TESTING & MEASUREMENT
Timing Considerations Using FFT-based
Measuring Receivers for EMI Compliance
Measurements
Jens Medler Rohde & Schwarz GmbH & Co. KG
Abstract
The use of FFT-based measuring receivers for EMI compliance measurements is motivated by the desire to reduce the scan time by
several orders of magnitude and to gain additional insights by applying longer measurement times.Usage of an appropriate measurement
time is the key to comprehensively record the disturbance characteristic of the equipment under test (EUT).
Keywords
EMI receiver; FFT-based measuring receiver; spectrum analyzer; EMI compliance measurement; CISPR 16-1-1; CISPR 16-2.
Introduction
signals.
This paper is the author's continuation of the topic "Use of FFT-
W
ith the publication of Amendment 1 to the 3
rd
Edition of CISPR 16-1-1 [1] in June 2010, FFT-
based EMI test receivers for EMI compliance measurements" which
was published in SAFETY & EMC in 2013[3].
based measuring receivers were introduced for EMI
compliance measurements. Usage of such receivers is motivated by
the desire to reduce the scan time by several orders of magnitude
Concept of CISPR 16-2
without entailing any degradation of the accuracy. To achieve this
The CISPR 16-2 basic standard series describes methods
significant improvement, FFT based receiversmeasure spectral
for measuring disturbance and immunity. For disturbance
segments that are much wider than the resolution bandwidth
measurements, the following parts are relevant:
during the measurement time by performing parallel calculations at
■
CISPR 16-2-1 for conducted disturbance measurements
multiple frequencies (Figure 1). In contrast, classic EMI receivers
■
CISPR 16-2-2 for measurements of disturbance power
measure the signal within the resolution bandwidth in a given
■
CISPR 16-2-3 for radiated disturbance measurements
measurement time, thereby requiring a longer scan time for the
In all three parts, definition of the minimum measurement
entire frequency range.
and scan times for continuous disturbances represents a major
requirement. In general, it is important to choose a setting that
allows measurement of the maximum emission while ensuring that
disturbance signals are not overlooked. To achieve this objective,
CISPR 16-2 requires usage of minimum measurement times which
are equivalent for stepping and FFT-based EMI receivers (see
Table 1). In addition, the frequency step size must be equal to half
of the resolution bandwidth (band A: 200 Hz, band B: 9 kHz, band
Figure 1 FFT-based measurement versus classic stepped scan
C/D: 120 kHz) or less.
These requirements are also applicable to spectrum analyzers,
Related specifications for the measurement methods to be used
resulting in the minimum scan times shown in Table2. Note:
with FFT-based measuring receivers were published in the relevant
The terms sweep and scan time are used interchangeably within
parts of CISPR 16-2 (see section II) . This article focuses on the
CISPR 16-2. The sweep/scan time Ts is defined as the time between
selection of an appropriate measurement time using an FFT-based
the start and stop frequency of a sweep or scan. Tables1 and Tables 2
receiver for measuring broadband disturbance and intermittent
apply to CW signals.
[2]
22 SAFETY & EMC 2015 TESTING & MEASUREMENT
Table 1 Minimum measurement times for CW signals
acc. to CISPR 16-2-3[2]
approach for this purpose. It is essential to use a sufficient number
Minimum measurement time Tm
Frequency band
A
9~150 kHz
1 0 ms
B
0.15~30 MHz
0.5 ms
C and D
30~1 000 MHz
0.06 ms
E
1~18 GHz
0.01 ms
of sweep points and a long sweep time (> 15 s). For CISPR band B
(150 kHz to 30 MHz), around 6 000 sweep points are sufficient,
which corresponds to a step size of about one half of the resolution
bandwidth. The thick curve in Figure 3 is a good indication that
intermittent signals exist. As long as the spectrum display continues
changing, there may be more intermittent signals to discover.
Table 2 Minimum scan times for CW signals in the three CISPR bands with peak
and quasi-peak detectors acc. to CISPR 16-2-3[2]
Frequency band
A
Scan time Ts for
9~150 kHz
Scan time Ts for quasi-peak
peak detection
detection
10 ms
2 820 s=47 min
B
0.15~30 MHz
0.5 ms
5 970 s=99.5 min=1 h 39 min
C and D
30~1 000 MHz
0.06 ms
19 400 s=323.3 min=5 h 23 min
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 increased to 15 s if the level of the
observed emission is not steady.
Figure 3 Frequency sweep in spectrum analyzer mode; sweep
time > 15 around 6 000 sweep points; EUT: touch dimmer of
halogen lamp
The measurement time used in stepping and FFT-based
Next, select a frequency of interest and switch to Zero Span
measuring receivers and the sweep time used in spectrum analyzers
mode to analyze the signal in the time domain. The example in
are the key to ensure that disturbance signals are not missed during
Figure 4 shows an interval between pulses of 20 ms.
automatic scans or sweeps over frequency spans. Therefore, a timing
analysis of the EUT disturbance characteristic has to be performed
before executing automated measurements (Figuer 2).
