2011 EMC Directory & Design Guide - Interference Technology

H o f f m a nn, Br aun, Frech, Rus s e r
filters
Transform (FFT) and weighted by digital
detectors like peak, quasi-peak, average or
rms. The calculated amplitude spectrum
is displayed.
Spectral Estimation
Discrete spectral estimation is performed
by the Discrete-Fourier-Transform (DFT).
A fast algorithm for the computation
of the DFT is the FFT. The FFT exploits
symmetry and repetition properties and
is defined as [7]
X[k] =
N−1
∑
n=0
x[n]e −j2πkn
N ,
where X[k] is the discrete amplitude spectrum of the discrete
time signal x[n].
The Short-Time-Fast-Fourier-Transform (STFFT) is
defined as an FFT over a limited time-interval. A Gaussian
window function w[n] is applied, corresponding to
the IF-filter of a conventional measurement receiver. By
application of the STFFT, a spectrogram is calculated. The
spectrogram is a FFT of a time-interval of the sampled timedomain
signal. It depends on the discrete time coordinate
of the window and the discrete frequency k. The STFFT is
calculated by [7]
X[τ, k] =
N−1
∑
n=0
x[n + τ]w[n]e −j2πkn
N .
(1)
(2)
Figure 4. Floating-point analog-to-digital converter.
sample is taken from the ADC that shows the maximum
not clipped value. By the multiresolution ADC-system, the
necessary dynamic range is achieved to fulfill the requirements
of CISPR 16-1-1. In comparison to the requirement
for sinusoidal signals, for the measurement of transient
signals, the input stage has to handle signals which require
additionally 50 dB higher dynamic range, regarding the
amplitude range. As an example the notch filter test, as
described in CISPR 16-1-1 Section 4.6 has been carried
out to verify the spurious-free dynamic range for pulses. A
notch filter with an attenuation of at least 40 dB at around
300 MHz was connected to the system input and a pulse
generator fed a pulse with a pulse width of 300 ps to the
Digital Down-Conversion
In order to process the signal continuously and to enable the
calculation of a real-time spectrogram, the frequency range
from DC to 1.1 GHz is subdivided into eight subbands with
a bandwidth of 162.5 MHz each. Every subband is digitally
down-converted to the baseband and the subbands are processed
sequentially [2]. The block diagram is shown in Figure
3. A polyphase decimation filter is used for the inphase and
quadrature channel to reduce the sampling frequency and to
fulfill the Nyquist criterion. The output sampling frequency
is 325 MHz, while the bandwidth is 162.5 MHz.
Multiresolution Time-Domain EMI Measurement
System
In Figure 4, the block diagram of the floating point ADC,
which is comprised of several ADCs, is shown [2]. The input
signal is distributed into three channels by an asymmetrical
power splitter. 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 5 V. The second channel is
used to digitize the intermediate amplitude range from
0 to 200 mV. The signal is recorded in all three channels
simultaneously.
A floating-point representation is calculated from the
data of all three ADCs. The values are take in a way that the
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