Improving the accuracy of
EMI emissions testing
James Young
Rohde & Schwarz
Q&A
™ Who uses what for EMI?
⇒ Spectrum Analyzers (SA)
⇒ Test Receivers (TR)
™ CISPR, MIL-STD or Automotive?
™ Software or front panel ?
™ Novice, Capable, Fluent or Expert ?
Overview
™ EMI Equipment comparison
⇒ Spectrum Analyzers (SA)
⇒ Test Receivers (TR)
™ Making accurate measurements
⇒ RF / IF overload and Preselection
⇒ EMI Detectors and Filters
⇒ Preamps, where and when
Spectrum Analyzers for EMI
Spectrum Analyzers for EMI
Level [dBµV]
120
100
80
60
40
20
0
-20 30M
50M
70M
100M
200M
300M
500M 700M
1G
™ SA setup for EMI Example
- Set Start freq, stop freq, RBW and detector from standard
- Span of 1G-30MHz = 970 MHz / 1000 = 1 MHz resolution
- x samples in RBW are stored, 500 or 1000 are displayed
- samples within RBW analyzed or weighted by the detector
- QP “integrates” voltage in RBW, applies CISPR weighting
Spectrum Analyzers for EMI
™ Question
- What was wrong with the previous setup?
30M
50M 70M 100M
200M 300M
500M 700M 1G
™ Answer
- Frequency and amplitude accuracy depend on many
samples falling within each measurement bin (also called
display pixel).
- Next “bin” will be 1 MHz away (nearly 10 x 120 kHz)!
- Frequency resolution much too course for EMI
- Solution?
Spectrum Analyzers for EMI
Level [dBµV]
120
100
80
60
40
20
0
-20
30M
50M
70M
100M
200M
300M
500M
™ Example Revisited
- Span of 970 MHz / 1001 = 1 MHz resolution
- If using 120 KHz RBW, CISPR recommends
60 KHz “bin” points (17 x finer than 1 MHz)
- Solution: subrange in the span
700M
1G
Spectrum Analyzers for EMI
™ Frequency Accuracy of SA
- SA frequency resolution is far too course for EMI
without sub-ranging the CISPR span
- SA frequency accuracy when exploring peaks
influenced by Span, RBW, VBW, marker accuracy
™ Amplitude Accuracy of SA
-
6 dB (EMI) filters
QP and AVE detector times are observed
Data correction for system transducers
EUT specific timing issues are considered
Subranges set properly for sample #
RF and IF stages are not overloaded
Spectrum Analyzers for EMI
™ Conclusion
⇒ SA lacks 2 control parameters for EMI
- STEP SIZE between measurement bins
- DWELL TIME at each measurement bin
- Lacks dynamic range and overload protection (later slides)
⇒ Sub-ranging and zero span is an attempt
to make SA measure like TR
Test Receivers for EMI
Test Receivers for EMI
™ Receivers for EMI
- Frequency Span (start / stop)
- RBW Filter and detector
- DWELL TIME at each measurement point
- FREQUENCY INCRIMENT (step size independent of RBW)
- TR adjusts sample x and bins depending on span
- x is often 16,000 – 100,000+
- Span / x = frequency resolution
Sample Points Example
120
100
80
60
40
20
0
30M
50M
70M
100M
200M
300M
500M
700M
1G
Level [dBµV]
57.00
50.00
40.00
30.00
20.00
10.00
0.00
79.94M
85M
90M
95M
Frequency [Hz]
100M
109.63M
™ Example: TR (green) vs. SA (blue) sample points
Samples and Bins
samples
Freq error
RBW
RBW
Integration of samples
Peak Lost
detector
Peak
detector
peak
peak
QP
QP
Ave
Ave
RMS
RMS
Display BIN
•
•
Bin amplitude is detector value, Bin freq is reported in center
SA has 1001 bins, TR accesses 100,000 bins (from memory)
QP
Display BIN
Receiver Measurements
• CISPR 16 recommends Step size ≤ RBW / 2
Frequency Range
6-dB Bandwidth
150 kHz to 30 MHz
9 kHz
4 kHz
7,463
30 MHz to 1 GHz
120 KHz
50 KHz
19,400
Step Size
(<1/2 RBW)
# of meas
Bins
VS
80M
85M
90M
95M
Frequency [Hz]
100M
110M
Test Receivers for EMI
™ Conclusion
⇒ TR incorporates EMI control parameters
- STEP SIZE between measurement bin
- DWELL TIME at each measurement bin
- # of measurement bins as necessary for accuracy
™ Time Penalty ?
