The HF Current Probe: Theory and Application KENNETH WYATT Wyatt Technical Services Woodland Park, Colorado, USA T his article describes one of the most valuable tools in the EMC engineers "bag of tricks" - the high-frequency current probe. Current probes are invalu- able for measuring high-frequency com- mon-mode (or "antenna") currents flowing on wires or cables. Experience has proven that poorly terminated (bonded or filtered) cables are the number-one cause for radi- ated emissions failures at a test facility. By measuring the common-mode (CM) cur- rents (sometimes referred to as "antenna" currents) on these cables it's possible to troubleshoot and apply fixes to a product right there in your development lab. You can also predict, to a good degree of ac- curacy, whether a given cable current will pass or fail in the measurement chamber. This will save you tons of time trying to apply fixes at the test facility while the clock is ticking away your test time. I'll also show you several ways to create do- it-yourself (DIY) probes that are quick to make and very useful i n a pinch. COMMON-MODE CURRENTS Let's consider CM currents and how they are generated, because it is not intuitive as to how current may travel the same direc- tion through both the signal and signal- return wires i n a cable or PC board. Re- ferring to Figure 1, note that due to finite impedance in any grounding system (in- cluding circuit board signal/power return planes), there will be a voltage difference between any two points within that return plane. This is denoted by ^Q^jy, and V^j^^^ in the figure. This difference in potential will drive CM currents through common cabling or circuit traces between circuits or sub-systems. In addition, unbalanced geometries - for example, different lengths or path routings for high-speed differential pairs - can create voltage sources that drive associated CM currents. Finally, routing a high-speed clock trace across a split in the return plane or referencing it to multiple planes, can also be a source of CM current. Because the current phasors in Figure 1 are additive, the resulting radiated pha- sor may be quite large compared to those generated by differential-mode (DM), or signal currents, which are opposite in di- rection, and so tend to cancel. Therefore, CM emissions tend to be more of an issue than D M emissions. CURRENT PROBES: THEORY OF OPERATION The RF current probe is an "inserted- primary" type of radio frequency current transformer. When the probe is clamped over the conductor or cable in which current is to be measured, the conductor forms the primary winding. The clamp-on feature of this probe enables easy place- ment around any conductor or cable. This is essentially a broadband high-frequency transformer. High-frequency currents can 14 INTERFERENCE TECHNOLOGY EMC DIRECTORY S. DESIGN GUIDE 2 0 1 2
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The HF Current Probe: Theory and Application
KENNETH WYATT Wyatt Technical Services Woodland Park, Colorado, USA
This article describes one of the most valuable tools in the E M C engineers "bag of t r i cks" - the high-frequency
current probe. Current probes are invaluable for measur ing h igh- frequency c o m mon-mode (or "antenna") currents f l o w i n g on wires or cables. Experience has proven that poor ly te rminated (bonded or f i l tered) cables are the number-one cause for r a d i ated emissions fai lures at a test fac i l i ty . By measur ing the c o m m o n - m o d e ( C M ) currents (sometimes referred to as "antenna" currents) on these cables it's possible to t roubleshoot and apply fixes to a p r o d u c t r i g h t there i n your development lab. You can also predic t , to a good degree of accuracy, whether a given cable c u r r e n t w i l l pass or fa i l i n the measurement chamber. T h i s w i l l save you tons of t i m e t r y i n g to apply fixes at the test f a c i l i t y w h i l e the clock is t i c k i n g away y o u r test t i m e . I ' l l also show you several ways to create d o -i t -yoursel f ( D I Y ) probes that are quick to make and very useful i n a p i n c h .
