EMC’14/Tokyo
13P1-A4
Timing Considerations using FFT-based measuring
receivers for EMI compliance measurements
Jens Medler, Christian Reimer
Rohde & Schwarz GmbH & Co. KG, Test and Measurement Division,
Rohde & Schwarz International Operations GmbH, Regional Development Asia North,
Muehldorfstrasse 15, D-81671 Munich, Germany
jens.medler@rohde-schwarz.com, christian.reimer@rohde-schwarz.com
Abstract — The use of FFT-based measuring receivers for EMI
compliance measurements is motivated by reducing the scan time
by several orders of magnitude and to get more insight due to the
possibility of applying longer measurement times. Using an
appropriate measurement time is the key for a comprehensive
recording of the disturbance characteristic of the equipment under
test (EUT).
Key words: EMI receiver, FFT-based measuring receiver,
Spectrum Analyser, EMI compliance measurement, CISPR 16-1-1,
CISPR 16-2.
I. INTRODUCTION
With the publication of Amendment 1 to 3rd Edition of
CISPR 16-1-1 [1] in June 2010 FFT-based measuring
receivers were introduced for EMI compliance measurements.
The use of such receivers is motivated by reducing the scan
time by several orders of magnitude without degradation of
accuracy. To achieve this significant improvement FFT-based
receivers are measuring spectral segments much wider
than the resolution bandwidth during the measurement
time by parallel calculation at several frequencies (Fig. 1).
In contrast classic EMI receivers are measuring the signal
within the resolution bandwidth in a given measurement time,
which results in a long scan time for the entire frequency
range.
II. CONCEPT OF CISPR 16-2
The CISPR 16-2 basic standard series describes the
methods of measurements of disturbances and immunity. For
disturbance measurements the following parts are relevant.
• CISPR 16-2-1 for conducted disturbance measurements.
• CISPR 16-2-2 for measurements of disturbance power.
• CISPR 16-2-3 for radiated disturbance measurements.
Major requirements in all three parts are the definition of
the minimum measurement and scan times for continuous
disturbances. In general it shall be set to measure the
maximum emission and to ensure that disturbance signals are
not overlooked. To do so CISPR 16-2 requires the application
of minimum measurement times which is equivalent for
stepping and FFT-based EMI receivers as shown in Table I. In
addition the frequency step size shall be equal to half of the
resolution bandwidth (Band A: 200 Hz, Band B: 9 kHz, Band
C/D: 120 kHz) or less.
These requirements are also valid for spectrum analyzers,
which results in the minimum scan times as shown in Table II.
Note: The terms sweep and scan time are used in the same
right within CISPR 16-2. The sweep/scan time Ts is defined as
time between start and stop frequency of a sweep or scan.
Tables I and II apply for CW signals.
TABLE I
MINIMUM MEASUREMENT TIMES FOR CW SIGNALS ACC. TO CISPR 16-2-3 [2]
TABLE II
MINIMUM SCAN TIMES FOR CW SIGNALS IN THE THREE CISPR BANDS WITH
PEAK AND QUASI-PEAK DETECTORS ACC. TO CISPR 16-2-3 [2]
Fig. 1 FFT-based measurement versus classic stepped scan
Related specifications for the measurement methods to be
used with FFT-based measuring receivers were published in
the relevant parts of CISPR 16-2, see next clause. Focus point
in this article is the selection of an appropriate measurement
time using an FFT-based receiver for measuring broadband
disturbance and intermittent signals.
Copyright 2014 IEICE
Depending on the type of disturbance, the measurement
time Tm or the scan time Ts may have to be increased – even
for quasi-peak measurements. In extreme cases, the
measurement time Tm at a certain frequency may have to be
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increased to 15 s, if the level of the observed emission is not
steady.
The used measurement time for stepping and FFT-based
measuring receivers and the used sweep time for spectrum
analyzers is the key to ensure that disturbance signals are not
missed during automatic scans or sweeps over frequency
spans. Therefore, a timing analysis of the EUT disturbance
characteristic has to be performed before doing automated
measurements (Fig.2).
Next, select a frequency of interest and switch to Zero Span
mode to analyse the signal in the time domain. The example in
Fig. 4 shows an interval between pulses of 20 ms.