- Time dependant on detector and EUT, not
measurement speed of instruments
Best Instrument for EMI?
™ Pros and Cons of Each
- SA is faster for initial preview
- SA can also be used for RX and TX measurements
- TR has little
little use
use outside
outside EMI,
EMI, expensive
expensive
unit for one use
- SA sub-ranging negates any speed
advantage over TR for EMI
- SA frequency / amplitude accuracy easily skewed by
improper settings and interpretation
Which one to use?
™ Use SA or TR to develop “hit list”
Use SA or TR for maximization
Att 30 dB AUTO
FREQUENCY
LEVEL QPK
10
dBµV
20
100 MHz
60
200 MHz
30
300 MHz
RBW
120 kHz
MT
1 s
PREAMP ON
931.9200000 MHz
dBµV
40
400 MHz
50
500 MHz
60
600 MHz
70
700 MHz
800 MHz
80
90
900 MHz
50
FCC15RB
1 PK
CLRWR
SGL
40
30
20
10
™ TR is optimum for final
(dwell time, step size and
auto attenuator)
0
-10
-20
30 MHz
Date:
1 GHz
8.SEP.2003
14:13:12
Making accurate measurements
Overload protection
Detectors for EMI
RBW Filters for EMI
Preamps
Accurate Measurement
™ Dynamic range of SA / TR is ~160 dB
™ 160 dB = 10e8 or 8 orders of magnitude
™ EMC engineers don’t know what signals
they are looking for initially
™ Accuracy killers
- Overloads (RF and IF)
- Incorrect detector settings
- Preamplifiers improperly used
- Improper RBWs
RF Overdrive
™ RF: Watch for harmonics of large signals
- Use attenuator to set mixer input level
Max Input Level
Ref Level
RF Attenuation sets mixer
input level
1st
mixer
Input
Mixer Level
RF Overload Example
MARKER 1
496 MHz
Ref
0 dBm
1
* Att
RBW 3 MHz
VBW 10 MHz
SWT 10 ms
10 dB
0
Marker 2 [T1 ]
-48.80
996.000000000
Marker 3 [T1 ]
-48.60
1.500000000
-10
1 SA
AVG
Marker 1 [T1 ]
3.03 dBm
496.000000000 MHz
-20
dBm
MHz
dBm
GHz
-30
-40
2
3
-50
-60
-70
-80
-90
-100
Center 1 GHz
Date:
23.JUN.2004
200 MHz/
20:33:37
™ Example: amplified signal at 500 MHz
Span 2 GHz
A
IF Overdrive
™ IF: Watch for overload flags
- Use Ref Level to set IF Gain
Ref Level
- Set IF gain using “ref Level”
Ref Level sets IF
Gain
IF Gain
1st
mixer
Input
Mixer Level
IF Overload Example
RF ATTENUATION
10 dB
Ref
0 dBm
* Att
10 dB
1
RBW 3 MHz
VBW 10 MHz
SWT 5 ms
Marker 1 [T1 ]
6.13 dBm
502.053410569 MHz
0
OVLD
1 AP
CLRWR
A
-10
-20
-30
-40
-50
-60
-70
-80
-90
-100
Center 502.0534106 MHz
Date:
23.JUN.2004
99.02160153 MHz/
20:55:20
™ Example: +6 dBm Pulse
Span 990.2160153 MHz
Preselection Filtering
™ Preselector is a “tracking” RF filter
- ALL RF power (noise & signals) go into mixer
- high amplitude signals outside displayed span can
influence amplitude and may be aliased
™ Cure: preselect filtering of signals before RF or IF
- SA may not warn of RF or IF overdrive
- IF overload won’t show on display