COMMON-MODE CURRENTS Let's consider C M currents and how they are generated, because i t is not i n t u i t i v e as to how c u r r e n t may travel the same direct i o n t h r o u g h b o t h the signal and s ignal-r e t u r n wires i n a cable or PC board. Ref e r r i n g to Figure 1, note that due to f i n i t e
impedance i n any g r o u n d i n g system ( i n c l u d i n g c i r c u i t board signal/power r e t u r n planes), there w i l l be a voltage dif ference between any t w o points w i t h i n that r e t u r n plane. T h i s is denoted by ^Q^jy, and V^j^^^ i n the f igure . T h i s dif ference i n potent ia l w i l l dr ive C M currents t h r o u g h c o m m o n cabl ing or c i r c u i t traces between c i r c u i t s or sub-systems. I n a d d i t i o n , unbalanced geometries - for example, d i f ferent lengths or path routings for high-speed di f ferent ia l pairs - can create voltage sources that drive associated C M currents . Finally, r o u t i n g a high-speed clock trace across a spl i t in the r e t u r n plane or referencing it to m u l t i p l e planes, can also be a source of C M current . Because the c u r r e n t phasors i n Figure 1 are addi t ive , the r e s u l t i n g radiated phasor may be qui te large compared to those generated by d i f f e r e n t i a l - m o d e ( D M ) , or signal currents , w h i c h are opposite in d i rec t ion , and so tend to cancel. Therefore , C M emissions tend to be more of an issue t h a n D M emissions.
CURRENT PROBES: THEORY OF OPERATION T h e RF c u r r e n t probe is an " i n s e r t e d -p r i m a r y " type of radio frequency c u r r e n t t ransformer . W h e n the probe is c lamped over the c o n d u c t o r or cable i n w h i c h c u r r e n t is to be measured, the conductor forms the p r i m a r y w i n d i n g . The c l a m p - o n feature of this probe enables easy placement a r o u n d any conductor or cable. T h i s is essentially a broadband h igh- f requency transformer . H i g h - f r e q u e n c y currents can
! THE H F C U R R E N T PROBE: THEORY AND APPLICATION
Source Load Signal
Signal Return
V , GND1
Phasor from far wire Phasor from near wire Resultant phasor —
R .
V GND2 d = 3m
t o r o i d or c l a m p - o n core that offers good h igh- f requency character ist ics i n the 10 to 1000 M H z range. W i n d i n g a few (not too c r i t i ca l ) t u r n s and t e r m i n a t i n g w i t h a coax connector is a l l you need. Keeping the t u r n s as far apart as possible (as i n F i g u r e 4) w i l l reduce i n t e r - w i n d i n g capaci tance a n d y i e l d b e t t e r results at the higher frequencies. T h i s is one of the largest drawbacks i n per formance of the c l a m p - o n ferrites (as i n Figure 5).
Figure 1. Common-mode currents in a circuit loop. The source is a digital signal (with harmonics) and we'll assume a resistive load. Because the phasor current in the far wire is in the same direction as the phasor current in the near wire, the resultant phasor is relatively large compared to that produced by differential-mode current phasors. In this case, lowering the harmonic content (by slowing the digital rise/fall-times) or diverting/blocking the CM current is very important in limiting radiated emissions.
be measured i n cables w i t h o u t physical ly d i s t u r b i n g the c i r c u i t .
Since the c u r r e n t probe is i n t e n d e d for " c l a m p - o n " operat ion, the p r i m a r y shown i n Figure 2 is actual ly the electr ical conductor i n w h i c h C M currents are to be measured. T h i s p r i m a r y is considered as one t u r n since i t is assumed that the C M currents f l o w t h r o u g h the conductor and r e t u r n to the source via a r e t u r n conductor such as a f rame, c o m m o n g r o u n d plane, or e a r t h . O n some c u r r e n t probe models the secondary o u t p u t t e r m i n a l s are resis-t ively loaded i n t e r n a l l y to provide substant ia l ly constant transfer impedance over a wider frequency range.
TRANSFER IMPEDANCE The C M c u r r e n t (Ic) i n m i c r o a m p s in the conductor under test is d e t e r m i n e d f r o m the reading of the c u r r e n t probe o u t p u t (V) i n microvol t s d i v i d e d by the c u r r e n t probe transfer impedance (ZT).
I c = V / Z T (1) Or, i n dB
I c ( d B u A ) = V ( d B u V ) - Z T ( d B n ) (2) T h e t y p i c a l t r a n s f e r i m p e d a n c e of the c u r r e n t probe t h r o u g h o u t the frequency range is d e t e r m i n e d by passing a k n o w n RF c u r r e n t (Ic) t h r o u g h the p r i m a r y test
conductor and n o t i n g the voltage (V) developed across a 5 0 - O h m load. T h e n ,
Z T = V / I c ( in s tandard units) (3) O r
Z T ( d B n ) = V(dBjV) - Ic (dBjA) (4)
T h e Fischer F-33-1 probe is a c o m m o n l y used t r o u b l e shoot ing t o o l and has a f la t f requency response f r o m 2 to 250 M H z (Figure 6). The transfer impedance is about S n (approximately +14 d B f l on the graph), therefore, a 1 u A c u r r e n t w i l l produce a 5 uV o u t p u t voltage f r o m the c u r r e n t probe.
