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

14 INTERFERENCE TECHNOLOGY EMC DIRECTORY S. DESIGN GUIDE 2 0 1 2

! 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).

16 INTERFERENCE TECHNOLOGY EMC DIRECTORY & DESIGN GUIDE 2 0 1 2

4 THE H F CURRENT PROBE: THEORY AND APPLICATION

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.

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Nuclear

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Space

Telecom

Wireless

MIL-STD, DEF-STAN

CISPR

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IEEE

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

18 INTERFERENCE TECHNOLOGY EMC DIRECTORY & DESIGN GUIDE 2 0 1 2

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Figure 3. Examples of commercial current probes.

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.

interferenceteohnology.com INTERFERENCE TECHNOLOGY 19

THE H F CURRENT PROBE; THEORY AND APPLICATION

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

interferencetechnology.com INTERFERENCE TECHNOLOGY 21

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!

http://emcesd.com/tt2009/tt030309.htm

Part 2.

. [14] Doug Smith, Using Current Probes

to Measure Cable Resonance, http://emcesd.

com/tt2008/tt010108.htm.

. [15] Doug Smith, Measuring and In­

terpreting the Relative Phase of Common

Mode Currents, http://emcesd.com/tt2008/

tt030208.htm.

• [16] Doug Smith, Using a Comb Genera­

tor with a Pair of Current Probes to Mea­

sure Cable Resonance, http://emcesd.com/

tt2009/ttll0709.htm.

• [17] Doug Smith, Using Current Probes

to Inject Pulses for Troubleshooting (Board

Resonances), http://cmce,sd.com/tt2010/

tt010110.htm.

[18] Doug Smith, Predicting Cable Emis­

sions from Common Mode Current, http://

emcesd.com/tt2006/tt030106.htm.

• [19] Douglas Smith, High Frequency Mea­

surements and Noise in Electronic Circuits,

Van Nostrand Reinhoid, 1993, pages 41-44,

159-182, 192-209.

• [20] Allen WoifT, Building a Ferrite Core

Antenna Current Probe, Technical Corre­

spondence, QST, August 2009, page 53.

[21] Kenneth Wyatt, Troubleshooting

Radiated Emissions Using Low-Cost Bench-

Top Methods, interference Technology

(ITEM) - E M C Directory & Design Guide

2011, May 2011, page 10-21, http://www.in-

terferencetechnoiogy.com/upioads/media/

VX'yatt-DDGU.pdf.

• [22] Kenneth XX'yatt, Troubleshooting

Radiated Emissions - Three Case Studies,

incompliance Magazine, October 2011,

http://www.incompliancemag.com/inde,\

php?option=com_coiitent&view=article&i

d=818:troubleshooting-radiated-emis,sions-

three-case-studies&catid=27:testing&item

id=136.

REFERENCES - SUPPLIERS [23] Fischer Custom Communications

(FCC), Phone: (310) 303-3300, Email: sales®

fischercc.com, Web: www.fischercc.com.

They provide a very wide range of HF current

probes - their specialty.

. [24] Laird Technologies, Web: http://

www.lairdtech.com/Products/EMi-Solu-

tions/. Tliey offer a complete line of ferrite

cores and chokes.

• [25] Pearson Electronics, Phone: (650)

494-6444, Email: sales@pearsonelectronics.

com, XX'eb: www.pearsonelectronics.com.

They have a good selection of probes.

22 INTERFERENCE TECHNOLOGY EMC DIRECTORY S. DESIGN GUIDE 2 0 1 2

W Y A T T

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.

[26] Rhode & Schwartz USA, Phone: (888)

837-8772, Email: info@rohde-schwarz.com,

Web: www.rohde-schwartz.us. They have a

very limited selection.

[27] Solar Electronics, Phone: (800) 952-

5302, Email: sales@solar-emc.com, Web:

www.solar-emc.com. They have a limited

selection.

[28] Teseq USA, Phone: (732) 417-0501,

Email: usasales@teseq.com, Web: www.

teseq.us. They have a very limited selection.

[29] Thurlby Thander instruments, Phone:

+44-1480-412451, Email: sales@tti-test.

com, Web: http://www.tti-test.com/contact-

tti.htm. They offer a low-cost handheld

spectrum analyzer for under $2,000 USD.

[30] Wurth Electronics Midcom, (605)

886-4385, midcom@we-online.com, www.

we-oniine.com. I used one of their large

ferrite cores for my DIY current probe.

KENNETH WYATT, SR. EMC Engineer, Wyatt

Technical Service.^ LLC, holds degrees in biology

and electronic engineering and has worked as

a senior EMC engineer for Hewlett-Packard

and Agilent Technologies for 21 years. He also

worked as a product development engineer for

10 years at various aerospace firms on proj­

ects ranging from DC-DC power converters

to RF and microwave systems for shipboard

and space systems. A prolific author and

presenter, he has written or presented topics

including RF amplifier design, RF network

analysis software, EMC design of products

and EMC troubleshooting techniques. He has

been published in magazines such as, RF De­

sign, EMC Design & Test, Electronic Design,

Incompliance, Interference Technology, Micro­

wave Journal, HP Journal and several others.

Wyatt is a senior member of the IEEE and a

longtime member of the EMC Society where he

serves as their official photographer. He is also

a member of the dB Society and is a licensed

amateur radio operator. Contact Wyatt at

ken@emc-seminars.com. His website is www.

emc-seminars.com.

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Interferencetechnology.com INTERFERENCE TECHNOLOGY 23