Figure 2 Analysis steps for automated or semi-automated
measurements
Figure 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 must be considered:
To demonstrate such timing analysis, a dimmer of a halogen
lamp was measured using the spectrum analyzer's Zero Span
single and multiple scans or sweeps.
Single scans or sweeps
mode. As an alternative, an oscilloscope may be connected to the
For a single scan or sweep, the measurement time at each
receiver's IF output. First, an overview measurement was performed
frequency must be larger than the intervals between pulses for
with the peak detector to find critical frequencies for the timing
intermittent signals. If the measurement or sweep time is too short,
analysis. Performing multiple sweeps with maximum hold is a good
erroneous measurement results will be obtained.
SAFETY & EMC 2015 23
TESTING & MEASUREMENT
Multiple scans or sweeps
For performing multiple sweeps with maximum hold, the
observation time at each frequency needs to be sufficient to
steady.
The following equation can be used to calculate the minimum
sweep time for multiple sweeps:
intercept intermittent signals, i.e. the observation time has to be
Ts min=2×(∆f/Bres2)
selected according to the pulse repetition interval of the disturbance
where
signals (see timing analysis above). If the sweep timeper sweep
Ts min is the min. sweep time for multiple sweeps
is too fast, the probabilityof intercept per sweep is very low and
∆f is the frequency span
erroneous measurement results will be obtained (see Figure 5).
Bres is the resolution bandwidth
(1)
Increasing the sweep time per sweep gives a higher probability of
intercept (see Figure 6).
Timing Considerations using FFTbased Measuring Instruments
Today's FFT-based measuring receivers are limited by the
currently available analog-to-digital converters(ADC), e.g. for a
1 GHz measuring receiver an ADC is necessary with 2 GS/s sampling
rate to meet the Nyquist criterion. Furthermore, preselection filters and
high resolution ADCs (e.g. 16 bits for 30 MHz FFT bandwidth) are
necessary to meet the CISPR 16-1-1 overload requirements for
quasi-peak detection. Therefore, today's FFT-based receivers cannot
Figure 5 Visualization of intercepts – low probability of intercept per
sweep using very high sweep rate
sample and evaluate the complete CISPR bands C/D (30 MHz to
1 000 MHz) and E (1 GHz to 18 GHz) in one shot. Instead, they
combine parallel calculations at N frequencies with a stepped scan.
For this purpose, the frequency range of interest is subdivided into
several segments that are measured sequentially (see Figure 7).
Figure 6 Visualization of intercepts – high probability of intercept per
sweep using lower sweep rate.
In all cases, the fastest possible sweep time per sweep is the
multiplicative inverse of the resolution bandwidth based on a
step size of 0.5×RBW, e.g. 194 ms for RBW=100 kHz over the
frequency range 30 MHz to 1 000 MHz.This fast sweep must be
applied continuously with maximum hold for a period of time
equal to or greater than the time that would have been spent for
Figure 7 FFT scan in sequence;source: CISPR 16-2-3[2]
sweeping in line with Table 2, e.g. 0.97 s for peak detection of CW
signals corresponding to 5 sweeps of 194 ms each for the frequency
The scan time Tscan is calculated as:
range 30 MHz to 1 000 MHz. However, depending on the type of
Tscan= TmNseg
disturbance, the sweep time Ts may have to be increased, e.g. to 15
where
scorresponding to 77 sweeps of 194 ms each for the frequency range
Tscan Tm is the measurement time for each segment, and
30 MHz to 1 000 MHz if the level of the observed emission is not
Nseg is the number of segments.
24 SAFETY & EMC 2015 (2)
TESTING & MEASUREMENT
FFT-based measuring receivers have to meet the general
requirements for the minimum measurement time and step size as
defined in CISPR 16-2 (see section II). Therefore, the measurement
time Tm must be selected longer than the pulse repetition interval for
correct measurement of a broadband spectrum. If the measurement
time is too short, enormous measurement errors can result. In a
worst case scenario, the FFT-based measuring receiver may not
capture the disturbance signal at all. This is problematic if the
segment size has a large width, e.g. 30 MHz or more.
Fast Sweep versus Fast Time Domain
Scan
For demonstration purposes, the same EUT was used: a touch
dimmer of a halogen lamp. The frequency span was also reduced to
5 MHz for better visualization. First, a single fast sweep without sweep
time adjustment(sweep time SWT = 20 ms) was performed. It is clear that
the result does not match the spectrum envelope at all (Figure 8).
Figure 8 Spectrum analyzer – A single sweep without sweep time
adjustment does not matchthe spectrum envelope at all
Figure 9 Spectrum analyzer – A single sweep with sweep time
adjustment based on timing analysis of the EUT (a touch dimmer of a
halogen lamp).
1 s×20 ms pulse interval×1 000 sweep points). However, the
result in Figure 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.
