PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST · PAGE 10 • AUGUST 2004...

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FEATURE ARTICLE PAGE 10 • AUGUST 2004 MICROWAVE PRODUCT DIGEST How To Choose the Best Test Cable by Paul Tusini, Times Microwave W ith so many choices, how does one decide on the appro- priate test cables? Times Microwave introduces SilverLine Test cables, an all-around product designed for both production test and the lab, and priced to provide exceptional value. This article examines cable and cable assembly mechanical and electrical per- formance characteristics, and outlines a sound strategy for making the inevitable compromises typically required when choosing a product. In May of 2004, a test engineer from a well-known company called Times with the following 50 ohm test cable requirements: 1. Loss: <0.09db/ft @ 1 Ghz for 1 ft of cable (excluding SMA and N connectors) 2. Minimum bend radius: 0.75” (1.5” bend diameter) 3. Extremely flexible (low flex resistance or bending moment) 4. 250,000, 180 degree flex cycles before degradation 5. Good VSWR (“good” undefined) 6. Maximum frequency of operation: 4 GHz 7. High isolation (“high” not defined) 8. Low cost After summarizing the require- ments, the customer added: “I don’t want anything that’s too difficult to make because I need them right away.” Unfortunately, this individual failed to recognize that many of these require- ments conflict with each other to vary- ing degrees, necessitating compromise in one or more areas. Understanding the balance manufacturers must strike when building high performance coax can aid one in specifying a product that is practical to build and thus cost effec- tive. Since mechanical characteristics and RF performance are intertwined, it further helps to follow a pre-defined strategy for selecting the cable best suited for the application. The above example is a good place to begin our discussion. With the development of various manufacturing techniques that reduce attenuation with- out increasing diameter, cable makers imply that the lower the loss, the better the product. While not necessarily true (more on this later), it should be recog- nized that some applications do indeed require attenuation levels not easily achieved with flexible RG coax. Still, regardless of the need, many seeking a quality test cable begin with a require- ment for low loss as in this case. Cable attenuation generally increases with frequency but decreases as the diameter grows larger. Attenuation can also be decreased by raising the veloci- ty of propagation (Vp). Many flexible RG cables employ solid extruded PTFE (Teflon™) as the dielectric. With a Vp of approximately 70% (that is, signals travel through it at a speed equal to about 70% of that through a vacuum), solid PTFE coax allows one to make a consistent, relatively low cost cable with properties that are well understood. Knowing we have connector losses to add to the equation and attempting to keep costs down by using RG coax, our customer’s attenuation requirement can only be met with a cable such as RG393 (0.390” O.D) or larger. Several issues become immediately apparent. First, a cable this size will be very stiff indeed. Further, as a rule of thumb, one should limit the bend radius to ten times the outside diameter in a constant flexing application. Depending on braid construction, fre- quency of operation and performance requirements, the minimum bend radius can occasionally be reduced to 5 times the outside diameter. Beyond this, one risks permanently changing (stretching, then kinking) the braids, altering performance. 3.9 inches (almost 7.8 inches diameter) is signifi- cantly outside the maximum limit imposed by the customer. Second, RG393 is not well suited to SMA-sized connectors, making the attachment process more complex and perhaps yielding an unacceptable return loss. Finally, RG specifications do not call for particularly high shield- ing, so suppliers reduce the braid angle (that is, there is less wire contained in the braid per increment). While less wire generally equates to more flexibil- ity, another affect is reduced flex life because the braids move and loosen more easily. A search of RG cable specifications eliminates any other can- didates that won’t also require a signif- icant compromise in at least one area. We see above that increasing the cable diameter may be an unacceptable solution for achieving the desired attenuation. The next least costly method is to change the center conduc- tor and/or outer braid construction. Specifying a solid center conductor improves loss 10-15% depending on the frequency. However, many solid center conductors are silver plated cop- per clad steel; not the best choice for flexibility. Keeping in mind the flex requirement in our example, a