A probe for the measurement of electrical unbalance of networks and devices

IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 41, NO. 1, FEBRUARY 1999 3

A Probe for the Measurement of ElectricalUnbalance of Networks and Devices

Ian P. Macfarlane,Senior Member, IEEE

Abstract—A probe has been developed to measure residualelectrical unbalance of nominally balanced networks and devices.In the form described here, it makes convenient the use of single-ended 50 instruments to measure electrical unbalance of wirepair networks and two-terminal devices. For electromagneticcompatibility (EMC) planning purposes, a knowledge of theelectrical unbalance at signal interfaces in wire pair networksgenerally allows quantitative prediction of the conversion ofwanted differential mode signals into unwanted radiated distur-bances which can interfere with reception of radiocommunicationservices. The probe was first described to the CISPR in 1988.Since then, several other groups have also found it useful for mea-suring the electrical unbalance of telecommunication networksand devices. It has also been used to measure the differentialmode signals and common mode disturbances at signal interfaceson balanced data signaling networks.

Index Terms—Balanced conductor pairs, balanced networks,differential mode to common mode conversion, electrical balance,electrical unbalance measurement, longitudinal conversion loss(LCL), transverse conversion loss (TCL), twisted pairs, two-terminal devices.

I. INTRODUCTION

T HIS paper describes a probe which has proved very usefulfor measuring the electrical unbalance at signal interfaces

of wire pairs, in particular, twisted pairs and nominally bal-anced terminal devices. Originally described by the author ina 1988 contribution [1] to CISPR1 Subcommittee G WorkingGroup 2, it was developed to assist that Working Group’sconsideration of measuring methods at balanced signal portsof information technology equipment, for a revision of CISPRPublication 22 [2]. Use of the probe has been reported byseveral groups [3]–[5] to measure the electrical unbalance ofterminal equipment and, in particular, twisted pair networks(see Section V). It has been used to measure the longitudinalconversion loss (LCL) parameter (see Section II) originallydefined by the CCITT [6], [7], and—more recently—an alter-native parameter of electrical unbalance called the conversionadmittance , which has been defined by Goedbloed [3].

Nominally balanced differential mode2 signals [3], [4],[8]–[11] (also described as transverse [6], [7], symmetrical [8],

Manuscript received October 16, 1997; revised September 15, 1998.The author is an EMC Consultant at North Ringwood, Victoria, 3134

Australia.Publisher Item Identifier S 0018-9375(99)01529-X.1Comite International Special des Perturbations Radioelectriques (Interna-

tional Special Committee on Radio Interference).2The International Electrotechnical Vocabulary [10] at 161-04-08 defines

differential mode voltage (or symmetrical voltage) as, “The voltage betweenany two of a specified set of active conductors.”

[10], or metallic [9] signals) are used to transmit informationon unshielded conductor pairs in many digital and analoguesystems. When perfectly balanced differential mode currentsflow on a pair of closely spaced conductors the two currentsare equal in amplitude and have a 180phase difference.The fields radiated by the two closely spaced currents cancel(except in the very near-field region where cross talk toother nearby conductors may take place), and such perfectlybalanced conductor pairs do not create significant levels ofunwanted radiated disturbances. However, unwanted electricalunbalance at the nominally balanced signal generator, or onthe pair of conductors, or at the receiver, converts a portion ofthose differential mode signals to unwanted common mode3

signals [3], [4], [8]–[10], (also described as longitudinal [6],[7], [9] or asymmetrical [8], [10] signals). Residual commonmode current on a group or bundle of conductors can bedefined as the phasor sum of the currents flowing in theindividual conductors, taking account of the amplitudes andrelative phase angles of those individual currents. The commonmode current flowing on a cable bundle can also be veryaptly described as an “antenna mode” current because it isanalogous to the current generated on a wire antenna by a radiotransmitter. The common mode current similarly producesa radiated electromagnetic field. Unwanted common modecurrents produce unwanted radiated disturbances, which caninterfere with the reception of radiocommunication servicesof all kinds.

The concept of the LCL probe and a method of cali-bration for its original purpose—the determination of theelectrical unbalance of two-terminal devices by measuringtheir LCL—are described. A knowledge of LCL, as a mea-sure of electrical unbalance of nominally balanced pairs andterminal equipment, allows estimates to be made of howmuch conversion from a wanted differential mode signalto an unwanted common mode disturbance will take place(see Appendix A). Paradoxically, the measurement of LCLactually involves the reverse process, i.e., it measures theamount of differential mode signal produced at a pair ofnominally balanced terminals by conversion, through electricalunbalance, of an applied common mode signal. However, itprovides a useful measure of the unbalance present at theterminals. A precise definition and a description of LCL areprovided in Section II.

3The International Electrotechnical Vocabulary [10] at 161-04-09 definescommon mode voltage (or asymmetrical voltage) as, “The mean of the phasorvoltages appearing between each conductor and a specified reference, usuallyearth or frame.”