- Signals outside display ruin amplitude reading
Preselection Simple
Mixer stage
Pre-selection
V (f)
V(f)
B2
B1
f
Overdrive and Preselection
MARKER
1
1.000272533
Ref
0 dBm
GHz
* Att
20
dB
RBW
VBW
SWT
3 MHz
10 MHz
10 ms
0
Marker
[T1 ]
-43.41
1.000272533
Marker
[T1 ]
-54.36
1.501204189
-10
1 PK *
CLRWR
1
dBm
GHz
2
dBm
GHz
-20
-30
-40
1
-50
2
-60
-70
-80
-90
-100
Start
Date:
400
MHz
8.SEP.2003
141.6135906
MHz/
Stop
1.816135906
GHz
13:03:46
™ Example: +10 dBm signal at 500 MHz causing overdrive
A
Overdrive and Preselection
START
FREQUENCY
600 MHz
Ref
0 dBm
* Att
20
dB
RBW
VBW
SWT
3 MHz
10 MHz
10 ms
0
Marker
[T1 ]
-43.41
1.000272533
Marker
[T1 ]
-54.58
1.501204189
-10
1 PK *
CLRWR
1
dBm
GHz
2
dBm
GHz
A
-20
-30
-40
1
-50
2
-60
-70
-80
-90
-100
Start
Date:
600
MHz
8.SEP.2003
121.6135906
MHz/
Stop
1.816135906
GHz
13:04:13
™ Example2: Moving overload off screen won’t clear overload
Overdrive and Preselection
MARKERFREQUENCY
START
1
1.000272533
600
MHz
Ref
0 dBm
GHz
* Att
20
dB
RBW
VBW
SWT
3 MHz
10 MHz
10 ms
0
Marker
[T1 ]
-43.41
-62.14
1.000272533
Marker
[T1 ]
-54.36
-54.58
-63.57
1.501204189
-10
1 PK *
CLRWR
1
dBm
GHz
2
dBm
GHz
A
-20
-30
-40
1
PS
1
-50
2
1
-60
2
2
-70
-80
-90
-100
Start
Date:
400
600
MHz
8.SEP.2003
141.6135906
121.6135906
MHz/
Stop
1.816135906
GHz
13:04:47
13:03:46
13:04:13
™
™ Example3:
Example3: Activating
Activating Preselector
Preselector clears
clears overload
overload by
by filtering
filtering the
the fundamental
fundamental
PRF, Dwell time and Detectors
™ EUT / Detector dwell time requirements
- Must capture “worst case” emissions of EUT
- Cycle time, modulation, and pulse repetition frequency
may require extended dwell in each subrange (QP,AVE, RMS)
- Test Receivers include this parameter
-Since tuning each bin, can dwell as long as necessary
- Spectrum Analyzers have a workaround
-Zero Span is a way to trick SA into dwelling at tuned bin
-Must do calculation since overall sweep time is controlled
-Dwell time = sweep time / measurement bins
Detector Settings
™ Quasipeak ????
⇒ QP is an attempt to quantify a signals
Impact on a radio receiver (annoyance)
- Factors: amplitude, frequency, pulse repetition frequency
™ Quasipeak restrictions
- Dwell time (per step or subrange) at least 1 second
- PRF issues increase dwell time requirements
Detector Settings
™ Peak Detector
- Peak gives “worst case”
- Safest detector: Often fast enough to see most signals
even if instrument settings are not “optimum”
™ Average or RMS Detector
- Average detector used above 1 GHz for FCC and CE tests
- RMS for ultra wide band, dwell time critical for integration
- dwell for at least 100 mS at each bin for proper integration
- Watch RBW (1MHz or 120 KHz above 1GHz?)