high strand-count silver plated copper center conductor would be called for, but this is not optimum for attenuation. A typ- ical trade off is to replace the solid steel center conductor with more malleable (but solid) copper. Some of the flex life and flexibility of a stranded con- ductor is restored without negating the improvement in attenuation. Most popular RG cables use one or two layers of either tin or silver plated copper round wire braids for the outer conductor. When looking for improve- ments in outer conductors, manufactur- ers take their clues from copper jacket- ed semi-rigid cable. Ironically, for its core diameter semi-rigid is as low loss as as can be achieved using solid PTFE for the dielectric. Cable makers are constantly experimenting with new braid constructions and manufacturing methods in an attempt to achieve the ultimate flexible cable while mimick- ing the smooth, solid construction of copper jacketed semi-rigid cable. A common strategy to improve overall performance including attenua- tion is to supplement a single round braid with a 50% overlay helical wrap as the inner layer over the core. While providing a surface more conducive to RF propagation, helical wraps have the added benefit of increasing a cable’s shielding (isolation). However, this structure also has a weakness. RF sta- bility typically degrades more quickly with flexure. To help, many times a third, flat ribbon braid is added under the helical wrap. Applied against the dielectric, flat ribbon braids provide some of the smooth, regular surface of a helical wrap but hold up better under flexing. Individually or combined, flat ribbon and helical braid structures can improve attenuation up to 10%, depending on the frequency. In summary, three-braid-layer cables provide improved attenuation, increased shielding and longer flex life, albeit at the expense of bending moment (stiffness). However, close attention to braid tension and angle (angles are typically defined by “pics” per inch, or the number of times the braid wires cross per increment) and tightly controlled processes result in cables that also transmit at much high- er frequencies. They also maintain their RF performance with flexure much longer than RG cables depending on the frequency and the minimum bend radius required. The most involved method of lower- ing attenuation includes not only the braid improvements and center conduc- tor changes above but also changing the core construction. Expanded PTFE tape, foamed PTFE, spline PTFE and foamed polyethylene (PE) are all meth- ods by which manufacturers increase the percentage of air in the dielectric, thereby decreasing its dielectric con- stant. More air generally translates into softer, more pliable core and therefore more flexibility. To maintain the proper impedance, larger center conductors are needed. A larger center conductor reduces resistance and helps achieve lower attenuation. It is not unusual to find low loss cables that exhibit Vp’s of 90% or higher. These cables are much lower in attenuation and sometimes even smaller in diameter and more flex- ible than the higher attenuation, solid PTFE alternative. It would appear that low loss cable is the proper choice for our requirement above but again, there are trade-offs. The custom nature of low loss products makes using off-the-shelf connectors difficult at best and sometimes impossi- ble. It is likely connectors designed solely for the selected cable might be the only choice, potentially lengthening lead times and raising costs. Low loss core can be slightly hygroscopic, absorbing flux or cleaning solutions. Some low loss cores don’t strip as cleanly as solid core. These issues can complicate the attachment process, fur- ther increasing costs. The somewhat softer core can make achieving superior performance at higher frequencies consistently more difficult. For example, “dimpled” core from braids being applied too tightly will cause higher return loss. If the core is soft enough, braid components could move relative to each other more readily causing RF instability, particu- larly the phase or electrical length. The center conductor could move off-center and change the impedance (noticeable in some spline dielectric designs). Finally, low loss cable can potentially be more susceptible to damage from external physical pressure or other flexing dynamics. The higher the Vp, the more exacerbated are these issues. It may seem like low loss cable has too many problems to be considered, but in practice many of these issues might only be noticed at the very edge of the performance curve. Times LMR™, T-Flex™, SF™ and SFT™ series are prime examples of low loss and/or improved braid structure, high performance cables that squeeze the most from the loss budget yet provide Times, Cont on pg 42