0018–9375/99$10.00 1999 IEEE

4 IEEE TRANSACTIONS ON ELECTROMAGNETIC COMPATIBILITY, VOL. 41, NO. 1, FEBRUARY 1999

Maximizing the LCL of balanced transmission media andterminal equipment (i.e., minimizing the amount of unwantedmode conversion that will take place) helps to minimize theinterference caused to radio services by differential mode tocommon mode conversion of fast digital signals, which caninclude, for example, basic rate ISDN carrying 192 kb/s, 10Mb/s Ethernet and 10Base-T, 16 Mb/s Token Ring, and otherhigh-speed signal transmission protocols. By reciprocity, ifthe unbalance mechanisms are linear, maximizing the LCL ofbalanced networks also proportionally improves their immu-nity to the common mode disturbances induced by ambientelectromagnetic fields, by minimizing their conversion tounwanted differential mode disturbances. In the case of twistedpair media, minimal specifications for LCL of commonlyavailable categories of unshielded twisted pairs and shieldedtwisted pairs are now available [12].

The LCL probe in the form described here is optimized foruse with balanced devices or conductor pairs having 100transverse characteristic impedance. However, the reader willfind it easy to deduce the minor changes of component valuesrequired to make LCL measurements of conductor pairs orbalanced devices having other common values of characteristicimpedance, for example 120 or 150 .

The measurement of transverse conversion loss (TCL) [6],[7] using the probe is also described, see Section IV. The mea-surement of TCL involves the measurement of the commonmode disturbance created at a pair of terminals by an applieddifferential mode signal. Of course, in general practice, TCLmeasurements are limited in application by the necessity tomake them in a shielded measurement environment. This isbecause the (desirably small) common mode disturbance to bemeasured will be obscured by ambient electromagnetic signalsand noise received as antenna mode disturbances at the deviceunder test if the measurement is not made in an electricallyquiet environment. This point is also made by Goedbloed in[3] and is discussed further in Section IV-A.

Using the component values mentioned in this paperLCL/TCL probes have been constructed that have a usefulmeasurement frequency range from less than 40 kHz tomore than 30 MHz. Appropriate scaling of the inductancevalues used in the probe allows coverage of other frequencyranges (see Section V). In general, increased inductance valuesare required to extend measurements to lower frequencies.Physically small inductive components, together with verycareful attention paid to minimizing parasitic capacitancesand leakage inductances, are necessary if one wishes toextend measurements to the higher frequencies mentioned inSection V while maintaining the inherently high self balanceof the probe.

II. L ONGITUDINAL CONVERSION LOSS

The probe allows the measurement of LCL and TCL inthe manner specified by the CCITT (now the ITU-T) inRecommendations G.117 or O.9 [6], [7]. In this section, wedeal with the LCL. According to the ITU-T the measurementof LCL is to be made in accordance with the circuit diagramshown in Fig. 1.

Fig. 1. ITU-T circuit diagram defining the LCL measurement method. Thedashed component is only required if the two-terminal equipment undertest is a bridging device which does not provide the necessary transversecharacteristic impedance termination (see [7, Section 3.2]). See the text inSec. II for (1) and (2), which define the LCL. (Adapted from the ITU-T’sFig. 4/G.117.)

Referring to Fig. 1, the ITU-T defines the longitudinalconversion ratio as

(1)

and the longitudinal conversion loss (LCL) as

longitudinal conversion loss (LCL)

dB.

(2)

It can be seen from the equations and Fig. 1 that the LCL isa measure of the amount of transverse (or differential) modevoltage converted from the applied common mode voltage

by the electrical unbalance of the two-terminal deviceunder test.

The center-tapped inductance depicted in Fig. 1 representsan ideal auto transformer. Fig. 2 illustrates how, if desired, theauto-transformer network in Fig. 1 can be transformed into asimpler equivalent network of discrete impedances (normallyresistive), shown in Fig. 2(c).

Consider an example where the transverse impedanceis100 . A carefully balanced resistive network together witha single-ended signal generator and balanced voltmeter canbe used to perform LCL measurements, using the discreteimpedance network depicted by the final network transforma-tion in Fig. 2(c)4. Indeed, our first measurements of LCL usedsuch a simple resistive network. However, it can be difficultto create, and maintain at frequencies up to 30 MHz, the veryclose matching (including the degrading effects of parasitic Land C) that is required of the two impedances, and theextremely good balance required of the wide-band balancedvoltmeter used to make measurements across terminalsin

4Note that the ITU-T definition of LCL and the network transformationsillustrated in Fig. 2 do not seem to have been strictly observed in thedevelopment of the LCL test procedures described in Annex A of ISO/IEC11 801:1995 [12].

MACFARLANE: PROBE FOR THE MEASUREMENT OF ELECTRICAL UNBALANCE OF NETWORKS AND DEVICES 5

(a) (b) (c)

Fig. 2. Transformation of the auto-transformer network in Fig. 1 into a simpler discrete impedance network. The transformation from (a) showing in (b) thecancellation of the generator impedanceZ=4 with the transformed�Z=4, leading to the probe illustrated in (c), also indicates why the source impedanceof the EL voltage generator used for LCL measurement was originally specified by the CCITT to beZ=4.

