Understanding
Impulse Bandwidth
of EM1 Receivers
Specifications
Werner Schaefer
Hewlett-Packard
Company
SantaRosa SystemsDivision
1400 Fountaingrove Parkway
SantaRosa,CA 95403-1799,USA
Abstract - When measuring emissionswith an EM1 receiver or
spectrum analyzer, different signal types may be encountered.
Their classification as narrowband or broadband is dependent
on the signal’s spectral distribution relative to the receiver’s
resolution bandwidth used for the measurement.It is essential
to know which type of signal is being measured to avoid
erroneoustest results and the wrong interpretation of data. In
this context, the knowledge of the instrument’s impulse
bandwidth is necessary to correctly determine the absolute
amplitude value of an emission. The determination of the
receiver’s impulse bandwidth can be done with a series of
measurements,but not simply by multiplication of its 3dB or
6dB bandwidth with a constant factor. Only the accurate
knowledge of this parameter allows a meaningful comparison
of the measuredemission levels to a limit line and verification
of a product’s compliance with an EM1 standard.
Introduction
When measuring impulsive signals, it is necessaryto know the
impulse bandwidth of the EMI receiver’s or spectrum
analyzer’s IF filter. Due to the broadband nature of impulsive
signals, someof their spectral componentsare not contained in
the IF filter passbandat any one time. As the filter bandwidth
becomeswider, more spectral componentsare included and the
amplitude of the signal at the filter output increasesuntil the
filter is wide enough to enclosethe entire signal spectrum.
The filters implemented in EMI receivers or spectrumanalyzers
are usually defined by their 3dB or 6dB bandwidths, which is
suitable for measuring CW or narrowband signals. However,
this specification is insufficient for broadband signal
measurements because the filter’s amplitude and phase
responsewill affect the results. A comparisonof test data,taken
with different EM1 receivers, is very difficult (if not impossible) to make.
EUT passes or fails a compliance test, depending on the
receiver being used. Amendment 2 to CISPR 16 Part 1
acknowledges this issue and defines the EM1 receiver’s
resolution bandwidth for measurementsabove 1GHz as the
1MHz impulse bandwidth.
In the following paper, definitions of broadbandsignals and the
impulse bandwidth of a filter will be presented,the effect of
filtering on a broadband signal discussed,and an easy method
to determinethe impulse bandwidth of a receiver described.
Defining Broadband Signals and Impulse Bandwidth
Broadband impulse signals are signals of short duration and a
frequency spectrum exceeding the resolution bandwidth of a
calibrated receiver. They will have a repetition tiequency
substantially less than the receiver bandwidth. They are
measured,in terms of spectral intensity, in units of [V/MHZ]
or, in logarithmic terms, in [dBpV/MHZ]. A broadband
signal’s spectrum always exceedsthe receiver’s IF bandwidth;
therefore, the complete signal spectrum cannot be contained in
the filter passbandat any one time. It is important to recognize
that the receiver IF bandwidth is the only criteria that classifies
a signal as narrowband or broadband.If its setting is changed,
narrowband emissions may be detected as broadband signals
and vice versa.
Impulsive signals causetransient responsesin a receiver. The
peak value of these responsesis proportional to the spectral
intensity of the impulsive signal and to the bandwidth of the
receiver. The exact value of this impulse bandwidth must be
known in order to measurebroadband impulse signals. Unlike
the 3dB or 6dB bandwidth, this value is not easily derived from
CW responsemeasurements.The transient behavior of the IF
filter dependson the exact shapeof the frequency response,the
design of the bandpass,any logarithmic amplifiers used, their
gain-shaping performanceand any video bandwidth used.
A study conducted in Japan[l] in 1996 shows that differences
in measurement results of up to 6.4dB are possible when Impulse bandwidth BWi is defined as the ratio of the peak
measuringbroadbandimpulsive signals,basedon a 3dB or 6dB value of the detectedtransient VP and the spectral intensity of
bandwidth specification. This could lead to situations where an the impulse signal S causing the transient:
0-7803-5057-X/99/$10.00 © 1999 IEEE
958
BWi=v’
(1)
S
Because spectral intensity is specified in RMS volts, V, must
be measured in EMS volts; the absolute voltage peak of the
transient must be divided by J2. This correction is not
necessaryif the receiver display is calibrated in RMS values.