Transcript of PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST · PAGE 10 • AUGUST 2004...

Page 1: PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST · PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST How To Choose the Best Test Cable by Paul Tusini, Times

FEATURE ARTICLEPAGE 10 • AUGUST 2004 MICROWAVE PRODUCT DIGEST

How To Choose the Best Test Cableby Paul Tusini, Times Micro w a v e

With so many choices, howdoes one decide on the appro-priate test cables? Ti m e s

Microwave introduces SilverLine Te s tcables, an all-around product designedfor both production test and the lab, andpriced to provide exceptional value.This article examines cable and cableassembly mechanical and electrical per-formance characteristics, and outlines asound strategy for making the inevitablecompromises typically required whenchoosing a product.

In May of 2004, a test engineer froma well-known company called Ti m e swith the following 50 ohm test cabler e q u i r e m e n t s :

1 . Loss: <0.09db/ft @ 1 Ghz for 1 ft of cable (excluding SMAand N connectors)

2 . Minimum bend radius: 0.75” (1.5” bend diameter)

3 . Extremely flexible (low flex resistance or bending moment)

4 . 250,000, 180 degree flex cycles before degradation

5 . Good VSWR (“good” undefined)6 . Maximum frequency of

operation: 4 GHz7 . High isolation (“high” not defined)8 . Low cost

After summarizing the require-ments, the customer added: “I don’twant anything that’s too difficult tomake because I need them right away. ”U n f o r t u n a t e l y, this individual failed torecognize that many of these require-ments conflict with each other to vary-ing degrees, necessitating compromisein one or more areas. Understandingthe balance manufacturers must strikewhen building high performance coaxcan aid one in specifying a product thatis practical to build and thus cost eff e c-tive. Since mechanical characteristicsand RF performance are intertwined, itfurther helps to follow a pre-definedstrategy for selecting the cable bestsuited for the application.

The above example is a good placeto begin our discussion. With thedevelopment of various manufacturingtechniques that reduce attenuation with-out increasing diameter, cable makersimply that the lower the loss, the betterthe product. While not necessarily true(more on this later), it should be recog-nized that some applications do indeedrequire attenuation levels not easilyachieved with flexible RG coax. Still,regardless of the need, many seeking aquality test cable begin with a require-ment for low loss as in this case.

Cable attenuation generally increaseswith frequency but decreases as thediameter grows larg e r. Attenuation canalso be decreased by raising the veloci-ty of propagation (Vp). Many flexibleRG cables employ solid extruded PTFE( Teflon™) as the dielectric. With a V pof approximately 70% (that is, signalstravel through it at a speed equal toabout 70% of that through a vacuum),solid PTFE coax allows one to make aconsistent, relatively low cost cable withproperties that are well understood.

Knowing we have connector losses toadd to the equation and attempting tokeep costs down by using RG coax, ourc u s t o m e r’s attenuation requirement canonly be met with a cable such as RG393(0.390” O.D) or larg e r.

Several issues become immediatelyapparent. First, a cable this size will bevery stiff indeed. Further, as a rule ofthumb, one should limit the bendradius to ten times the outside diameterin a constant flexing application.Depending on braid construction, fre-quency of operation and performancerequirements, the minimum bendradius can occasionally be reduced to 5times the outside diameter. Beyondthis, one risks permanently changing(stretching, then kinking) the braids,altering performance. 3.9 inches(almost 7.8 inches diameter) is signifi-cantly outside the maximum limit

imposed by the customer. Second, RG393 is not well suited to

SMA-sized connectors, making theattachment process more complex andperhaps yielding an unacceptablereturn loss. Finally, RG specificationsdo not call for particularly high shield-ing, so suppliers reduce the braid angle(that is, there is less wire contained inthe braid per increment). While lesswire generally equates to more flexibil-i t y, another affect is reduced flex lifebecause the braids move and loosenmore easily. A search of RG cablespecifications eliminates any other can-didates that won’t also require a signif-icant compromise in at least one area.

