Far-End Crosstalk Reduction in Adjacent PCB Traces employing High/Low-Z Configurations

8/11/2019 Far-End Crosstalk Reduction in Adjacent PCB Traces employing High/Low-Z Configurations

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Far-End Crosstalk Reduction in Adjacent PCBTraces employing High/Low-Z Configurations

Mohammad Almalkawi #1, Zulfiqar Khan #2, Vijay Devabhaktuni #3, Charles Bunting *4 # EECS Department, University of Toledo, Toledo, OH 43606, USA.

[email protected]

[email protected]

[email protected]

* Department of Electrical and Computer Engineering, Oklahoma State University,

Stillwater, OK 74078, USA.4 [email protected]

Abstract This paper introduces a new High/Low-Z traceconfiguration to reduce far-end crosstalk in adjacent printedcircuit board (PCB) interconnects. The emphasis is made toreduce crosstalk without using additional PCB components in thedesign. Specifically, we employ stepped impedance elements ofuniform length on the victim traces to suppress high-frequencyelectromagnetic interference. The overall design is much simplerthan the usual low-pass filter configurations which are difficultto implement in the prototype testing. The proposed approachshows remarkably better results as compared to conventionalintervening guard trace schemes. Further, the proposedconfiguration can be combined with the conventional schemes toachieve even higher reduction in crosstalk.

I. INTRODUCTION Crosstalk in PCB traces is a well known EMC and signal

integrity problem. A typical scenario involves coupling of ahigh-frequency signal ( e.g. a digital clock) onto a lowfrequency trace ( e.g. a power or a ground trace). Since low-frequency traces are typically long traces routed on multiplelayers on a PCB, high-frequency interference may easilyradiate from multiple and/or little suspected locations on theprinted board. Therefore, it is usually very difficult todetermine the actual location of the radiating area on the board.Normally, the first line of defence is to mitigate the radiationsource by reducing the current strength, slewing the rise/falltime of the digital signal ( e.g. by adding series resistors orferrites [1]) or shortening/straightening the high-frequencytrace. Though such countermeasures may provide a limitedimprovement, they may not even be an option for a givendesign ( e.g. for signal integrity constraints).

Another way to reduce crosstalk is to exploit the inherentparasitic effects ( i.e. mutual capacitance and inductance)among PCB traces [2-4]. To this end, different configurations

of guard traces have been employed in between adjacentinterconnects [2, 5, 6]. However, routing additional traces maynot be possible in a crowded layout. Furthermore, crosstalkreduction provided by guard traces is limited and is usuallynot sufficient [2, 7]. Therefore, it is highly desirable to explorenew trace configurations that may provide better crosstalkreduction and can be routed in a crowded layout. One way isto employ the low-pass filter (LPF) configuration on thevictim traces. A typical LPF employs multiple stepped

impedance elements on the victim trace with much betterperformance compared to that achieved by using guard tracesalone. However, since stepped elements have non-uniformlengths and widths, it is very difficult to implement differentLPF configurations on PCB traces in a prototype testing.

To this end, this paper introduces a new High/Low-Z

victim trace configuration to reduce far-end crosstalk couplingbetween two adjacent PCB traces. The far-end crosstalk is ofparticular interest due to the significantly stronger coupling ascompared to that at the near-end [8]. The proposedconfiguration has LPF characteristics [9] as it employsstepped impedance elements. However, here elements haveuniform lengths and, therefore, are easier to implement in theprototype testing in EMC pre-compliance/compliancechambers. Furthermore, the proposed configuration showssimilar performance to the LPF design and can be combinedwith the guard traces for even better results.

The conventional and proposed crosstalk reductionconfigurations are given in Section II. Section III comparesthese PCB configurations in terms of crosstalk reduction inadjacent PCB traces, while conclusions are given in SectionIV.

II. CROSSTALK REDUCTION SCHEMES Fig. 1 illustrates a PCB structure (40 mm x 40 mm) with

two adjacent traces that are spaced 8 mm apart. Both tracesare 3 mm wide and 40 mm long. As illustrated, the PCBstructure is supported by a 1.575 mm thick FR4 substrate.

We assume that one trace (acting as an aggressor) iscarrying digital or high-frequency signals. These signals maycouple onto the adjacent trace (hereby referred as the victimtrace) carrying low-frequency or DC currents. To reduce thiscoupling, referred to as crosstalk, various configurations can

be employed. Below, we discuss conventional configurationsas well as our proposed configuration.

