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Traditional spectrum
analyzers are scalar measurement devices, where the primary display is
a scalar value (signal amplitude) plotted versus frequency. The most
common measurements are signal power and frequency of spectral
components such as harmonics, intermodulation and spurious. The power
versus frequency values can be integrated or corrected for resolution
bandwidth (RBW) to yield noise power (and phase noise), band power,
adjacent channel power, and occupied bandwidth. Spectrum analyzers have
also been frequently adapted to measure time domain or modulation
domain characteristics.
While traditional spectrum analyzers have many uses and benefits,
today’s communications systems and MIL-STD applications require modern
spectrum analyzers with their higher accuracy, performance and speed,
as well as increased functionality. One area where modern spectrum
analyzers are especially helpful is in electromagnetic interference
(EMI) testing – a critical part of any product lifecycle. EMI
measurements (e.g., pre-compliance and full-compliance testing) are
made after a product’s completion to evaluate whether or not its EM
emissions are within specified limits.
In pre-compliance testing, the main goal is to determine whether or not
a product or device will pass full-compliance testing at an accredited
test facility, and without the need for additional problem solving
which could translate into added expense. In this case, the closer the
engineer’s pre-compliance measurements are to full-compliance
measurements, the greater the confidence that the product in question
will pass final evaluation.
This is exactly where the modern spectrum analyzer comes in. Its
performance and functionality make pre-compliance and full-compliance
measurements easier, faster and as accurate for the novice as for the
expert engineer. Of course, fully realizing these benefits, demands
that the engineer first selects the right spectrum analyzer for his or
her specific needs.
Understanding EMI Measurements
Before choosing a spectrum analyzer, it is helpful to first gain a
clearer understanding of how the instrument will be utilized for EMI
testing.
First and foremost, achieving the goal of a close match between
pre-compliance and full-compliance measurements requires a careful
evaluation of both conducted and radiated emissions. Here, the spectrum
analyzer serves as an excellent pre-compliance and diagnostic tool,
helping the engineer to quickly and accurately determine the emissions
profile of the device under test (DUT) and whether or not it will pass
full compliance testing with a high level of confidence.
With the proper capabilities, it can then also qualify as a fully compliant EMI measurement receiver.
Conducted Emissions
Commercial conducted emissions tests are performed over the 9 kHz to 30
MHz frequency range on power and data lines. Power line testing offers
some unique challenges, depending on the type of power supply used in
the DUT. The traditional power supply, for example, which uses a
transformer and rectifiers along with filtering, has a tendency to
conduct harmonics of 60 Hz onto the power lines (see Figure 1).
Figure 1: Conducted emissions
This type of emission concentrates most of the energy in the area below
1 MHz. It is interesting to note that not all conducted emissions are
produced by the DUT power supply. Energy from other areas of the DUT
can be coupled to the power lines though the power supply as well.
Placing some temporary shielding around the power supply elements can
help the engineer understand if there is any coupling into the power
supply.
For a closer look at the conducted emissions of this type of power
supply, the engineer can reduce the stop frequency so that the
emissions are spread out over the visible span as shown in Figure 2.
The signals can be stabilized by switching the trigger to “line” mode.
Figure 2: Reduced stop frequency display
Another class of power supplies is the switcher power supply. Its
frequency is usually above 20 kHz to avoid any annoying hum. Because
the switcher’s emissions tend to be broadband in nature and therefore
require different measurement techniques, they pose an additional
measurement challenge for the engineer. Figure 3 provides an example of
a switcher broadband signal.
Figure 3: Broadband emissions
The goal is to make sure that the conducted emissions are below the
appropriate limit lines. Most conducted emissions have two limits,
quasi-peak and average. Today’s spectrum analyzers display the limit
line, plus any margin the operator may wish to set as shown in Figure
3. The display must be corrected for any loss that the transducer
(e.g., the line impedance stabilization network or current probe) may
have.
Since the quasi-peak detector is a very slow responding detector,
measurements are performed using peak detection first. The engineer
then zooms in on signals above the quasi peak (QP) limit and performs a
measure-at-marker measurement as shown in Figure 4.
Figure 4: Zone sweep and measure-at-marker
The measure-at-marker shows the peak, quasi-peak and average value. To
measure broadband signals, the engineer must reduce the span down so
that the signal covers most of the display and switch on the QP
detector. As an alternative method, the engineer can go to zero span
and set the step frequency size to the bandwidth specified by the EMI
band.