CDMMERCIAL CURRENT PROBES W h i l e c o m m e r c i a l c u r r e n t probes are pricey, the advantage is that they can open up and snap around a cable, r a t h er than having to be threaded onto the cable to be measured. See Figure 3. They are also a lot more rugged and can take a lo t of abuse as compared to the " d o - i t - y o u r s e l f " ( D I Y ) versions below. Finally, they are also accurately character ized, a l l o w i n g very precise measurements of cable currents .
DIY CURRENT PROBES I n a p i n c h , you can make your o w n c u r r e n t probe . Examples of several D I Y probes are s h o w n i n Figures 4 and 5. I t y p i c a l l y t r y to f i n d a ferr i te
Noise Current
Case Ground
Primary V^inding (wire under test)
Electrostatic Shield (Case)
Secondary Winding
Coax Connector (bO Ohms)
Figure 2. The basic current probe (high-frequency current transformer).
PROBE CALIBRATION T he accurate c a l i b r a t i o n of RF c u r r e n t probes is a
c o m p l e x process. C h a r a c t e r i z a t i o n is a m o r e c o r r e c t t e r m to use t h a n ca l ibra t ion . The probe must be p r o p erly character ized to ref lect how the user uses the probe. Probe m a n u f a c t u r e r s usual ly sell a c a l i b r a t i o n f i x t u r e that a t tempts to m a i n t a i n a 5 0 0 impedance. A 5 0 O load is connected to the o u t p u t p o r t and a cal ibrated RF generator (or n e t w o r k analyzer) is connected to the i n p u t p o r t . The probe to be character ized is c lamped a r o u n d
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the f i x t u r e and the frequency is swept w h i l e measur ing the probe o u t p u t .
M y test setup was a l i t t l e more r u d i m e n t a r y (Figure 7), but for t r o ub le sho o t ing purposes, it's good enough. I used a short piece of s t i f f wi re across the o u t p u t p o r t w i t h a 5 0 0 resistive load i n series. I then adjusted the generator for zero d B m - a convenient a m o u n t . T h i s is equivalent to an output voltage of 224 mV (or 73 d B u A of current) in to 5 0 0 . T he actual generator o u t p u t doesn't matter, so long as the resu l t ing probe voltage is large enough to be seen readily
i n the receiver or spec t rum analyzer. I m o n i t o r e d the probe o u t p u t w i t h a T h u r l b y Thander T T i PSA2701T handheld s p e c t r u m analyzer.
K n o w i n g the current t h r o u g h the w i r e i n d B u A and the probe o u t p u t i n dBuV, the t r a n s f e r i m p e d a n c e may be p l o t t e d graphica l ly by subt r a c t i n g : V ( d B u V ) - I c ( d B u A ) (expressed i n dB). I n this case, Z T ( d B O ) = V(dBuV) - 73. W h i l e th i s may be use fu l for educat ional purposes, I w o u l d n ' t be too i n c l i n e d to use the D I Y probes to predic t "pass/fail", as described f u r t h e r d o w n . However , because t h e y c o m p a r e f a v o r a b l y to the c o m m e r c i a l probes as far as o u t p u t voltage, I believe (and have p r o v e n i n pract ice) t h a t they are complete ly sui ted for t roubleshooting. You only need to k n o w w h e t h e r an E M C design f i x made the cable c u r r e n t better or worse.
Medical
Nuclear
Rail
Space
Telecom
Wireless
MIL-STD, DEF-STAN
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Telcordia, FCC, iC
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PREDICTING PASS/FAIL I t is possible to predic t whether a part icular cable w i l l pass or fa i l radi ated emissions by measuring the C M c u r r e n t at the o f f e n d i n g frequency, reading o f f the transfer impedance of the probe, Z t (dBH) i n Figure 6, and so lv ing for Ic (using Equat ion 2 above). P l u g g i n g I c ( A m p s ) i n t o Equat ion 5 w i l l calculate the E-f ie ld level i n V/m. T he l e n g t h of the cable is L(rn) and the o f f e n d i n g h a r m o n i c f requency is f (Hz) . Use a test d i s tance, d, of either 3 or 10m to predict the outcome at those test distances.