Frequency
Filtering
of a Pulsed RF Signal
A pulsed R.F signal with a short pulse duration and a low
repetition frequency, such as that used in radar applications, is
classified as a broadband signal. So it is necessaryto use the
concept of impulse bandwidth to get useful information from
the IF filter’s impulse responseshown on the receiver’s display.
The signal has a carrier frequency f, with an EMS amplitude Figure 2: Filtering with RBW smaller than PRF
V,. The pulse width at the 50 percent amplitude points is r, and
the pulse period T (the pulse repetition frequency fJ. Figure 1
depicts this signal, which is applied to the input of an EM1 of the EMI receiver, as depicted in figure 3, many spectral
componentswill be in the filter’s passband.If the width of the
receiver.
main lobe of the pulsed RF spectrum is large comparedto the
IF filter bandwidth (2/r >> SW), then the spectral components
within the passbandhave approximately constantamplitude. In
Time
’
I-T-I
r: pulse width (at 50% points)
c =;
; pulse repetition frequency (ITS)
hItage
1
RMS value of peak
Figure 1: Pulsed RF signal applied to an EM1 receiver
Depending on the ratio of IF filter bandwidth to the pulse
repetition frequency of the test signal, the result shown on the Figure 3: Filtering with an RBW greaterthan the PRF
receiver display needs to be interpreted differently. Figure 2
illustrates the casewhere the EM1 receiver bandwidth is much this case the spectral intensity of this signal is given by:
smaller than the pulse repetition frequency. Here only one
s= v1*z
(3)
spectral line is in the IF filter’s passbandat any one time. If the
receiver is tuned to the carrier frequency so that the IF filter
will be centered around the maximum of the spectrum main The maximum output voltage of the RMS value of the peak of
the transient responseis:
lobe, the RMS amplitude V, can be calculated as:
v2
= VI*
T* fr
VP= S*BWi
(2)
If the pulse repetition frequency is larger than the IF bandwidth
(4)
Using equation 3, the maximum output voltage can be
959
expressedas:
In the secondstep,the pulse repetition frequency is reduced to
a rate much less than the resolution bandwidth of the receiver
VP= VI* T* BWi
(5) f, (minimum a factor of ten). The pulse shape must remain
constant.The amplitude of this signal, V,, is measuredagain at
This expression is valid as long as the resolution bandwidth is the sametuned frequency. Several spectral componentswill be
much greater than the pulse repetition rate, and the main lobe in the filter’s passband,and the amplitude V,, = V,*z*BWi. The
width is much greater than the filter bandwidth as well (factor equations for VP and V, are combined and the impulse
bandwidth can be calculated as shown in figure 4:
10 for practical purposes).
BWi = &I* VP
It is obvious from this example that knowledge of the impulse
bandwidth is necessaryto correctly interpret the measurement
result of the broadband case.
v2
V, is the maximum detectedamplitude of the transientresponse
of the receiver to an impulsive signal. V, is the actual peak
amplitude of the carrier signal. These are related by the
expression:
VP=a*Vl
The factor o is called desensitization
expressedin logarithmic terms:
(6)
factor
and often
CL= 20* log(T* BWi)
Determining
(7)
bzc BWm
fn= BW, fax> BWm
an EMI Receiver’s Impulse Bandwidth
To measurethe impulse bandwidth of an EM1 receiver, use a
pulse generator with an accurately known and settable pulse
repetition frequency. The pulse shapemust remain constant as
the repetition frequency is changed.When applying the pulse to
the input of the receiver to make the impulse bandwidth
measurement,the receiver must be usedin linear display mode,
becausethe logarithmic mode would distort the filter impulse
response. The receiver input attenuator must be set to an
appropriate value to avoid frontend overload. An overload
condition can be determined by varying the attenuator setting
and observing the corresponding signal level change.
Furthermore, the averaging or video bandwidth must be set at
least three times as wide as the resolution bandwidth to avoid
any averaging effect on the pulse signal.
There are two stepsin measuringthe impulse bandwidth. First,
the pulse generator is set to a pulse repetition frequency fR,that
is much higher than the resolution bandwidth of the receiver
(minimum a factor of three). The amplitude of the signal, V,, is
measuredwith the receiver center frequency tuned to the pulse
repetition frequency or one of its harmonics. Only one spectral
component will be in the filter passbandand the amplitude can
be calculated as V, = 2*V,*z*f,,. In this caseit is assumedthat
a basebandpulse is used for the measurement;therefore the
constant factor of 2 is introduced by the Fourier analysis.