In this case, the bandwidth is 9 kHz for band B. The engineer simply
switches on the QP detector, selects center frequency and uses the
up/down key to step through the broadband signal to compare the signal
to the limit line or margin.
Radiated Emissions
The goal of pre-compliance radiated measurements is to emulate, as
close as possible, full-compliance measurements. This means that the
DUT must be placed on a table at the specified height. The antenna,
whether biconical, log periodic or broadband, must be placed at the
prescribed distance -usually three meters. Then, if possible, the DUT
must be operated in worst case mode.
The technique for measuring radiated emissions is similar to conducted
measurements with one major exception. If the ambient environment in
which the radiated emissions measurements are being made is minimal
(e.g., well below the regulatory limit), then the engineer must make
emissions measurements without recording the existing ambient
environment. Figure 5 provides an example of the local ambient
environment.
Figure 5: Ambient environment
If the ambient environment is such that signals are above the limit,
then the engineer will need to make a copy of the screen over the span
of the antenna with the DUT turned off. This will provide something to
compare against the emission with the DUT turned on. If necessary, the
engineer can then switch the DUT off and on to be sure of a signal.
Figure 6 shows a typical measurement with the DUT turned on. Note that,
once the clock frequencies generated within the DUT are known, the
engineer has a good place to start looking for emissions.
Figure 6: Ambient and DUT signals
All sides of the DUT must be measured. It is recommended that the DUT
be rotated 45 degrees for each measurement, over 360 degrees, copying
the screen for each position. Different positions may have different
frequency and amplitude radiation emissions; therefore the engineer
will need to record the frequency and amplitude of suspect signals for
each position tested.
After the initial scan is completed, the engineer simply returns to
each of the suspect signals and performs a measure-at-marker. If, after
the measure-at-marker is performed, the signal is still above the QP
limit or established margin, then signal must be reduced via a
redesign.
Isolating Problem Emissions
The next step in pre-compliance measurements is to isolate the signals
marked earlier. To do this, the engineer can use close field probes
attached to an amplifier and a spectrum analyzer.
One technique is to set the spectrum analyzer to the signal frequency
of interest and move the probe slowly over the surface of the DUT until
the point of emission is found. Once the signal has been identified the
engineer can attempt to find the source within the DUT.
Solving the problem at the source is less expensive than adding
shielding to each unit sold. Some places to look for the source include
ground paths, shielded cable grounds and unshielded pigtails. Cabinet
vent holes can also allow emissions if not designed correctly -- that
is short narrow holes versus one long narrow vent. Short holes do a
much better job of shielding.
Selection Criteria
With a better understanding of how the spectrum analyzer is used for
EMI testing, it’s now time to look more closely at the measurement
instrument itself. But what features or characteristics must the
spectrum analyzer have in order to ensure successful EMI testing? The
answer to that question depends in part on whether the measurement
instrument will be used for pre-compliance testing, full-compliance
testing or both.
To begin with though, it’s important to note that the architecture of
today’s modern spectrum analyzers differ greatly from yesterday’s
traditional spectrum analyzers (see Figure 7). Today’s spectrum
analyzers use high-speed digitizers and digital signal processing (DSP)
technology to gain speed, dynamic range and accuracy over legacy
equipment. The DSP is used to set resolution bandwidths, video
bandwidths and detectors. The engineer can choose from a wide range of
resolution bandwidths including CISPR (a standard which places
stringent requirements on receiver dynamic range), MIL-STD and 10
percent step bandwidths. The DSP implemented detectors include peak,
negative peak, sample, average and quasi peak. Peak and negative peak
detectors are used to identify impulsive signals.
Figure 7: Modern spectrum analyzer block diagram
Pre-Compliance Measurements
Over all, a modern spectrum analyzer with the correct feature set of
detectors, correction factors, and troubleshooting tools is an
excellent EMI pre-compliance and diagnostic tool. It allows the
engineer to quickly determine with excellent accuracy the emissions
profile of the DUT, and whether or not it will pass full-compliance
testing with a high level of confidence.
As mentioned previously, the closer the spectrum analyzer is to a
full-compliance receiver in performance, the more confident the
engineer will be that the product in question will pass full-compliance
testing at a test facility following completion of pre-compliance
testing. The large variable here in testing is the site. A
fully-compliant test facility tests the performance of the site (e.g.,
site attenuation).