Fig. 3. LCL probe schematic circuit diagram and the measurement connections required to make an LCL measurement. Note that switch S1 remains openwhen making LCL measurements (see the text referring to S1 andR3 in Sections III-A and IV-A). ResistorR6 represents the transverse impedanceof the balanced device or conductor pair under test.

order to achieve the high value of probe self LCL needed whenmeasuring the LCL of well-balanced devices and conductorpairs (see Appendix A). It should be borne in mind thatthe self LCL of the measuring probe (including the balancedvoltmeter) should be at least 10 or 20 dB greater than theLCL that is to be measured. The LCL probe described in thispaper only requires the use of readily obtainable single-endedsignal generators and voltmeters and inherently self-balancinginductive components.

III. LCL PROBE DESCRIPTION

A schematic diagram of the complete LCL probe, with themeasuring equipment connections necessary for making LCLmeasurements, is shown in Fig. 3.

The transformers in the probe, T1, T2, T3, and T4, arewound as transmission-line transformers on small ferrite

toroids using twisted pair transmission lines with, in this case,a transverse characteristic impedance of approximately 100.

The windings of transformers T1 and T3 are connectedas auto transformers. They offer a very low impedance tocommon mode signals (asymmetric mode, longitudinal mode,or antenna mode signals), while at the same time presenting ahigh impedance ( 100 ) to differential mode signals (sym-metric mode, transverse mode, or balanced signals). Becauseof the high impedance they offer to the differential modesignals impressed across them, such auto transformers may becalled differential mode chokes.5 The connection sense of theseauto transformers is signified by the elongatedZ symbol and

5The differential mode choke performs a function similar to that of adevice called a transformer exciting network (TEN). Induction neutralisingtransformers (INT’s) excited by TEN’s have been used for many decades bythe telephone industry to reduce power line interference in telephone networks[14]. The differential mode choke is also used in several impedance stabilizingnetworks (ISN’s) in [2].


thus, by allusion to the old TV serial “The Mark of Zorro,” theconnection sense can be referred to as the “Zorro” connection.Having a warped sense of humor, we have taken to referringto transformers used in this way as Zorro windings.

The Zorro winding T1 in Fig. 3 provides the center-tappedauto transformer function of the CCITT’s LCL measure-ment circuit depicted in Fig. 1. By the nature of its twistedpair transmission line construction, the Zorro winding isbifilar wound and—if its terminations are carefully broughtout—inherently self balancing. To achieve near-ideal perfor-mance of the Zorro winding, great care must be taken to ensureequal conductor lengths in the conductor pair and a center tapthat is as close to perfect as possible.

Transformers T2 and T4 are connected as current com-pensated chokes, also called common mode chokes [8], [11],[13], or longitudinal chokes [9], or neutralizing transformers[9], or noise chokes [14]. They provide a high impedanceto common mode signals and a very small attenuation ofdifferential (transverse) mode signals.

To extend the probe’s useable frequency range, at bothlow and high frequencies, the small toroidal ferrite coresused in the transformers had an initial relative permeabilityof at least 2000 or more. The high permeability helps tominimize the numbers of turns necessary to achieve the largeinductance values required for an extended low-frequency re-sponse. Minimizing the numbers of turns reduces the resultingparasitic capacitances of the windings, thereby also extendingthe high-frequency response. In the version of the probedescribed here each individual winding of T1 and T3 has aninductance of approximately 0.3 mH. Thus, the Zorro windingsof the auto transformers T1 and T3 provide a transverseinductance of approximately 1.2 mH. With careful attentionto the winding and termination details the residual commonmode inductance of the Zorro windings can be reduced to lessthan 20 nH. Ideally, of course, we would like it to be zero.The common mode chokes T2 and T4 in this version of theprobe provide a common mode inductance of approximately0.28 mH.

When constructing the LCL probe the components shouldbe assembled inside a metal box to minimize stray couplingsto the surroundings, for example, hand capacitance.

Coaxial connectors for the single-ended 50signal gener-ator measuring receivers or RF voltmeters or the 50coaxialloads, can be provided at the common mode input port atand at the differential mode output ports atand . A two-pin socket for connection to a nominally balanced device orconductor pair under test (for example a balanced twisted pair)can be provided at the balanced test port, pointsand .

A. LCL Measurement

Referring to Fig. 3, the LCL of a two-terminal device canbe measured as follows. The signal generator injects acommon mode signal (corresponding with the ITU-T’s

, see Fig. 1), which is directly measured at the commonmode input port at point. is applied through and T1 tothe device or twisted pair under test connected between points

at the balanced test port. The Zorro winding T1 ideally

offers no attenuation to the common mode signal, but presentsa high transverse impedance ( ) to any transverse modesignal developed across . Note that the switch S1 acrossis left open during LCL measurements, so that providesthe source impedance specified for the generatorbythe ITU-T (see Figs. 1 and 2).