We see above that increasing thecable diameter may be an unacceptablesolution for achieving the desiredattenuation. The next least costlymethod is to change the center conduc-tor and/or outer braid construction.Specifying a solid center conductorimproves loss 10-15% depending onthe frequency. However, many solidcenter conductors are silver plated cop-per clad steel; not the best choice forf l e x i b i l i t y. Keeping in mind the flexrequirement in our example, a high

strand-count silver plated copper centerconductor would be called for, but thisis not optimum for attenuation. A t y p-ical trade off is to replace the solid steelcenter conductor with more malleable(but solid) copper. Some of the flexlife and flexibility of a stranded con-ductor is restored without negating theimprovement in attenuation.

Most popular RG cables use one ortwo layers of either tin or silver platedcopper round wire braids for the outerc o n d u c t o r. When looking for improve-ments in outer conductors, manufactur-ers take their clues from copper jacket-ed semi-rigid cable. Ironically, for itscore diameter semi-rigid is as low lossas as can be achieved using solid PTFEfor the dielectric. Cable makers areconstantly experimenting with newbraid constructions and manufacturingmethods in an attempt to achieve the

ultimate flexible cable while mimick-ing the smooth, solid construction ofcopper jacketed semi-rigid cable.

A common strategy to improveoverall performance including attenua-tion is to supplement a single roundbraid with a 50% overlay helical wrapas the inner layer over the core. W h i l eproviding a surface more conducive toRF propagation, helical wraps have theadded benefit of increasing a cable’sshielding (isolation). However, thisstructure also has a weakness. RF sta-bility typically degrades more quicklywith flexure. To help, many times athird, flat ribbon braid is added underthe helical wrap. Applied against thedielectric, flat ribbon braids providesome of the smooth, regular surface ofa helical wrap but hold up better underflexing. Individually or combined, flatribbon and helical braid structures canimprove attenuation up to 10%,depending on the frequency.

In summary, three-braid-layercables provide improved attenuation,increased shielding and longer flex life,albeit at the expense of bendingmoment (stiffness). However, closeattention to braid tension and angle

(angles are typically defined by “pics”per inch, or the number of times thebraid wires cross per increment) andtightly controlled processes result incables that also transmit at much high-er frequencies. They also maintaintheir RF performance with flexuremuch longer than RG cables dependingon the frequency and the minimumbend radius required.

The most involved method of lower-ing attenuation includes not only thebraid improvements and center conduc-tor changes above but also changing thecore construction. Expanded PTFEtape, foamed PTFE, spline PTFE andfoamed polyethylene (PE) are all meth-ods by which manufacturers increasethe percentage of air in the dielectric,thereby decreasing its dielectric con-stant. More air generally translates intos o f t e r, more pliable core and thereforemore flexibility. To maintain the properimpedance, larger center conductors areneeded. A l a rger center conductorreduces resistance and helps achievelower attenuation. It is not unusual tofind low loss cables that exhibit V p ’s of90% or higher. These cables are muchlower in attenuation and sometimeseven smaller in diameter and more flex-ible than the higher attenuation, solidPTFE alternative.

It would appear that low loss cable isthe proper choice for our requirementabove but again, there are trade-off s .The custom nature of low loss productsmakes using off-the-shelf connectorsd i fficult at best and sometimes impossi-ble. It is likely connectors designedsolely for the selected cable might bethe only choice, potentially lengtheninglead times and raising costs. Low losscore can be slightly hygroscopic,absorbing flux or cleaning solutions.Some low loss cores don’t strip ascleanly as solid core. These issues cancomplicate the attachment process, fur-ther increasing costs.

The somewhat softer core can makeachieving superior performance athigher frequencies consistently mored i fficult. For example, “dimpled” corefrom braids being applied too tightlywill cause higher return loss. If thecore is soft enough, braid componentscould move relative to each other morereadily causing RF instability, particu-larly the phase or electrical length. T h ecenter conductor could move off - c e n t e rand change the impedance (noticeablein some spline dielectric designs).F i n a l l y, low loss cable can potentiallybe more susceptible to damage fromexternal physical pressure or otherflexing dynamics. The higher the V p ,the more exacerbated are these issues.

