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
Figure 1. Heterodyne EMI receiver.
Figure 3. Digital down-conversion.
Figure 2. Time-domain EMI measurement system.
HETERODYNE MEASUREMENT RECEIVERS
Since the beginning of the 20th century, measurement
receivers based on the heterodyne principle have been
predominantly used to characterize EMI. As an example,
companies like General Electric and Siemens have started
the development of such systems in the 1920s [5][6].
The block diagram of a heterodyne measurement receiver
is shown in Figure 1. The EMI input signal is bandpass
filtered by a variable preselection filter. Thus, the dynamic
range is increased by attenuating out-of-band narrowband
and transient broadband interference signals. The preselection
typically consists of several selectable bandpass filters
that are tunable within a certain frequency range. By means
of a variable attenuator, the level of the input signal to the
mixer is set in order not to overdrive the mixer and minimize
distortion of the IF-signal. Every considered frequency is
then consecutively down-converted to a fixed IF frequency.
The IF-signal is filtered by the chosen IF-filter. Thereby it is
assured, that only a specific band of the IF signal is reaching
the detector input. CISPR 16-1-1 dictates the use of
several IF-filters of different bandwidths that have to fulfill
the given critical masks. The output signal is evaluated by
a given set of detectors for the selected dwell-time. The average,
CISPR-Average, peak, quasi-peak and rms detectors
are used to measure EMI signals according to CISPR 16-1-1.
The amplitude spectrum is displayed.
Heterodyne EMI receivers offer high dynamic range
through a complex preselection, high sensitivity through
low-noise preamplifiers and are commonly available up
to millimeter wave frequencies. The major drawback of
this technology are the long scan times. The scan time can
easily reach hours or days, when wide measurement bands
shall be measured with high frequency resolution and high
sensitivity.
TIME-DOMAIN MEASUREMENT SYSTEM
The block diagram of the time-domain EMI measurement
system is shown in Figure 2. The EMI input signal in the
frequency range from 9 kHz-1.1 GHz is low-pass filtered to
ensure Shannon’s theorem is fulfilled. The filtered signal
is sampled by a floating-point analog-to-digital converter
(ADC) [2]. The spectrum is calculated by the Fast-Fourier-
116 interference technology emc Directory & design guide 2011