The dotted resistor 6 represents the 100 transversecharacteristic impedance of the device or conductorpair under test in this example. Therefore, to comply withthe ITU-T’s measurement requirements for the commonmode source generator impedance, is 25 .

Any transverse mode signal (corresponding with theITU-T’s , see Fig. 1) developed at the balanced test port

by unbalance of the device or conductor pair under testwill pass readily through current compensated chokes T2 andT4 and will appear across the differential mode output ports

. Conversely, the current compensated chokes T2 and T4present high impedances to the common mode signal.The Zorro winding T3 presents a high transverse impedanceto support the transverse mode signal across , but itpresents a very low impedance path, less than 20 nH, forcommon mode signals to the reference ground. Therefore, thetransformers T2, T3, T4, and the loads and connectedto the reference plane, constitute a very effective attenuator ofthe common mode signal . Thus, the signal appearingat the receiver/voltmeter will be, ideally, half the transversemode signal created by unbalance of the test object at.And, ideally again, no residual portion of the common mode

will reach the differential mode output ports to contributeto at the measuring receiver.

The amount of attenuated common mode test signal thatleaks through to the measuring receiver together with anyresidual probe unbalance and the receiver noise level, deter-mines the ultimate sensitivity that can be achieved in detecting

during LCL measurements.The resistive loads and appear in series as a trans-

verse impedance across , by transformer action through T4,T3, and T2. Thus, and present a 100 characteristicimpedance to the device or twisted pair under test (and thedashed component shown in Fig. 1 is not required).

B. LCL Calibration

A method of calibrating the LCL probe, by attaching adevice with a known unbalance at the balanced measuringport, is also illustrated in Fig. 3. A known value of unbalancingimpedance can be connected to one side of the transverseload , as shown.

This kind of calibration provides an indication of themeasurement accuracy of the probe or, alternatively, it allowsan estimate to be made of the uncertainty associated withmeasurements made with the probe, i.e., it indicates thedifference that might exist between the LCL measured by theprobe and the true LCL of a two-terminal device under test.

After some tedious algebraic analysis of the circuit in Fig. 3it can be shown that the numerical LCL (which, in (2), is thequantity defined by the ITU-T) created by a device undertest composed of and —when measured with an ideal


Fig. 4. MeasuredEL and VT curves (including the adjustment toVT so thatVT = VP + 6 dB), measured with an example of the LCL probe,in the case of a two-terminal device under test having a true LCL calibration value of 26.1 dB (Rcal = 468:9 in Fig. 3). Note that the verticalscale intervals are 5 dB per division.

lossless LCL probe in the manner illustrated in Fig. 3—willbe given by

numerical LCL

(3)

where signifies in parallel with.

Note that the derivation of (3) involves the assumptionthat the inductive components (common mode chokes andZorro windings) are “perfect,” i.e., they each have infinitemagnetizing inductance, no leakage inductance, perfect elec-trical balance, no stray couplings with their surroundings,ideal center taps, no parasitic self capacitance, etc., and whenmaking the algebraic analysis it must be remembered thatwhile an ideal Zorro winding creates no common mode voltagedrop, its windings will support differential mode voltages (andit offers an infinite impedance to differential mode currents).

To calibrate the LCL probe, known values of andare attached at the balanced test port and then and

are measured. The measured LCL is obtained by insertingthe measured and (the measured is obtained fromthe measured by assuming that ) in (2). Themeasured LCL is then compared with the LCL calculatedtheoretically for and using (3). For example, withthe values , , ,and , the LCL of the unbalanced device comprising

with when calculated using (3) has a numerical valueof 20.26, expressed logarithmically as an LCL of 26.1 dB.

Using the above resistance values, the assumedand values which were measured using an example of thisLCL probe have been plotted over the frequency range 40kHz to 30 MHz in Fig. 4.

The measured LCL obtained at 1 MHz is 26.5 dB, calculatedby substitution of the measured voltage values in (2), orby observing the difference between the traces in Fig. 4, orby performing “trace arithmetic” to obtain a trace differencepicture of the LCL on the screen of the measuring receiverwhen it is used to measure and store both and . Thus,compared with the LCL of 26.1 dB calculated for the network

and using (3), the probe measurement at 1 MHzdisagrees with the calculated LCL by0.4 dB. Of course,measuring receiver errors have not been included so this figureserves as a guide only.

At 40 kHz, the measured LCL is 27.3 dB, which disagreeswith the calculated value by 1.2 dB. At 30 MHz, themeasured LCL is 27.1 dB, a disagreement of1.0 dB.

With the balanced test port terminals attached to abalanced 100 termination as depicted in Fig. 3, butwithremoved, the residual self-LCL limit of the probe—determinedby residual probe unbalance, leakage of to , and thereceiver noise—can be obtained, and an example has beenplotted in Fig. 5. It can be seen that the residual LCL of theprobe remained greater than 80 dB at frequencies betweenabout 100 kHz and 16 MHz, and greater than 60 dB at 30MHz. Note that the plotted curve is the measured with6 dB added, as is required to obtain the true LCL value.