It may seem like low loss cable hastoo many problems to be considered,but in practice many of these issuesmight only be noticed at the very edgeof the performance curve. Ti m e sLMR™, T-Flex™, SF™ and SFT™series are prime examples of low lossand/or improved braid structure, highperformance cables that squeeze themost from the loss budget yet provide

Times, Cont on pg 42

Page 2: PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST · PAGE 10 • AUGUST 2004 FEATUREARTICLE MICROWAVEPRODUCTDIGEST How To Choose the Best Test Cable by Paul Tusini, Times

excellent flexibility and high frequency performance.Going back to our example, it becomes clear that a

low loss cable may be required to meet or at leastapproach the loss budget but still meet the minimumbend radius/flexibility requirements. Trying to keepthe customer’s price and delivery targets in mind, wefirst look at catalog products starting with PE dielec-tric technology. . . . . Times LMR UltraFlex™ products.Because of its popularity in telecom and wireless,foamed PE cables and many connectors designed forit are readily available and very low cost. At 80-87%Vp foamed PE is also very low loss for its size. T h eUltraFlex™ series has a lower bending moment thanstandard LMR™ or generic foamed PE cables. A0.200” diameter cable calculates out to be 0.12db ofloss at the frequency of interest; very close to the cus-t o m e r’s requirement. While it appears we have theperfect solution, using our rule of thumb for minimumbend radius, this cable is too large in diameter.

Another product we consider is Times SFT316,but with a stranded center conductor. This is a 0.120”d i a m e t e r, expanded tape wrapped PTFE cable with a76% Vp. The softer PTFE core is more likely to meetthe flexibility and flex life requirements and comesclose to meeting the bend radius, but at 0.18db it iswell above the loss budget. LMR is a two-layer prod-uct while SFT is a high isolation, three-layer product.Each uses a different core technology. Clearlyachieving all the requirements in one product isn’t aseasy as the customer anticipates.

Gaining the Best Performance and Most Value From a Test CableThis example aside there are other aspects of a testcable that result in a superior product. Try to choosea connector series appropriately sized for the cable.One way to do this is to compare the cable centerconductor diameter to the connector center contactd i a m e t e r. Similarly sized components will havefewer transitions within the connector and at theattachment area, resulting in the best return loss.

Ty p i c a l l y, the connector/cable attachment area isthe first to fail so the strain relief system should pro-vide for a smooth transition from a rigid attachmentsection to the flexible cable. A strain relief systemthat does not hold the attachment area immobile orcreates a point of high leverage immediately behindit on the cable is ineffective and the assembly will failp r e m a t u r e l y. If possible, opt for solder attachment toboth the braid structure and center contact for longestlife and best electrical stability.

Unless the system or component under test haspassive intermodulation considerations, the connec-tors should be machined from a quality stainless steelversus brass to achieve long life. Using steel increas-es the mating life cycle count and reduces metal wearand metal particles from being imbedded in the con-nector dielectric, changing the impedance of the con-n e c t o r. If possible, SMA series should have a thickwall outer conductor design to reduce crushing in theevent a properly calibrated torque wrench is not used

during mating. Armoring a cable will help prevent

damage from crushing, over-bending orkinking, but generally flexible armor willnot prevent damage from torque....themost common reason for connector/cableattachment failure. Finally, the singlemost effective way to get the most valueand longest life from your purchase is bytraining the user in proper handling, careand maintenance of the product.

A Strategy for Choosing aSuitable Test CableOur customer’s elusive goals precipitat-ed several calls and emails over thecourse of about four weeks as the priori-ties on specifications and performancegoals changed. This is not uncommon.It’s natural to want the best in all areas.Still, to make sense of it all, following apre-defined strategy helps one quicklynarrow the selection of both vendors andproducts. We’ll start by categorizing themore common parameters in Table 1.