Figure 10 Spectrum analyzer – multiple sweep (20x) with sweep time
adjustment based on timing analysis of the EUT.
Next, a single sweep with sweep time adjustment based on the
Finally, a timedomain scan with measurement time adjustment
above timing analysis was performed; this results in a sweep time
based on the above timing analysis was performed. An FFT based
of 20 s (20 ms pulse interval×1 000 sweep points). The result in
measuring receiver with 30 MHz segment size can measure CISPR
Figure 9 exhibits a good match with the envelope.
band B (150 kHz to 30 MHz) in one shot. According to Equation (1),
If the pulse repetition interval is unknown, many users perform
this results in a pure scan time of 20 ms; in reality, however, a high-
multiple sweeps with fast sweep times using the maximum hold
performance receiver can generate the result in about 100 ms. The
function to determine the spectrum envelope. For low repetition
achieved curve is a full match with the envelope (see Figure 11). An
impulsive signals, many sweeps are necessary to fill up the spectrum
overall comparison of the different approaches is shown in Table 3.
envelope of the broadband component. The correct sweep time can
also be determined by increasing it until the difference between
More Speed with the Time Domain Scan
maximum hold and clear/write displays is below 2 dB, for example.
For demonstration purposes, a multiple sweep with sweep time
FFT-based measuring receivers can attain measurement speeds
adjustment based on the above timing analysis was performed; this
a few thousand times faster than can be achieved in conventional,
results in a total observation time of 20 s for 20 sweeps (20×SWT =
stepped frequency scan mode. As a consequence, frequency scans
SAFETY & EMC 2015 25
TESTING & MEASUREMENT
Figure 13 Measurement speed of timedomain scan versus stepped
frequency scan with the R&S®ESR EMI test receiver
Figure 11 FFT-based receiver – time domain measurement with
measurement time adjustment based on timing analysis of the EUT
Conclusions
Table 3 Match with envelope versus scan time
Single sweep
Pulse
repetion
Measurement RBW=9 kHz
time to get
frequency interception
50 Hz
20 ms
Match with envelope
Span=5 MHz
Single sweep
RBW=9 kHz
Span=5 MHz
20x Multiple
20x Multiple
Time domain
single sweep
single sweep
scan
receivers can be used for EMI
compliance measurements in
RBW=9 kHz
RBW=9 kHz
RBW=9 kHz,
Span=5 MHz
Span=5 MHz
0.15~5 MHz
SWT=1 s
SWT=2 s
Step=2.25 kHz
SWP=1 000
SWP=1 000
MT=20 ms
SWT=20 ms
SWT=20 s
SWP=1 000
SWP=1 000
0.020 s
20 s
20 s
40 s
0.1 s
no
good
not all
good
good
FFT-based measuring
accordance with Amendment 1
to the 3rd Edition of CISPR 161-1. The use of FFT-based
measuring receiversis motivated
by the desire to reduce the
in the CISPR bands using the peak detector can be performed
scan time by several orders of magnitude without loss of accuracy
in only a few milliseconds, and even with the quasi-peak and
while gaining deeper insight by applying longer measurement
average detector it only takes seconds, thereby rendering preview
times.Another benefit is that the disturbance spectrum can be
measurements with the peak detector obsolete (see Figure 12).
directly measured with the final detector, making measurement of
fluctuating disturbances more reliable.For precise and reproducible
measurements, the use of an appropriate measurement or sweep time
based on timing analysis of the EUT's disturbance characteristic is
essential.
Figure 12 Analysis steps for automated or semi-automated
measurements with an FFT-based measuring receiver
References
[1]
Amendment 1:2010-06 to CISPR 16-1-1:2010-01 (Edition 3)
Specification for radio disturbance andimmunity measuring apparatus
Rohde & Schwarz has introduced a new generation of FFT-
and methods - Part 1-1: Radio disturbance and immunity measuring
based EMI test receivers for disturbance measurements compliant
with CISPR 16: the R&S®ESR[4]. Its FFT-based time domain scan
apparatus - Measuring apparatus.
[2]
Amendment 1:2010-06 to CISPR 16-2-3:2010-04 (Edition 3)
can deliver measurement speeds up to 6 000 times faster than can
Specification for radio disturbance andimmunity measuring apparatus
be achieved with the traditional single-channel filtering approach
and methods - Part 2-3: Methods of measurement of disturbances
(see Figure 13).
and immunity - Radiated disturbance measurements.
The ultra-fast measurement speed is particularly useful if the
[3]
MEDLER, J., More speed, more insight - Use of FFT-based EMI test
equipment under test can be operated only during a short period of
receivers for EMI compliance measurements, SAFETY & EMC 2013,
time, e.g. a starter motor in cars. Part of the time savings can also
38-42.
be exploited to apply longer measurement times in order to reliably
detect narrowband intermittent signals or isolated pulses.
26 SAFETY & EMC 2015 [4]
®
R&S ESR EMI Test Receiver, Product Brochure Version 02.00,
www. rohde-schwarz. com