Fig. 5. Residual self-LCL limit curve for the example of the LCL probe, which provided the calibration curves in Fig. 4. Note that the vertical scaleintervals are now 10 dB per division.

Fig. 6. ITU-T circuit diagram defining the TCL measurement method. Thedashed component is only required if the two-terminal equipment under test isa bridging device that does not provide the necessary transverse characteristicimpedance termination. See the text in Section IV for the (4) and (5) whichdefine TCL. (Adapted from the ITU-T’s Fig. 3/G.117.)

IV. TRANSVERSE CONVERSION LOSS (TCL)

The measurement of TCL has to be made according tothe schematic diagram and definitions provided in the ITU-TRecommendations G.117 or O.9 [6], [7]. They are reproducedhere as Fig. 6 and (4) and (5).

Referring to Fig. 6, the ITU-T defines the transverse con-version ratio as

(4)

and the transverse conversion loss as

transverse conversion loss (TCL)

dB. (5)

As was the case for LCL measurements, the center-tappedinductance depicted in Fig. 6 represents an ideal auto trans-former. And recall that Fig. 2 in Section II illustrates how theauto-transformer network can be replaced by an equivalentnetwork of discrete impedances (normally resistive).

The measuring equipment connections required to makeTCL measurements with the probe we are describing areshown in Fig. 7. Note that the only difference between theTCL probe and the LCL probe, as such, is that in the TCLprobe the switch S1 is closed and resistor is thereforebypassed.

A. TCL Measurement

TCL measurements are less frequently used and are muchless popular than LCL measurements. TCL measurements onlybecome practical if they are made in a shielded environment.The reason is not difficult to see. The determination of theTCL of a twisted pair or other balanced device generallyinvolves the measurement of very low levels of commonmode voltage . Such common mode signals are also knownas antenna mode signals for a very good reason. Unlessthe TCL is measured in an electrically quiet environment(e.g., inside a shielded room) the measurement results willbe contaminated by antenna mode voltages induced onto theconductor pair or equipment under test by ambient RF signals


Fig. 7. TCL probe schematic and measurement connections used to measure the TCL of the balanced device represented byR6. The only differencebetween the LCL probe (see Fig. 3) and the TCL probe is that in the LCL probe switch S1 is open and in the TCL probe it is closed. See the text inSections III-A and IV-A describing the purpose of switch S1 and resistorR3.

and other radiated electromagnetic disturbances (this pointis also made by Goedbloed in [3]). Nonetheless, for somepurposes a direct measurement of TCL sometimes becomesnecessary. For example, a set of TCL measurements mightbe specified in order to resolve a dispute over the amount ofmode conversion to be expected at a two-terminal device portintended for connection to a balanced pair network.

Referring to Fig. 7, the TCL of a balanced two-terminaldevice can be measured as follows. The signal generatorinjects a signal at the differential mode input port at point

which, by transformer action through T4, T3, and T2,creates the transverse mode signal at the balanced testport—terminals . If the transformers in the probe are idealand the device connected at presents a perfectly balanced100 , i.e., the impedance is made infinite (removed),the transverse voltage will be exactly half the generatorvoltage . The voltmeter will then read exactly half thebalanced test port signal voltage drop. This means that thevoltmeter connected at the differential mode input port-point

, to monitor the generator voltage , will read a voltagewhich is equal to one quarter of .

When the transverse signal is applied to a device orconductor pair under test any common mode signal createdby unbalance of the test object will pass through the Zorrowinding T1 to the common mode measurement port at point

where will be read by the voltmeter. Note that theswitch S1 is closed, and it bypasses resistorduring TCLmeasurements. Thus, at the common mode measurement point

, the load impedance across which is measured is 25( in parallel with ), as required by the ITU-T for a testobject having a transverse characteristic impedance of 100( in Fig. 6).

When comparing this method of TCL measurement with theITU-T method illustrated in Fig. 6, note that it is only in the

case of a perfectly balanced test object that correspondswith . See Section IV-B for a discussion of the relationshipbetween and when the test object is unbalanced.However, there remains a direct correspondence between the

and in Fig. 6 (the ITU-T circuit diagram) and theand associated with the TCL probe in Fig. 7.

The generator impedance and the voltmeterimpedance appear in series at as a balanced 100source impedance for the Thevenin equivalent generator atthe balanced test port, which develops the transverse voltage

across the test object.

B. TCL Calibration

A method of calibrating the probe for TCL measurements isdepicted in Fig. 7. A known value of unbalancing impedance

can be connected as shown, thereby introducing a knownunbalance to the test object connected at the balanced test port.

The calibration method described here provides an indica-tion of the measurement accuracy of the probe or, alternatively,an indication of the uncertainty that will be associated withmeasurements made when using the probe. It indicates thedifference which might be expected between the TCL mea-sured by the probe and the true TCL of a two-terminal deviceunder test.