Fig. 4 Zero span measurement in spectrum analyzer mode shows a pulse
interval of 20 ms; EUT: touch dimmer of halogen lamp
For automated measurements two cases have to be
considered: single and multiple scans or sweeps.
Fig. 2 Analysis steps for automated or semi-automated measurements
To demonstrate such timing analysis, a dimmer of a
halogen lamp was measured using the Spectrum Analyser
Zero-span mode. As an alternative an oscilloscope may be
connected to the receiver’s IF output. First an overview
measurement with Peak detector was performed with
maximum hold to find critical frequencies for the timing
analysis. It is essential to use a sufficient number of sweep
points and a long sweep time >15 s. For CISPR Band B (150
kHz to 30 MHz) around 6000 sweep points are sufficient,
which corresponds to a step size of about half of the resolution
bandwidth. The thick trace in Fig. 3 is a good indication that
intermittent signals exists. As long as the spectrums display
changes, there may still be intermittent signals to discover.
A. Single scans or sweeps
For a single scan or sweep the measurement time at each
frequency must be larger than the intervals between pulses for
intermittent signals. If the measurement or sweep time is too
short erroneous measurement results will be obtained.
B. Multiple scans or sweeps
For doing multiple sweeps with maximum hold the
observation time at each frequency need to be sufficient for
intercepting intermittent signals, i.e. the observation time has
to be selected according to the pulse repetition interval of the
disturbance signals (see timing analysis above). If the sweep
time per sweep is chosen too fast the probability of intercepts
per sweep is very low and erroneous measurement results will
be obtained, see Fig. 5. Increasing the sweep time per sweep
gives a higher probability of intercept, see Fig. 6.
Fig. 3 Frequency sweep in spectrum analyzer mode; sweep time > 15 sec;
around 6,000 sweep points; EUT: touch dimmer of halogen lamp
Copyright 2014 IEICE
Fig. 5 Visualization of Interceptions – Low probability of intercept per sweep
using very high sweep rate
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where
Tm is the measurement time for each segment, and
Nseg is the number of seegments
FFT-based measuring receivvers have to meet the general
requirements on the minimum measurement
m
time and step size
as defined in CISPR 16-2, see
s
clause II. Therefore, the
measurement time Tm is to be selected longer than the pulse
repetition interval for a correctt measurement of a broadband
spectrum. If the measurement time
t
is to short it can result in
enormous measurement result errors.
e
In a worst case the FFTbased measuring receiver mayy not capture the disturbance
signal at all. This is in particularr fatal if the segment size has a
large width, e.g. 30 MHz or morre.
Fig. 6 Visualization of Interceptions – High probabbility of intercept per
sweep using lower sweep rate
In all cases the fastest possible sweep time per sweep is
the multiplicative inverse of the resollution bandwidth
based on a step size 0,5 x RBW, e.g. 194 ms for RBW=100
kHz over the frequency range 30 to 1000 MHz.
M
III. TIMING CONSIDERATIONS USING FFT-BASED
B
MEASURING
INSTRUMENTS
Today’s FFT-based measuring receivers are limited by the
currently available analog to digital convertters (ADC), e.g. for
a 1 GHz measuring receiver an ADC is neccessary with 2 GS/s
sampling rate to meet the Nyquist criterrion. Furthermore,
preselection filters and high resolution AD
DCs, e.g. 16-bit for
30 MHz FFT-bandwidth are necessary to meet
m the CISPR 161-1 overload requirements for quasi-peak detection.
d
Therefore,
todays FFT-based receivers cannot samplee and evaluate the
entire CISPR Bands C/D (30 to 1000 MH
Hz) and E (1 to 18
GHz) in one shot. Instead they com
mbine the parallel
calculation at N frequencies and a steppped scan. For this
purpose the frequency range of interest is subdivided into
several segments that are measured sequentiially, see Fig. 7.
IV. FAST SWEEP VERSUS FAST TIME-DOMAIN SCAN
For demonstration the same EUT was used: touch dimmer
of halogen lamp. Also the freequency span was reduced to
5 MHz for a better visualizatiion. First a single fast sweep
without sweep time adjustment (sweep time SWT = 20 ms)
was performed. It can be seen that the result does not match
with the spectrum envelope at all
a (Fig. 8).