= 1.257 x i r * d (5)
Once you've d e t e r m i n e d a part i c u lar cable has C M currents that may cause a RE fa i lure , you should to e x a m i n e the c o n n e c t o r w h e r e the cable is attached to the p r o d u c t
Figure 4. Examples of DIY current probes based on a large toroid core. These photos were taken prior to installing the E-field shield which consists of a layer of copper tape overthe windings, leaving a small gap around the inside of the toroid. 14 turns of Teflon-insulated wire wound around a Wiirth Electronik #74270097 ferrite core (4W620 material) was used, which is useful from 10 to 1000 MHz.
enclosure. Very of ten, I f i n d poor or non-exis tent b o n d i n g between the connector shield and enclosure shield. These points must be bonded w e l l to p e r m i t the C M currents to f l o w back to their source w i t h i n the product , avoiding associated cable r a d i a t i o n . Please refer to my previous articles on t r o u b l e s h o o t i n g radiated emissions for more i n f o r m a t i o n (references below).
REAL-WORLD TROUBLESHOOTING EXAMPLE As previous ly m e n t i o n e d , one of the most c o m m o n
sources of radiated emissions is due to p o o r l y bonded c o n n e c t o r s m o u n t e d o n shie lded p r o d u c t enclosures. T h i s occurs especially i f the connectors are c i r c u i t board m o u n t e d and penetrate loosely t h r o u g h the shielded enclosure. Poorly bonded connectors a l low in terna l ly generated C M currents to leak out and f l o w on the outside of I/O, mouse or keyboard cables. T h i s w i l l also a l low ESD discharges inside the p r o d u c t - more bad news. I f these currents are a l lowed out of the enclosure, the attached cables w i l l act as r a d i a t i n g antennas - o f ten resonat ing a r o u n d 300 M H z , due to their t y p i c a l I m length .
T h i s was the case for a new d i g i t i z i n g osci l loscope p r o t o t y p e I w o r k e d on recently. The I/O connectors were al l soldered onto the PC board and the board was fastened to the rear ha l f of the enclosure. The connectors s imply poked up t h r o u g h cutouts i n the rear meta l shield.
W h i l e us ing a c u r r e n t probe to measure the C M current f l o w i n g on the outside of the USB cable under test, I s imply j a m m e d the screwdriver blade of my Swiss A r m y k n i f e between the connector b o n d i n g f ingers and meta l chassis enclosure and was able to d r o p the overal l cable currents by 10 to 15 dB.
T h e s o l u t i o n was to fabr icate a c u s t o m s h i m w i t h spr ing-f ingers that w o u l d slip over a l l the connectors creat ing a f i r m b o n d between the connector g r o u n d shell and inside of the shielded enclosure. M o r e and more low-cost products are r e l y i n g on PC board m o u n t e d I/O connec-
FigureS. Examples of DIY current probes based on clamp-on ferrite chokes. I used a couple sample Steward (now a unit of Laird Technologies) chokes - a round one (model 28A3851-0A2) and a square one (model 28A2024-0A2), They each had 7 turns of Teflon-insulated wire wound around one-half and glued down on the inside to hold the windings. I later epoxied a PC board-style BNC connector to the outside, making sure there was enough epoxy to hold the outer turns together. Type 28 material was used, which is useful from 10 to 1000 MHz.
Figure 6. Transfer impedance (ZT) graph of an F-33-1 current probe (courtesy of Fischer Custom Communications). The x-axis is frequency, while the y-axis is dBCl. Use this to calculate the value of Ic (Equation 2), given the measured voltage at the probe terminals (V,JandZT.
Figure 7.1 used a short wire and 50O load (two parallel WOO resistors) across the generator output for probe characterization. Obviously, there are shortcomings at higher frequencies, due to the inductance of the wire. In fact, the system impedance starts to go capacitive at 100 MHz and it's difficult to keep a fixed 224 mV across the load resistor with frequency.
tors as a c o s t - c u t t i n g measure. A n y t i m e you see this , be prepared to careful ly examine the b o n d i n g between the connector g r o u n d shell and the shielded enclosure.