This list is not meant to be all-inclu-sive. However, not every parameter needbe considered for a test cable either.F u r t h e r, many are linked to each other.For example, power handling decreasesas altitude increases. If power is a primeconsideration and the chosen cable’smaximum power handling is close to theminimum requirements, one may need toconsider if the product will be used inhigh altitude areas vs. at sea level. Acable used largely outdoors will (orshould) have moisture ingress considera-tions and perhaps require ultra-violetresistance. Moisture resistance aff e c t sboth the choice of connector construction(internal and external sealing gaskets recommended)and cable (Times LMR-db water block braid con-struction recommended). UV resistance wouldrequire specific chemicals to be included in the jack-et material. The final application will determinewhich ones to include.

First, from the list of the parameters, choose thosethat m u s t be satisfied given the application. Next, setthe minimum or maximum specifications limits.Certainly it makes sense to guard band the limits, butd o n ’t go overboard. It causes needless expense andeliminates choices that might be perfectly suitable.Once this is accomplished prioritize the parametersfrom the most important to the least important, recog-nizing that those at the bottom of the priority list mayneed to be compromised.

Once this is accomplished identify the parametersand their min/max specification limits that are desir-able but not absolutely required, and prioritize theseas well. Why do this if the “must” list has been iden-

tified? Because it makesclear not only that forwhich you are willing topay extra but just asi m p o r t a n t l y, that forwhich you are unwillingto pay extra.

Consideration of themost suitable cable tech-nology is the first steptowards narrowing thevendor base. With a lit-tle research, determinewhich technology bestmeets the “must” cate-gory of parameters andgoals. In our example,if a 250,000 flex life is ahigh priority “must,”then vendors best

known for semi-rigid coax probably aren’t goingto offer many choices. This technology and thevendors that specialize in it can be eliminated.L a rger vendors like Times make cable using morethan one technology or material like solid andfoamed PE, solid and tape wrapped PTFE and noweven silicon dioxide, and therefore offer manymore choices under one roof.

In conclusion, remain realistic. Keep in mindthat generally, the more top-priority parameterswhich one insists be included and the more stringentthe specification the more exhaustive the search willtend to be and more costly and/or longer lead timethe item.

In the example used throughout this article, thiscustomer insisted attenuation, bend radius and flexlife all had to be the absolute best modern technolo-gy had to offer. While these were important, anexamination of critical needs revealed that the atten-uation for a one-foot long cable at 1 ghz was notsubstantially more in solid PTFE than low loss coreand the flex life need not be nearly so high or thebend radius so tight. Also revealed was that electri-cal stability with flexure, a parameter most oftenoverlooked, was paramount and that the typical fail-ure mode of the existing cable was that the connec-tors broke off before the cable wore out from flex-ure. SilverLine test cables met the customer’s trueneeds, addressed the typical failure mode, fell wellwithin budget constraints and were in stock forimmediate delivery. Today SilverLine test cablesare used throughout this customer’s test and repairfacility in various configurations and lengths.

Contact Times or your local, authorized Times dis -tributor for more information on SilverLine Te s tC a b l e s .

TIMES MICROWAV ECIRCLE READER SERVICE NO. 3

FEATURE ARTICLEPAGE 42 • AUGUST 2004 MICROWAVE PRODUCT DIGEST

Armored SilverLine with SMA, Type N and 7-16 Stainless Steel Connector Options

Times, Cont from pg 10

Table 1

Electrical Mechanical EnvironmentalFrequency Range Length Operating temp

rangeAttenuation (loss) Flexibility Operating

(bend moment) altitudeReturn loss (VSWR) Flex Life Moisture

resistancePassive IM Bend radius UV resistanceShielding (isolation) Outer Jacket or armorRF Stability Connector (with temp or flex) seriesPhase length Attachment methodPower handling Mating life cycleImpedance