Figure 4: Calculation of the impulse bandwidth
In steptwo, the pulse width must be selectednarrowly enough
to ensurea flat amplitude spectrumwithin the passbandof the
receiver’s IF filter. The pulse width must also be wide enough
that the signal will be sufficiently above the noise floor to
allow an accuratemeasurement.The advantagesof this method
of determining the impulse bandwidth are: the spectral
intensity of the pulse doesnot needto be known, and any pulse
shape (modulated or unmodulated) can be used. The only
requirements on the pulse signal are that the amplitude
spectrumis flat in the area of the filter under investigation and
the pulse repetition frequency can be changedindependently of
the pulse shape.
Measurement Example
A measurementwas madeto determine the impulse bandwidth
of spectrum analyzer IF filters using the method suggested
above. A pulse generator meeting the previously discussed
requirementsis the only additional test equipmentneeded. The
following pulse parameterswere chosen:
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pulse width z: 50 ns
pulse amplitude VP: 5 V
rise time 6: 2 ns
fall time tf: 2 ns
PRF (narrowband) fR,: 3 *RBW
PRF (broadband) fR2:0.1 *RBW
~18:39:50
l2 JAN 1393
NRRKER
2!.Fi6 NH2
18.351 mu
Lltl
In the fast part of the measurement,a PRF is usedthat is much
higher than the selectedresolution bandwidth of the spectrum
analyzer. In figure 5, the test result for the 1OOkHzbandwidth
(3dB value) is shown, using a PRF of 300kHz. The receiver is
put into linear display mode and sufficient input attenuation is
used to avoid front-end overload and ensure linear operating
conditions of the instrument. Furthermore,the video bandwidth
was set to its maximum value so that it has no effect on the
measuredamplitude. The voltage level, 19.61mV, measuredat
a specific tuning frequency, was recorded.
Last Hrd
Key Nenu
RCTU DLT: PERK
MCRS DET: PEAK !P RUG
SPRH
NKR 29.456 NH7
18.35j mU c
NARKCR
IIORNAL
RLF 11 21 mU
CENTER 29.946 MHz
RL
wIF BW 188 kHz
WJG B11 3 HHz
SPAN 'I.088 Ntl?
rsw 28.8 "SXC
Ifore
1 of 2
Figure 6: “Broadband” measurement
I
~15:ki?:83
12 JRN 1399
RCTU DET: PERK
HERS DET: PERK QP RUG
NKR 29.9'6 NHz
19.610 mU L
LIH
RCF 19 91 mv
selecting a pulse amplitude where both measurementscan be
made without changing instrument settings, especially the
reference level. Due to the ratioing of the two measurement
values, most of the instrument’s amplitude uncertainty factors
cancel out, leaving the display fidelity as the predominant
contributor.
lLas1 Hrd
Rey Nenu
SPRH
WdiKER
lOANAL
Summary
NRRKCR
RHPlD
Since EMI receivers are usedto measureboth narrowband and
broadband signals, it is essential to know the impulse
bandwidth to correctly interpret the broadband measurement
data. As discussedin the example of measuring a pulsed RF
signal, the broadband signal amplitude is directly proportional
SPflN 'I.088 flth
NUCZ
CCCTCR 2!.W!6 MHz
to the filter’s impulse bandwidth, and not its 3dB or 6dB
rsw 2.08 sec.
I or ?
nlf BW 1HE kHz
dRU0 BW 3 #Hz
RL
bandwidth. A very simple and reliable method for determining
Figure 5: “Narrowband” measurement
impulse bandwidth, which involves minimal additional test
equipment, was introduced. Using this method, the impulse
In the second part of the measurement,the PRF was set to 10 bandwidth of any IF filter implemented in an EM1 receiver or
kHz, to ensure that multiple spectral componentsof the pulse spectrumanalyzer can be measuredquickly and accurately.
are within the IF filter passbandat all times. The pulse width
was selectedso that the spectrum within the filter passbandis
References
flat; thus no errors caused by amplitude variations are
introduced. Figure 6 depicts this measurement.The recorded
amplitude was 10.35mV.
CISPR/A/WGl (ShinozukaNamanaka) 96-l: Responses of
spectrumanalyzersto pulsive and Gaussiannoise inputs
This data allows the determination of the impulse bandwidth
using equation (8):
EELCCl
1234
The accuracy of this procedure can be improved further by
961