It can be shown (once again after some tedious algebraicanalysis) that the numerical TCL (which in (5) is the quantity

defined by the ITU-T) created by a device under testcomposed of and —when measured in accordancewith the ITU-T method—will be given by

numerical TCL (6)

where signifies in parallel with .


Fig. 8. MeasuredVL andVT curves for a TCL probe (the plottedVT was obtained by adding 6 dB toVP on the assumption that2VP � VT , see the textin Section IV-B), when the TCL calibration value is 31.9 dB (Rcal = 468:9 in Fig. 7). Note that the vertical scale intervals are 5 dB per division.

The derivation of (6) involves the same assumptions of“perfect” inductive components used in deriving (3).

However, it must be noted that in the case of TCL measure-ments using the TCL probe in general, becausesome of the common mode current created by the presenceof flows in in a direction opposed to the transversemode current (generated by ) that flows in and .

It can also be shown that if the TCL probe is ideal andlossless then

(7)

so that in the case of , .However, in practice, the assumption that producesonly a small error. The error in measured TCL caused byassuming will remain dB if the actual TCLbeing measured is 24 dB; that is, when remains 175

.Note that in the special case of a single-ended test object

(which, by definition, is completely unbalanced) ,and then .

With the resistance values shown in Fig. 7, and an, the TCL of the unbalanced device comprising

with when calculated using (6) has a numerical valueof 39.5, which can be expressed logarithmically as a TCLvalue of 31.9 dB. We can, therefore, assume that with theexample resistance values, with less than 1-dBerror. Using a typical example of the TCL probe we havethereby obtained the plotted values of and shown inFig. 8. The plotted curve is actually the measuredadjusted by the approximate correction factor of6 dB.

From the difference between the plotted curves in Fig. 8 itis seen that the TCL measured by the probe is 32.7 dB at 1MHz, which disagrees with the calculated TCL by0.8 dB.

At 40 kHz, the measured TCL disagrees with the calculatedvalue by 1.4 dB. At 30 MHz, the disagreement is1.9 dB.However, 0.4 dB of these disagreements is contributed by theassumption that . From (7) above, it was calculatedthat the actual so that is actually only 5.6dB less than instead of the 6-dB correction factor, whichwe assumed for simplicity above. And, of course, measuringreceiver errors are not accounted for, so the numbers areincluded here only for the sake of guidance to the accuracythat may be achieved with the probe.

With the balanced test port terminals attached to abalanced 100 termination as depicted in Fig. 7 (with perfectbalanceachieved by removing the unbalance impedance)the residual self TCL of the probe—determined by the residualunbalance of the probe itself and the measuring receivernoise—can be obtained. A typical example of the result ofsuch a residual TCL measurement, measured in a shieldedenvironment, is shown in Fig. 9.

It can be seen that the residual self TCL of the probe is ofthe order of 90 dB or more at frequencies between about 100kHz and 16 MHz, and is almost 70 dB at 30 MHz.

V. APPLICATIONS SUMMARY

The LCL probe has proved to be very useful for themeasurement of electrical balance of nominally balanced two-terminal devices and, in particular, twisted pairs. When con-structed with the inductance values stated in Section III ithas been used over the frequency range from less than 40


Fig. 9. Residual self-TCL limit curve obtained with the TCL probe, which provided the calibration curves in Fig. 8. Note that the vertical scale intervalsare 10 dB per division.

Fig. 10. Measurement plots for the LCL measurement of a twisted pair in a quad telephone cable installed for use as a data transmission medium in alaboratory building. The measurement frequency range shown in this example is 90 kHz to 30 MHz. Physical length of the cable under test was 198 m. Thetwo pairs in the quad cable were each terminated in transverse impedances of 100, but were left unterminated to common mode signals. At the frequency of800 kHz, indicated by the marker on theVT plot, the measured LCL at the balanced test port interface to the pair under test is seen to beEL�VT � 48 dB.

kHz to more than 30 MHz. An example of the result of ameasurement of the LCL of a balanced twisted pair is shownin Fig. 10.

The pair under test was one of the two balanced pairs in astar quad telephone cable, 198 m in length, installed as part of a

data transmission network in a laboratory building. During thetest, both pairs were open circuited to common mode signalsat the remote end, and transverse terminated in 100. Themeasurement setup is illustrated in the simplified diagram inFig. 11.


Fig. 11. Simplified diagram of the measurement setup used to measure theEL andVT (VT = 2VP ) traces illustrated in Fig. 10. Note that an antenna,supporting common mode conduction and displacement currents excited by theVL generator, has been created by the combination of the protectiveearth conductor, the pair under test, and the ground.

The shallow dips in the measured trace occurred atfrequencies where the pair under test together with the pro-tective earth conductor behaved as a resonant antenna in themeasurement setup illustrated in Fig. 11. The dips are causedby the resultant common mode impedance of the pair undertest falling to values comparable with the effective 25-sourceimpedance ( in parallel with ) of the generator . Atresonance the antenna offers a low input impedance at thecommon mode input port where is monitored. Note thatthe antenna length comprises the length of the protective earthconductor together with the length of the cable under test.Those two lengths are unequal, in general, and the antenna is,therefore, usually being excited off center.

