2011 EMC Directory & Design Guide - Interference Technology

filters
M e a s ur e m e n t s a b o v e 1 GHz in Time-Domain
µ
yielding a total frequency shift of
δf = 6 · 10 MHz = 60 MHz (9)
for the 6th harmonic.
CONCLUSION
The theory and application of time-domain EMI measurement
systems according to CISPR 16-1-1 have been presented.
Such measurement systems allow to reduce test time, and
to perform investigations of the emission of ISM equipment
above 1 GHz. As such emission measurement systems allow
for real-time measurements over large frequency bands, the
non-stationary behavior of the electromagnetic interference
can be measured and weighted.
Figure 8. Emission spectrum of a microwave oven.
system’s real-time capability allows for the examination of
the time-behavior of the magnetron’s fundamental. The
spectrogram is shown in Figure 9. The microwave oven
was set to a medium power level, where the magnetron is
periodically turning on and off. After a short broadband
switching pulse, the magnetron turns on at around 3 s in
time and turns off at around 9 s. The magnetron’s output
frequency changes by about 10 MHz over this time-period,
as the oscillator is freerunning.
The microwave oven’s magnetron exhibits a non-linear
transfer function and therefore, higher harmonics can be
seen in the radiated emission spectrum. Figure 10 shows a
spectrogram of the microwave oven’s emission at the 6th
harmonic, located at around 14.74 GHz. The maximum
electric field strength of the microwave oven’s radiated
emission is about 70 dBμV/m at this frequency. With its
ultra-low system noise floor and the corresponding high
sensitivity, the time-domain EMI measurement system is
able to measure this low-level emission in real-time.
The frequency shift of the free-running oscillator’s
6th harmonic equals around 60 MHz. The microwave
oven’s magnetron can be described as a non-linear system
[12]. Thus, the output signal y(t) contains harmonics
of the sinusoidal input signal which can be described as
y(t) = A n · cos(n2πft), n = 1, 2, ... .
(6)
The frequency shift of the magnetron’s fundamental resembles
a frequency modulation of the output signal y(t)
according to
y(t) = A · cos[2πt(f + δf)].
(7)
Thus, the frequency modulated magnetron’s harmonics can
be described as
y(t) = A n · cos[n2πt(f + δf)], n = 1, 2, ... ,
(8)
REFERENCES
• [1] CISPR 16-1-1, Ed. 3.1 Am. 1, "Specification for Radio Disturbance
and Immunity Measuring Apparatus and Methods Part 1-1: Radio
Disturbance and Immunity Measuring Apparatus – Measuring Apparatus,"
International Electrotechnical Commission, 2010.
• [2] S. Braun, T. Donauer, and P. Russer, "A Real-Time Time-Domain
EMI Measurement System for Full-Compliance Measurements According
to CISPR 16-1-1," IEEE Transactions on Electromagnetic
Compatibility, Vol. 50 No. 2 May 2008: 259 - 267. Print.
• [3] CISPR 11, Ed. 5.1, "Industrial, Scientific and Medical Equipment
–Radio-Frequency Disturbance Characteristics – Limits and Methods
of Measurement," International Electrotechnical Commission, 2010.
• [4] C. Hoffmann, S. Braun, and P. Russer, "A Broadband Time-Domain
EMI Measurement System for Measurements up to 18 GHz," EMC
Europe 2010, Wroclaw, Poland, pp. 34 - 37, 2010. Print.
• [5] C.R. Barhydt, "Radio Noise Meter and its Application," General
Electric Rev., Vol. 36 1933: 201-205. Print.
• [6] K. Hagenhaus, "Die Messung von Funkstörungen," Elektrotechnische
Zeitschrift, Vol. 63 1942: 182-187. Print.
• [7] J. G. Proakis, and D. G. Manolakis, "Digital Signal Processing, Third
Edition," Pearson Prentice Hall, 1996. Print.
• [8] G. D. Vendelin, A. M. Pavio, and U. L. Rohde, "Microwave Circuit
Design using Linear and Nonlinear Techniques, Second Edition," John
Wiley & Sons, 2005. Print.
• [9] S. Braun, C. Hoffmann, and P. Russer, "A Realtime Time-Domain
EMI Measurement System for Measurements above 1 GHz," IEEE
EMC Society Symposium on Electromagnetic Compatibility, Austin,
USA, 2009.
• [10] W. B. Davenport, and D. L. Root, "An Introduction to the Theory
of Random Signals and Noise," John Wiley & Sons, 1987. Print.
• [11] RF Spin, “Broadband Quad-Ridged Horn Antenna QRH20,” Data
Sheet.
• [12] W. Sansen, "Distortion in Elementary Transistor Circuits," IEEE
Transactions on Circuits and Systems II: Analog and Digital Signal
Processing, Vol. 46 No. 3 March 1999: 315 - 325. Print.
Christian Hoffm ann was born in Ulm/Donau , Germany.
He received the Dipl.-Ing. degree in Electrical Engineering from
the Technische Universität München (TUM), Munich, Germany,
in 2008. He is currently working towards the Dr.-Ing. degree at
the Institute for High-Frequency Engineering at TUM, Germany.
120 interference technology emc Directory & design guide 2011