TROUBLESHOOTING TIPS USING CURRENT PROBES
Here are a few t r o u b l e s h o o t i n g t ips u s i n g c u r r e n t probes.
r^ir"r- fiD,ni Ari(ir.„.-. C!ii::„
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ITEM
Commerc ia l versus DIY Current Probe (Wire Loop]
— Comnercial Probe
— D:V Probe
Frequency (MH2)
Figure 8 Transfer impedance (ZT) graph of a commercial current probe versus the DIY toroidal probe. The x-axis is frequency, while the y-axis is dBCl. Note that the commercial probe is only designed and characterized to 250 MHz, so the data above that, while interesting, is probably not valid. The DIY probe, as well, performs poorly above 200 MHz and frankly, the wire loop used to introduce a "calibrated" current (while as short as possible) affects the measurement, as well.
1. W h e n evaluat ing the harmonics on a cable by using a c u r r e n t probe, i f s l i d i n g the probe back and f o r t h changes the harmonic levels, part of the coupl ing maybe near-field, rather t h a n conducted .
2. W h e n using a pair of c u r r e n t probes; one on each of t w o cables, i f the harmonics are the same i n each, the source is i n the m i d d l e . I f one cable has stronger h a r m o n ics, then y o u ' l l w a n t to w o r k on that side f i r s t . See Figure 12 below.
3. M e a s u r i n g the currents o n t w o suspect legs of a d ipo le s h o u l d read the same. Placing the t w o suspect legs t b r o u g b the same c u r r e n t probe should cause a big decrease due to c u r r e n t cancel lat ion. See Figure 12 below.
4. W h e n measuring video cable currents and large cable movements cause big changes i n a m p l i t u d e , the c o u p l i n g is l ikely induct ive - otherwise , it's more l ikely conduct ive .
5. I f you suspect induct ive c o u p l i n g , the phase at the v i c t i m w i l l be 180-degrees f r o m the source. T h i s may be observed o n an oscil loscope w i t h H - f i e l d probes or current probes. T r y syncing the scope tr igger at the source using a scope probe.
M y colleague, D o u g S m i t h , has many more examples on bow to use c u r r e n t probes for measur ing cable and PC board resonances, in j ec t ing pulses for t r o u b l e s h o o t i n g , i n t e r p r e t i n g the relative phase of c o m m o n - m o d e currents and t r o u b l e s h o o t i n g ESD issues. Refer to the references below.
SUMMARY Use of a c u r r e n t probe is v i t a l d u r i n g the t r o u b l e
s h o o t i n g process. Poorly bonded cable connectors can be readi ly i d e n t i f i e d and f ixed . The radiated E-f ie ld f r o m a p r o d u c t I/O cable may be calculated by measur ing the h igh- f requency c o m m o n - m o d e currents f l o w i n g i n the cable. A l l th is may be p e r f o r m e d r i g h t at the designer's
20 INTERFERENCE TECHNOLOGY EMC DIRECTORY S . DESIGN GUIDE 2012
W Y A T T
w o r k b e n c h and w i t h o u t the expense o f a t h i r d - p a r t y test f a c i l i t y or shie lded chamber.
REFERENCES - PAPERS . [1] Mat Aschenberg & Charles Grasso,
Radiation from Common-Mode Currents -
Beyond 1 GHz (Three Methods Compared)
[2] Dave Eckhardt, Homebrew Clamp-
On Current Probe, private correspondence
(January 2009), Email: davearea51@wild-
biue.net.
[3] Jasper Goodblood, Electromagnetic
Compatibility, 1990, Prentice Hall, pages
31-34.
. [4] Michel Mardiguian, EMI Trouble
shooting Techniques, McGraw-Hill, 2000,
pages 39-49.
[5] Montrose & Nakauchi, Testing for
E M C Compliance, 2004, VXAley Interscience,
pages 116-124, 143-145, and 159-161.
• [6] Henry Ott, Electromagnetic Com
patibility Engineering, Wiley, 2009, pages
690-693.
• [7] Henry Ott, Measuring Common-
Mode Currents on Cables, www.hottcon-
60 > a
40
20
-20
-40
Commerc ia l versus DIY Current Probe (Wire Loop)
100 T -
•Connercial • DIY Probe - Difference (cB)
robe
Frequency (MHz)
figure 9. Probe output voltage (V^J graph of a commercial current probe versus the DIY toroidal probe. The x-axis is frequency, while the y-axis is dBuV. This shows that the probes are very comparable in output voltage versus frequency. For troubleshooting purposes, absolute accuracy is not required - just consistency in measurements. All one really needs to know is, "did the fix 1 implemented make the CfYI current go up or down?" The DIY probe works well for this.