It can be seen that the LCL at the driven end of the pairunder test decreases to approximately 48 dB at the markerfrequency of 800 kHz. The bottom trace in Fig. 10 shows theself-LCL limit of the probe (the probe’s residual for thegiven , as described in Section III-B).

Other examples of the results of LCL probe measurementsof the LCL exhibited byin situ twisted telephone pairs, atfrequencies up to 2 MHz, are shown in Fig. 3 of van Maurik’spaper [4].

By minimizing parasitic capacitances and inductances, usingvery small toroidal ferrite cores, which have initial relativepermeabilities of 10 000 and more, some LCL probes havealso been constructed which are capable of measuring LCLvalues larger than 40 dB at frequencies as high as 300 MHz[15].

The probe has also proved useful as a general purposemeasuring tool for making electrically balanced two-terminalmeasurements. In addition to measuring LCL, it has been usedto measure the conversion admittance parameter definedby Goedbloed [3], and the results of his use of an LCL probeto measure on telephone subscriber lines at frequenciesup to 30 MHz are shown in [3, Fig. 10].

The LCL probe has been used to measure the differentialmode (symmetric) signals at balanced signal ports and thecommon mode (asymmetric) disturbances present at thoseports, including those induced by ambient transient distur-bances in telephone networks, during investigations of digitalsubscriber loop (xDSL) signals transmitted on telephone pairs[16].

Measurements of LCL of telephone pairs have also beenmade in the voice frequency (VF) range [16] using a versionof the probe in which the Zorro windings and common mode

chokes employed inductance values which were larger thanthose mentioned in Section III.

The version of the probe described here is suited for usewith 100 transverse terminal impedances, and is convenientfor use with signal generators and measuring receivers havingsingle-ended 50 ports. If necessary, the probe’s componentvalues can be altered to suit different operating impedances.

Finally, it should also be mentioned that the addition ofblocking capacitors at the balanced test port terminalsand

allows measurements to be made of the LCL and TCLof two-terminal devices and networks which incorporate dcpower-feeding arrangements.

APPENDIX A

As mentioned in the Introduction, unwanted common modeor antenna mode emissions created by unbalance of wanteddifferential mode or transverse signals must be limited ifinterference with the reception of radio signals of all kindsis to be minimized. The allowable level of transverse signalwhich can be transmitted without exceeding the common modedisturbance limits for balanced conductor pairs and signal ports[2] can be estimated by considering the amount of transversemode to common mode conversion created by LCL of thetransmitter, the receiver, or the transmission medium. Forexample, consider a nominally balanced signal port connectedto a nominally balanced twisted pair in a LAN or othertransmission network, depicted in Fig. 12.

Assume that the electrical unbalance of the combination isdominated by the unbalance of the twisted pair, which, in thisexample, is assumed to exhibit a smaller value of LCL thanthe transmitter, i.e., it is more unbalanced than the transmitter.Using the variables defined in Fig. 12, logarithmic LCL of thetwisted pair is given by6

LCL dB. (A.1)

Of course, in (A.1) can usually be ignored, becausefor balanced circuits in general. To obtain afeeling for the orders of magnitude involved, note that if

resistive and (for example, this canoccur at a common mode resonance frequency of the twisted

6Equation (A.1) is [4, eq. (11)], however (A.1) shows the correct minussign in place of the plus sign given in [4, eq. (11)].


Fig. 12. Equivalent circuit of a transformer-coupled balanced transmitterdriving a twisted pair with a nominal transverse characteristic impedanceZ0

of 100 .

pair line), then (A.1) shows that must be made less thanapproximately 0.2 to achieve a 60 dB LCL. Or, in the caseof the unbalanced test network depicted in Fig. 3, it can befound by substitution in (3) that must be made greaterthan approximately 25 k. If a capacitance is substitutedfor in Fig. 3 then, to obtain more than 60 dB LCL at30 MHz, a pF is required. And it is important torecognize that if or are associated with the measuringprobe instead of the device under test, then they determine theresidual self-LCL limit of the probe for LCL measurementpurposes.

The level of the common mode disturbances arising fromtransverse mode to common mode conversion created by theLCL of the twisted pair can be estimated approximately from

dB A dB V LCL (dB)

(A.2)

when estimating the common mode current caused by thetransverse signal voltage and

dB V dB V LCL (dB)

(A.3)

when estimating the common mode voltage caused bythe transverse signal voltage .

In the above equations, the impedance variables are:

• the common mode impedance presented by thetwisted pair (which, in this example, has the worst (small-est) value and, therefore, the dominant value of LCL);

• the common mode impedance presented by thetransmitter (which in this example has the higher LCL);

• the transverse or differential mode impedance atthe signal port.

The above expressions (A.2) and (A.3), which have beenderived from equations developed by van Maurik [4], assumethat both the transmitter and the twisted pair in the combinationpresent a transverse characteristic impedance of, as shownin Fig. 12.

