC in comparison to the ADC that digitizes the complete amplitude scale. The improvement of the SNR is described by (15). H1 = SN RS a (15) SN RM = SN RS Hn where H1 is the amplification of the channel with high sensitivity and Hn is the channel that digitizes the complete amplitude range. H1 enhances the SNR during the recording The factor a = H n between the pulses. In table II the requirements for a single ADC System are shown. In the following the requirements that apply to a MRTDEMI system are presented. For the MRTDEMI system we use 8-bit ADCs. The MRTDEMI Band OCISP R SN R8bit amin

aM RT DEM I OM RT DEM I

A 24 dB 0 dB > 24 dB 74 dB 74 dB

B 30 dB 0 dB > 30 dB 74 dB 74 dB

C,D 43.5 dB -4.5 dB > 48 dB 74 dB 69.5 dB

P N = PN 1 + P N n   2 1 2 = tp 1 + (tmin − tp ) n tmin 12 12   tp 1 tp 21 + 2− = 12 tmin a tmin a2 For a → ∞ we get PN

2 = 1 12



(19)



tp tmin

.

(20)

We see that the noise PN can be reduced by increasing the time tmin . In Table IV an overview of the parameters of the MTDEMI system is shown. The system fullfils the CISPR 16-1 Band SN Rmin SN R8bit tp tmin SN RM a → ∞ SN RM a = amin

A 36 dB 0 dB 2000 ns 10 ms 37 dB 37 dB

B 36 dB 0 dB 10 ns 200 µs 43 dB 43 dB

C,D 36 dB -4.5 dB 0.3 ns 20 µs 44 dB 44 dB

TABLE IV S IGNAL - TO - NOISE R ATIO OF A MRTDEMI M EASUREMENT S YSTEM DURING THE NOTCH FILTER TEST

TABLE III OVERLOAD FACTOR AND S IGNAL - TO -N OISE R ATIO OF A MRTDEMI M EASUREMENT S YSTEM

system fulfills the requirements to the overload factor given by the CISPR 16-1 completely. 2) Spurious free dynamic range: The measurement procedures to determine the spurious free dynamic range for transient signals have already been discussed in section VIB.2 for a measurement system with one ADC. In this case we consider the digitization of a single pulse. According to CISPR 16-1 the SNR has to be at least 36 dB. The minimum time tmin of the signal to obtain via FFT a spectrum with the given binwidth bw is described in (16). 1 tmin = (16) bw The quantization noise power of each ADC is given by (17). 2i (17) 12 We consider for a MRTDEMI System the factor a introduced in (15) and obtain the relation between the quantization steps according to (18) 1 (18) n = a PN i =

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We suppose that during the pulse digitization the quantization noise of the first ADC is added while during the rest of the signal the quantization noise of the ADC with highest sensitivity is added. We obtain (19).

392

in Band B,C and D in the quasi-peak, average and rms detector mode, when 8-Bit ADCs are used. For Band A the system fulfills the requirement in average and rms detector mode. In order to achieve additionally 3 dB more dynamic range ADCs with 10 bits can be used. Another possibility to obtain more dynamic range is to perform oversampling. VII. M EASUREMENT R ESULTS In order to investigate the SNR and the SFDR, a pulse generator with a pulse width of 500 ps was connected via a notch filter with an attenuation of more than 43 dB at 478 MHz to the MRTDEMI measurement system. The notch filter consists of one resonator. A measurement was performed with a pulse voltage of 10 V. The scanning time of the EMI Receiver was 10 ms. The MRTDEMI system performed a single shot measurement. Analog-to-digital conversion was performed with an oscilloscope that has three channels and 5 GS/s. The ENOB is below 6 bit. The result is shown in Fig. 7. A spurious free dynamic range of 40 dB has been achieved by the MRTDEMI Measurement System. The EMI Receiver shows a spurious free dynamic range of 48 dB. The maximum difference between the measurement performed with EMI Receiver and the measurement performed with the MRTDEMI system is 1 dB. The measurement by the the EMI Receiver was performed immediately after the self calibration. The EMI Receiver shows deviations at 550 MHz where a switching between two different amplifiers is performed. The

Magnitude / dBµV

60

peak detector mode. In the average detector mode all spurious signals are below 3.5 dBµV

50

VIII. C ONCLUSION The conformity of the MRTDEMI measurement system according to the CISPR 16-1 has been investigated. It has been shown that the IF-Filters are modelled in conformity to CISPR 16-1. The detectors fulfill the described behavior. The MRTDEMI measurement system does not provide an IFSignal and an output signal of the quasi-peak detector, but a statistical equivalent IF-Signal can be generated. A conventional disturbance analyzer cannot be used with that system. The MRTDEMI systems fulfills all requirements according to the dynamic range up to 800 MHz.