A. Guard Trace for Far-End Crosstalk ReductionAs illustrated in Fig. 2, a guard trace is usually introduced

(as a shield) in between the aggressor and the victim trace. Foradditional shielding, one may also ground the guard trace with

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Fig. 1. PCB structure with two uniform adjacent traces.

Fig. 2. PCB structure with a guard trace between two uniform paralleltraces.

via fences (see Fig. 3). In this paper, we will assume that theguard trace is 1 mm wide and is terminated at both ends by thematched impedance ( i.e. 87 ). We further assume that thespacing between each via on the guard trace is 2 mm.

B. Low-Pass Filter ( LPF ) ConfigurationIf victim trace is a low-frequency or a DC trace, one mayemploy a stepped impedance LPF configuration, i.e. usingelements of high and low impedance alternatively on the trace(see Fig. 4). Length Hil of the high impedance ( H Z ) elementcan be calculated using the electrical length defined as [9],

H Hi Z

LRl 0= , (1)

where 2= , corresponds to the cut-off frequency ofthe LPF, and 0 R represents the feed trace characteristic

impedance. Similarly, length Lil of the low impedanceelement ( L Z ) can be calculated as,

0 RCZ

l L

Li =

, (2)

where L and C are the lumped element values [10]. Thewidth of elements is chosen corresponding toimpedances H Z and L Z .

It is worth mentioning that the difference between the twoimpedances should be as high as possible for an excellentfilter performance ( i.e. sharper cutoff). For higher crosstalkreduction, LPF configuration can also be employed with guard

Fig. 3. Guard trace with ground vias.

Fig. 4. Stepped impedance configuration with 5 elements.

Fig. 5. Stepped impedance configuration along with a guard trace.

Fig. 6. Stepped impedance configuration with a guard trace grounded by vias.

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traces (Fig. 5) and grounded guard traces (Fig. 6). However, inour case, sharper cutoff is not an essential requirement. Fig. 7illustrates an example of a 5 element LPF design. Here, wechose H Z = 120 , L Z = 20 , and 0 R = 50 . As well, weselected a cutoff frequency of 4 GHz and 7 dB insertion lossat 4.4 GHz. The calculated values using (1) and (2) are givenin Table I.

Fig. 7. A 5 element LPF design.

TABLE I 5 ELEMENT LPF DESIGN FOR A 4 GHZ CUT-OFF FREQUENCY .

ElementValue

Impedance( )

Electrical Length

Length (mm)

Width (mm)

C 1=0.618 Z L=20 14.1630 lL1=1.492 w L=11

L2=1.618 Z H =120 38.6260 lH2=4.657 w H =0.4

C 3=2.000 Z L=20 45.836 0 lL3=4.828 w L=11 L4=1.618 Z H =120 38.626

0 lH4=4.657 w H =0.4C 5=0.618 Z L=20 14.163

0 lL5=1.492 w L=11

C. Proposed High/Low-Z ConfigurationThe proposed High/Low-Z configuration (Fig. 4) is based

on the LPF design. However, unlike LPF, the proposedstructure employs uniform lengths ( il ) for all elements. Thislength may be selected to lie within the minimum and themaximum values calculated for the LPF design. For example,for the 5 element design given in Table I, the length for anelement in the proposed configuration can be selected between1.492 mm and 4.828 mm. Choosing a particular length

uniformly for all elements will affect the cutoff frequency andthe return loss of the filter. However, since we are assuminglow-frequency or DC signals on the victim trace, cutofffrequency and return loss are not the primary concerns. Fixingthe element lengths simplifies the LPF design procedure andmakes it easier to implement it in EMC prototype testing ( e.g. using copper tape). Furthermore, selecting a particular numberof elements is arbitrary and affects the performance of theproposed configuration in terms of crosstalk. In ourinvestigation, however, we have selected 3, 5, 7 and 9elements for comparison with the LPF and guard traceconfigurations. The proposed configuration was alsoinvestigated by employing the guard traces (with and withoutvia fences) between the traces.

III. RESULTS AND D ISCUSSIONS

Both conventional and proposed PCB configurations weresimulated using Ansoft HFSS to calculate the far-end crosstalk(S 41) over a frequency range from 0 to 6 GHz. Below, wepresent a comparison of the calculated results.