By setting the common mode current or voltage variablesin the equations equal to the common mode current or volt-age disturbance limits respectively, the maximum permissibletransverse or differential mode signal levels can be estimated.

When making use of the above equations it must be recalledthat a common mode disturbance limit is a quantity that isspecified for comparison with disturbances measured in adefined bandwidth (for example, 9 kHz) using a specifieddetector function (quasi-peak, peak, or average). Therefore,for a given worst-case value of LCL the maximum allowedtransverse signal levels estimated using the above expressionsare those that are allowed to appear in the same bandwidthwhen measured differentially using the same specified detectorfunctions.

ACKNOWLEDGMENT

The author would like to thank S. Iskra, B. C. Gilbert, A.J. Cole, and W. S. Davies for their very useful discussions ofthis work.

REFERENCES

[1] CISPR/G/WG2 (Macfarlane) 3, “A probe for the measurement oflongitudinal conversion loss (LCL) and transverse conversion loss (TCL)in the frequency range 0.04–30 MHz,” Nov. 1988.

[2] CISPR Publication 22, 3rd ed.,Information Technology Equip-ment—Radio Disturbance Characteristics—Limits and Methods ofMeasurement. Geneva, Switzerland: CISPR, Nov. 1997.

[3] J. J. Goedbloed, “Aspects of EMC at the equipment level,” in12thInt. Symp. Electromagn. Compat., Zurich, Switzerland, Feb. 1997, pp.23–28 (supplement); also in Session 1D,IEEE Int. Symp. Electromagn.Compat., Workshop Tutorial Notes, Austin, TX, Aug. 1997.

[4] R. M. van Maurik, “Potential common mode currents on the ISDN Sand T-interface caused by cable unbalance,” inInst. Elect. Eng. 8th Int.Conf. Electromagn. Compat., Edinburgh, Scotland, U.K., Sept. 1992,no. 362, pp. 202–206.

[5] CISPR/G/WG2 (ad hoc ISDN/Macfarlane) 2, Longitudinal conversionloss (LCL) of in situ star quad telephone cables, Nov. 1988.

[6] CCITT (now known as the ITU-T), Recommendation G.117, “Trans-mission aspects of unbalance about earth,” Fascicle III.1, Blue Book,Geneva, Switzerland, 1989.

[7] CCITT (now known as the ITU-T), Recommendation O.9, “Measuringarrangements to assess the degree of unbalance about earth,” FascicleIV.4, Blue Book, Geneva, 1989; Recommendation O.121.

[8] J. J. Goedbloed,Electromagnetic Compatibility. London, U.K.:Prentice-Hall, 1992.

[9] H. W. Ott, Noise Reduction Techniques in Electronic Systems. NewYork: Wiley, 1976.

[10] IEC 50 (161), “International electrotechnical vocabulary, electromag-netic compatibility,” ch. 161, 1990.

[11] P. A. Chatterton and M. A. Houlden,EMC Electromagnetic Theory toPractical Design. Chichester, U.K.: Wiley, 1992.

[12] ISO/IEC 11801:1995, “Information technology—Generic cabling forcustomer premises,” reprint with corrections July 1995.

[13] T. Williams, EMC for Product Designers, 2nd ed. Oxford, U.K.:Newnes, 1996.

[14] R. Gundrum, Power Line Interference; Problems and Solutions.Geneva, IL: Lee’s Telephone ABC, Library Congress Cat. Card Number73-85629, vol. 14, 1982; .

[15] A. J. Cole and B. C. Gilbert, Telstra Res. Labs., Australia, privatecommunication, Mar. 1998.

[16] W. S. Davies, Telstra Res. Labs., Australia, private communication,1996.


Ian P. Macfarlane (M’77–SM’86) received thedegree from the Royal Melbourne Institute ofTechnology and the University of Melbourne,Victoria, Australia.

From 1959 to 1963, he worked on electronicdevelopment projects with Rola Co. Australia Pty.Ltd., and at the Australian Bureau of MineralResources, Geology and Geophysics. From 1963to 1996 he was with Telstra Research Laboratories,where he conducted research into time andfrequency standards, new semiconductor devices

and techniques in telecommunications, and electromagnetic compatibility oftelecommunication equipment and systems. Since 1996 he has been an EMCconsultant in private practice.

Mr. Macfarlane is a member of several Working Groups of the InternationalSpecial Committee on Radio Interference (CISPR) and Secretary of WG1of CISPR Subcommittee G, Interference Characteristics of InformationTechnology Equipment. He is a member of Standards Australia CommitteeTE/3, Electromagnetic Interference, and the Convenor of SubcommitteeTE/3/12, Information Technology Equipment. He is a Chartered ProfessionalEngineer and a Fellow of The Institution of Engineers, Australia, an Australiandelegate to Commission E of URSI, and is a member of the AustralianNational Committee for Radio Science.

A probe for the measurement of electrical unbalance of networks and devices

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