40

30

20

10 TDEMI Single Shot ESCS30 Peak 0

100

200

300

Fig. 7.

400 500 600 700 Frequency / MHz

800

IX. O UTLOOK

900 1000

Measurement of 0.5 ns Pulse

EMI Receiver shows further deviations at frequencies where the internal attenuation level is changed, e.g. 747 MHz. The 15 TDEMI Peak TDEMI Average ESCS30 Peak ESCS30 Average

Magnitude / dBµV

10

It has been demonstrated that the MRTDEMI measurement system already fulfills the CISPR 16-1 in almost all requirements. The dynamic range of the MRTDEMI measurement system can still be increased by an adjustment of the group delay of the various channels. Today disturbance analysis is done with conventional external disturbance analyzers. By disturbance analyzers the statistical behavior of the amplitude variation and pulse repetition frequency is investigated. A MRTDEMI measurement system already provides a statistical description of the signal. This description could be used for disturbance analysis. It could also be used to describe the disturbance to digital modulated signals and complex systems. Thus in the future simulations on the immunity of systems to a statistical described EMI signal will be possible.

5

ACKNOWLEDGMENT

0

The authors would like to thank Andreas Alt for the design and fabrication of the hybrid integrated high-frequency circuit. R EFERENCES

−5

−10 100

200

300

400 500 600 700 Frequency / MHz

800

900 1000

Fig. 8. Noisfloor of EMI Receiver and MRTDEMI System, Peak and Average Detector

noise floor of the MRTDEMI measurement system and a conventional EMI Receiver have been investigated. The result is shown in Fig. 8. The noise floor of the system in the average detector mode is below -7 dBµV. With the known equivalent noise bandwidth of 90.3 kHz of the 120 kHz IF-filter we obtain an average spectral noise density of -164 dBm/Hz. It is shown that the MRTDEMI measurement system shows a lower noise floor than the EMI-Receiver. The MRTDEMI system shows one spurious signal line at 700 MHz with a level of 7 dBµV which is the clock frequency of the internal PC of the used oscilloscope. Other spurious signals are below 4 dBµV in the

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[1] S. Braun, F. Krug, and P. Russer, “A novel automatic digital quasi-peak detector for a time domain measurement system,” in 2004 IEEE International Symposium On Electromagnetic Compatibility Digest, August 9–14, Santa Clara, USA, vol. 3, pp. 919–924, Aug. 2004. [2] CISPR16-1, Specification for radio disturbance and immunity measuring apparatus and methods Part 1: Radio disturbance and immunity measuring apparatus. International Electrotechnical Commission, 1999. [3] Rohde&Schwarz, Ultra Broadband Antenna HL562 ULTRALOG. Data sheet, 2001. [4] F. Krug, T. Hermann, and P. Russer, “Signal Processing Strategies with the TDEMI Measurement System,” in 2003 IEEE Instrumentation and Measurement Technology Conference Proceeding, May 20–22, Vail, USA, pp. 832–837, 2003. [5] A. V. Oppenheim and R. W. Schafer, Discrete–Time Signal Processing. ISBN 0-13-214107-8, Prentice-Hall, 1999. [6] F. Krug, D. Mueller, and P. Russer, “Statistical Physical Noise Behavior Analysis of the Time Domain EMI Measurement System,” in 2003 IEEE AP-S International Symposium on Antennas and Propagation, 2227.06.2003, Columbus, USA, vol. 3, pp. 212–215, 2003. [7] R. L. Belding, “Receiver Measurements near the Noise Floor,” in 1986 IEEE International Symposium On Electromagnetic Compatibility Digest, September 16–18, San Diego, USA, pp. 24–31, 1986.