Some of the key characteristics that the engineer should look for when
selecting the ideal spectrum analyzer include the following:
Measure at Marker Capability: With the measure-at-marker capability,
the engineer simply places a marker on a signal of interest, anywhere
on the display, and presses the measure-at-marker function to measure
with up to three different detectors. The specified amplitude accuracy
holds true for any point -- on or off the display -- without the need
to adjust reference levels. With a spectrum analyzer’s
measure-at-marker capability, the less experienced user can perform
accurate measurements as well as the veteran user
Accuracy: Frequency and span accuracy is very important in making EMI
measurements as it helps ensure the engineer can solve design
challenges with fewer iterations. It is important, therefore, for the
engineer to return to the same signal that was marked earlier for
further evaluation. Identifying the correct problem signal in a crowded
signal environment can be very challenging if the spectrum analyzer
being utilized has less frequency accuracy.
Speed: Measurement speed, like accuracy and performance, is critical
for communications and general purpose applications, as it accelerates
design verification and enables dramatic improvements in throughput and
manufacturing yields. Typically, modern spectrum analyzers feature
faster speeds, a much broader usable span and greater amplitude
accuracy than traditional spectrum analyzers. With over 8000 data
points in a single sweep, the user can measure signals using 120-kHz
resolution bandwidth; thereby capturing all signals within the 30 to
1000 MHz span.
Wide Breadth of Functionality: The ideal modern spectrum analyzer
should be able to correct for transducer factors, cable loss and
amplifier gains,. It should also have the ability to dynamically
display limit lines along with margins. For manual troubleshooting and
quick visual testing on screen, the spectrum analyzer should feature
margin and limit line fail indications.
Full-Compliance Measurements
If the engineer intends to use the spectrum analyzer for
full-compliance testing, as well as pre-compliance testing, then
additional functionality must be added to the selection criteria
specified above. Selecting a spectrum analyzer that will qualify as a
fully compliant EMI measurement receiver requires that the measurement
instrument meet two important criteria.
First, it must respond correctly to the CISPR pulse test using the
quasi-peak detector. The CISPR pulse generator produces DC pulses from
1000 Hz to 1 Hz and an isolated pulse. The pulse widths range from 270
ns to 0.3 ns (see Table 1).
| CISPR Bands | Freq Range (MHz) | Impulse area uVs | Reference PRI In Hz | Freq (MHz) | | A | .009 - .15 | 13.5 | 25 | .15 | | B | .15 – 30 | .316 | 100 | 30 | | C | 30 – 300 | .044 | 100 | 300 | | D | 300 – 1000 | .044
| 100 | 1000 |
Table 1: Pulse table
The results of performing the CISPR pulse test shows that the modern
spectrum analyzer does very well versus the CISPR requirements. There
are only a few exceptions and they are in the low repetition rate
range. Table 2 compares the CISPR requirements to the performance of
the modern spectrum analyzer. The areas of non-compliance with the
CISPR requirement is in band A (isolated pulse only), B, C and D when 2
Hz, 1 Hz or an isolated pulse is applied. In band C/D, the 10 Hz pulse
is not within specification.
Table 2: PSA response to CISPR DC pulse
Second, it must have an RF preselector. In order to be fully compliant
and therefore meet the CISPR 16-1-1 pulse test requirements, the
spectrum analyzer must have an RF preselector which can be used to
reduce the overloading from large pulses and improve the measurement’s
dynamic range. Some dedicated EMI receivers will feature a built-in
preselector. Others offer a separate RF preselector which, when used
with a specific spectrum analyzer, can turn the measurement instrument
into a fully compliant measurement system.
Having a separate RF preselector gives the engineer the flexibility to
operate the spectrum analyzer as a pre-compliance analyzer, utilizing
optional built-in diagnostic tools such as noise figure, phase noise or
EMI measurements, or to switch to using the preselector in the filter
or bypass mode.
Conclusion
The spectrum analyzer is a critical component for pre-compliance and
full-compliance EMI testing. In contrast to the traditional spectrum
analyzer, modern variations of the measurement instrument typically
feature excellent amplitude accuracy on or off display, span and
frequency accuracy, correction factors, and limit lines with margins
and pass/fail indicators. With over 8000 points across the display, the
user has the ability to look at wide spans without missing signals, even in CISPR 16-1-1 specified bandwidths.
Selecting a modern spectrum analyzer with these features, as well as
the other capabilities highlighted above, will help ensure successful
EMI testing -- whether pre-compliance and full-compliance. Couple that
with DSP capability over a wide range of bandwidths and detectors, and
the engineer has a spectrum analyzer that is suitable for use across a
wide range of commercial and MIL-STD applications. n
Dennis Handlon is a product manager in Agilent’s Wireless Business unit, and can be reached at dennis_handlon@agilent.com.
© 2007 Conformity
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