First, the crosstalk between adjacent PCB traces (Fig. 1)using conventional guard trace (Fig. 2) and guard trace with

vias (Fig. 3) was calculated. The results, plotted in Fig. 8,show that the guard trace alone does not provide anysignificant improvement in the crosstalk reduction. Similarly,using the guard trace with via fences shows a slight butinsignificant improvement in crosstalk reduction.

Frequency (GHz)

0 1 2 3 4 5 6

S 4 1 ( d B )

-80

-70

-60

-50

-40

-30

-20

-10

0

Uniform adjacent parallel traces (Fig. 1)With guard traces (Fig. 2)With guard traces grounded by vias (Fig. 3)

Fig. 8. Comparison of far-end crosstalk in adjacent PCB traces using guardtrace and guard trace grounded with vias.

Frequency (GHz)0 1 2 3 4 5 6

-80

-70

-60

-50

-40

-30

-20

-10

0 Uniform adjacent parallel traces (Fig. 1)3 elements proposed design5 elements proposed design7 elements proposed design9 elements proposed design

S 4 1 ( d B )

Fig. 9. Far-end crosstalk in PCB traces using the proposed configuration.

Next the performance of the High/Low-Z configuration(Fig. 4) employing 3 mm long elements was investigated. Ascan be seen in Fig. 9, the proposed design provides significantreduction in crosstalk as compared to guard traceconfiguration (Fig. 8). Moreover, it can be observed that thecrosstalk can be further reduced by increasing the number ofelements in the proposed configuration.

We compared the performance of the proposed scheme withthe conventional LPF design. To this end, Fig. 10 provides acomparison between a 5 element LPF design and a 5 elementproposed design. Results show relatively similar performancefor both designs in terms of far-end crosstalk. Moreover, thereturn loss ( S 33) levels are acceptable for the proposedconfiguration. A comparison of the near-end coupling, asplotted in Fig. 11, shows that the proposed configurationprovides crosstalk improvement similar to the LPF.

A 9 element High/Low-Z configuration along with theguard traces was also investigated. Fig. 12 shows that adding

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Frequency (GHz)

0 1 2 3 4 5 6

S 4 1 &

S 3 3 ( d B )

-80

-70

-60

-50

-40

-30

-20

-10

0

S41 for the Proposed designS41 for the LPFS33 for the proposed designS33 for the LPF

Fig. 10. Comparison of the proposed configuration with the LPF.

Frequency (GHz)

0 1 2 3 4 5 6

S 3 1 ( d B )

-80

-70

-60

-50

-40

-30

-20

-10

0

S31 of the 5 elements LPF (Fig. 4)S31 of the proposed 5 elements (Fig. 4)

Fig. 11. Comparison of the proposed configuration with the LPF for thenear-end crosstalk.

Frequency (GHz)

0 1 2 3 4 5 6

S 4 1 ( d B )

-80

-70

-60

-50

-40

-30

-20

-10

0

9 elements Proposed design (Fig. 4)With guard traces (Fig. 5)With guard traces grounded by vias (Fig. 6)

Fig. 12. Proposed configuration combined with guard traces.

guard trace with vias helps, in general, reducing the crosstalk.However, employing guard traces without ground vias, in fact,increases the S41 coupling. Finally, 5-element High/Low-Zconfigurations with different element lengths were compared.The results, as plotted in Fig. 13, show that increasing the

Frequency (GHz)

0 1 2 3 4 5 6

S 4 1 ( d B )

-80

-70

-60

-50

-40

-30

-20

-10

0

li =1 mmli =2 mmli =3 mmli =4 mmli =5 mm

Fig. 13. Proposed configuration with different element lengths.

element length shifts the resonances towards the lowerfrequency values.

IV. CONCLUSIONS

A new High/Low-Z configuration employing uniformelements was investigated for reduced far-end crosstalk inadjacent PCB traces. The performance of the proposed designwas found similar to the LPF design. However, unlike LPF,the proposed approach is easier to implement in the EMCprototype testing. The study shows that employing stand-aloneguard traces with ground vias along with the proposedconfiguration helps reduce the far-end crosstalk.

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Far-End Crosstalk Reduction in Adjacent PCB Traces employing High/Low-Z Configurations