Fig. 8 Spectrum Analyzer – Single sw
weep without sweep time adjustment
does not match with the spectrum
s
envelope at all
Next a single sweep with sweeep time adjustment based on
the timing analysis above was performed;
p
it results in a sweep
time of 20 s (20 ms pulse interrval x 1000 sweep points). The
result in Fig. 9 shows a good maatch with the envelope.
Fig. 7 FFT scan in sequence, Source: CISP
PR 16-2-3 [2]
The scan time Tscan is calculated as:
Tscan = Tm Nseg
Copyright 2014 IEICE
Fig. 9 Spectrum Analyzer – Single sweeep with sweep time adjustment based
on timing analysis of EUT: toouch dimmer of halogen lamp
(1)
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If the pulse repetition interval is unknown, quite a lot of
users performing multiple sweeps with fast sweep times using
maximum hold function to determine the spectrum envelope.
For low repetition impulsive signals, many sweeps will be
necessary to fill up the spectrum envelope of the broadband
component. The correct sweep time can also be determined by
increasing it until the difference between maximum hold and
clear/write displays is below e.g. 2 dB.
For demonstration a multiple sweep with sweep time
adjustment based on the timing analysis above was
performed; it results in a total observation time of 20 s for 20
sweeps (20 x SWT = 1 s x 20 ms pulse interval x 1000 sweep
points). But the result in Fig. 10 shows that critical
frequencies are still missed! For this EUT a SWT = 2 s for 20
sweeps gives a good match with the envelope.
TABLE III
MATCH WITH ENVELOPE VERSUS SCAN TIME
V. MORE SPEED WITH TIME-DOMAIN SCAN
FFT-based measuring receivers can deliver measurement
speed a few thousand times faster than can be achieved in
conventional, stepped frequency scan mode. As a
consequence frequency scans in the CISPR bands using the
peak detector can be performed in just a few milliseconds and
even with quasi-peak and average detector it takes just
seconds which makes preview measurements with peak
detector obsolete, see Fig. 12.
Fig. 12 Analysis steps for automated or semi-automated measurements with
FFT-based measuring receiver
Fig. 10 Spectrum Analyzer – Multiple sweep (20x) with sweep time
adjustment based on timing analysis of EUT
Finally a Time-Domain Scan with measurement time
adjustment based on the timing analysis above was
performed. An FFT-based measuring receiver with 30 MHz
segment size can measure the CISPR Band B (150 kHz to 30
MHz) in one shot. According to Equation 1 it results in a pure
scan time of 20 ms, in reality a high performance receiver can
present the result in about 100 ms. The achieved curve is a full
match with the envelope, see Fig. 11. An overall comparison
of the different approaches shows Table 3.
The ultra-fast measurement speed is particular useful if the
equipment under test can be operated only during a short
period of time, e.g. a starter motor in cars. A part of the time
saving can also be used for applying longer measurement
times in order to reliably detect narrowband intermittent
signals or isolated pulses.
VI. CONCLUSIONS
FFT-based measuring receivers can be used for EMI
compliance measurements in accordance with Amendment 1
to 3rd Edition of CISPR 16-1-1. The use of FFT-based
measuring receivers is motivated by reducing the scan time by
several orders of magnitude without losing accuracy and to
get more insight due to the possibility for applying longer
measurement times. As another benefit, it can measure the
disturbance spectrum with the final detector directly, which
makes the measurement of fluctuating disturbances more
reliable. For precise and reproducible measurements the use of
an appropriate measurement or sweep time based on a timing
analysis of the EUTs disturbance characteristic is essential.
REFERENCES
[1]
[2]
Fig. 11 FFT-based Receiver – Time-Domain measurement with measurement
time adjustment based on timing analysis of EUT
Copyright 2014 IEICE
96
Amendment 1:2010-06 to CISPR 16-1-1:2010-01 (Edition 3)
Specification for radio disturbance and immunity measuring apparatus
and methods – Part 1-1: Radio disturbance and immunity measuring
apparatus – Measuring apparatus
Amendment 1:2010-06 to CISPR 16-2-3:2010-04 (Edition 3)
Specification for radio disturbance and immunity measuring apparatus
and methods – Part 2-3: Methods of measurement of disturbances and
immunity – Radiated disturbance measurements