MULTIFUNCTION GENERATOR NSG 3040 -BIG THINGS COME IN SMALL PACKAGES It is small, smart and has a high-contrast 7" touchscreen color display and the rotary wheel for quick input with appealing ease of operation. With its open modular architecture, the NSG 3040 is the ideal immunity test companion for smaller engineering labs - with amazing capacities for demanding EMC testing companies and for easy integration into the production process. The electromagnetic pulses generated from this multipurpose unit are especially tailored for CE marking requirements of the EU in addition to the national and international standards. Like its big brother, NSG 3060 (6.6 kV), the NSG 3040 also has a SD memory card where test files can be saved easily and expanded at any time.
Modu l a r , e x p a n d a b l e s y s t e m Su rge v o l t a g e t o 4 .4 kV EFT /Bu r s t tO 4.8 kV /1 MHz PQT t o 16A / 260 VAC 8, DC Eas y - t o - o p e r a t e 7 " t o u c h s c r e e n c o l o r d i s p l a y TA (Tes t A s s i s t a n ce ) f o r r a p i d t e s t r e s o l u t i o n P a r a m e t e r s c a n be c h a n g e d d u r i n g t e s t
Teseq Inc. New Jersey 0883/ USA 1-̂ 1 732 417 0501 wv.w.tesequsa.com Advanced Test Solutions for EMC
THE H F C U R R E N T PROBE: THEORY AND APPLICATION
Commercia l versus DIY Current Probes (Wire Loop)
100
90
80
70
60
SO
40
30
20
10
I I I I I I I I I I I
— Commercial Probe — DIY Toroic #1
" DIY Torolo *2 »> DiY Square
— DIY Rounc
o o o o o o J-i J1 J\l
Frequency (MHz)
Figure W. Probe output voltage (V^J graph of a commercial current probe versus two DIY toroidal probes and two different clamp-on probes. The x-axis is frequency, while the y-axis is dBuV. This shows that all these probes are very comparable in output voltage versus frequency and therefore, useful for troubleshooting purposes. Just don't try using the DIY probes to determine "pass or fail" predictions. Commercial probes are better-suited for that.
sultants.com/techtips/tips-cm.html
[8] Clayton Paul, Introduction to Elec
tromagnetic Compatibility (2nd Edition),
Wiley Interscience, 2006, pages 518-532.
[9] Ridao, Carrasco, Galvin and Fran-
quelo, Implementation of low cost current
probes for conducted EMI interference mea
sure in Power Systems, EPE 1999 (Lausanne).
[10] H. Ward Silver, Hands-On Radio
column. Detecting RF - Part 2, QST, August
2011, page 54-55.
[11] Doug Smith, Current Probes, More
Useful Than You Think, IEEE EMC Sympo
sium 1998, http://emcesd.com/pdf/iprobe98.
pdf.
[12] Doug Smith, The Two Current Probe
Puzzle, http://emcesd.com/tt061999.htm.
[13] Doug Smith, Using Current Probes
to Inject Pulses for Troubleshooting, http://
emcesd.com/tt2007/ttl20307.htm Part 1,
Looking from 500 to 1000 MHz
R••••••••• 3!SiS«:SS •iSiiSBi!
iiiHHwaptnMOl
Tost setup. Current probe on USB cable. Connection between connector ground shell and chassis enclosure made with screwdriver blade.
Before After
Some harmonics dropped by 10-15 dB!
Figure 11. Cables should be tested individually. Here, I have a current probe clamped around the cable under test and am monitoring the harmonics with a simple hand-held spectrum analyzer. As I ground the connector shell to the chassis with the Swiss Army screwdriver blade, the harmonics were reduced 10-15 dB!
Figure 12. When measuring two cables from a system and the harmonic currents are approximately the same (point I is the same as point 2), the source is at the center (the BUT) and the two cables are acting as a dipole antenna. You may notice a peak in harmonic strength at the half-have length of the two cables combined. If the harmonic currents are larger in one side or the other, then you'll want to troubleshoot just that cable.