Detector Settings Example
Ref
-20
dBm
Att
10
RBW
VBW
SWT
dB
120 kHz
1 MHz
175 ms
Marker
1
[T1 ]
-49.64
280.000000000
dBm
MHz
-20
A
-30
1 RM *
CLRWR
-40
1
-50
-60
-70
-80
-90
-100
-110
-120
Start
Date:
0
Hz
23.JUN.2004
50
MHz/
Stop
500
MHz
21:44:15
™ Example: -40 dBm signal at 280 MHz measures with RMS
Detector Settings Example
Ref
-20
dBm
Att
10
RBW
VBW
SWT
dB
120 kHz
1 MHz
175 ms
Marker
1
[T1 ]
-56.89
-49.64
280.000000000
dBm
MHz
-20
A
A
-30
-30
1
1 AV
RM *
*
CLRWR
CLRWR
-40
-40
1
-50
-50
1
-60
-60
-70
-70
-80
-80
-90
-90
-100
-100
-110
-110
-120
-120
Start
Start
Date:
Date:
0
0
Hz
Hz
23.JUN.2004
23.JUN.2004
50
50
MHz/
MHz/
Stop
Stop
500
500
MHz
MHz
21:44:41
21:44:15
™ Example2: Same -40 dBm but with Ave and same settings
Detector Settings Example
Ref
-20
dBm
Att
10
RBW
VBW
SWT
dB
120 kHz
1 MHz
175 ms
Marker
1
[T1 ]
-56.89
-76.57
-49.64
280.000000000
dBm
MHz
-20
A
A
-30
-30
1
AV
1 QP
RM *
*
CLRWR
CLRWR
-40
-40
1
-50
-50
1
-60
-60
-70
-70
1
-80
-80
-90
-90
-100
-100
-110
-110
-120
-120
Start
Start
Date:
Date:
0
0
Hz
Hz
23.JUN.2004
23.JUN.2004
50
50
MHz/
MHz/
Stop
Stop
500
500
MHz
MHz
21:45:04
21:44:41
21:44:15
™ Example3: Same -40 dBm but with QP and same settings
Transducers and PRF
™ Transducer correction
- CISPR needed a way to eliminate effects of variability in
chambers, antennas and cable losses
- Standards require normalization of these
effects to compare results to limit line
- Chamber ruled by Normalized Site Attenuation (NSA)
- Transducers and connections used correction factors derived
from actual calibrations
Resolution Bandwidth Filters
Ref Lvl
9 7 d B ÌV
RBW
VBW
SWT
100 kHz
10 MHz
5 ms
RF Att
Uni t
20 dB
d B ÌV
97
A
90
3dB RBW
80
70
60
6dB RBW
I N1
1 MA
2 MA
1M A X
2V I EW
50
40
30
20
10
0
-3
Center 100 MHz
100 kHz/
Span 1 MHz
Preamps
™ Preamps: When are they needed?
- Generally needed above 7 GHz
- Below 7 GHz ONLY if stringent limit line or long cables
™ Preamps: Where to put them?
- Preamps amplify signal and noise
- Real
Real goal
goal of
of preamp
preamp is
is PRESERVE
PRESERVE signal
to noise ratio (best at the antenna)
antenna)
- Low level signals require amplification at antenna, before
signal is subjected to path loss of cables
Preamps
™ Preamp S/N Ratio example
S N
S N
S N
Preamp near EMI receiver
Common Cables loss
½ dB per meter at 1GHz
S N
3 dB per meter at 40 GHz
Preamp at antenna
S N
S N
Preamps
™ Preamps
- Noise Figure equation states the 1st gain/loss
encountered has the most impact on s/n ratio
- Must have high gain & low noise figure (amp contributed noise)
- 1-18 GHz gain around 30 dB with NF of 3.2 dB or less
- 18-26 GHz gain around 35 dB with NF of 3.0 dB or less
- 26-40 GHz gain around 50 dB with NF of 2.8 dB or less
Noise contributed by preamp (NF)
S
N
RF Thermal Noise (N0)
Preamps
™ Know your Preamp overload range and behavior
- Must keep preamp in linear region
- Know the preamp range (what is the max signal
input without compression or damage)
- Watch RF input levels to 1stst mixer system check if unsure
(variable attenuator)
- Non-wave guide
guide
antennas have a direct
connection to the FET,
watch static discharge
Preamps
Conclusion
™ TR vs. SA
Span
Subranges
step size
resolution
dwell time
overloading
preselection
™ RF & IF Overloads
RF -> Harmonics
Attenuator
IF -> If overload flag
Ref Level
™ Preamps
amplify at antenna
know linear region
system check (attenuator)
static
below 1gz