TEST & MEASUREMENT FOCUS 42 a Table 1: Attenuation andvoltage ratios  Voltage Ratio Measurement Attenuators for x
Attenuation March 2019 (United States) Jonathan Novick By

Fresh From the Bench audioxpress.com B1 B1 B1 B2 B2 B3 B40dB 30dB 24dB 20dB 18dB 12dB 10dB 6 dB : .: : : 011: 21100:1 32:1 16:1 10:1 8:1 4:1 3.2:1 2:1 ratio of the input voltage to the output voltage, voltage, which can output be determined using Equation the 1: to voltage input the of ratio expressed in decibels (dB). ratings, voltage resistor choices, andand added measurement noise. handling power topology, designing attenuators for higher voltages, including s when to ﬀ consider only. are tradeo several There use lowvoltage for designed typically are they as application this in choice good a not attenuators usually are offtheshelf analyzer. audio commercial an However, of voltage input the maximum above signals measure to way ective ﬀ e cost This makes the Lpad and Upad topologies ideal ideal topologies Upad and Lpad the makes This attenuator with a low output impedance is desirable. an For audio needs. noise measurements, erent diﬀ to ering suit each ﬀ o erent attributes diﬀ topologies, Topology Attenuator Attenuation values are typically given as a ratio are typically values Attenuation and effective an are attenuators External Attenuators can be constructed using manyusing constructed be can Attenuators Attenuatio nL =− 20 handy to have multiple attenuators of similar attenuation but attenuator is of primary concern. For this reason, it can be At high test voltages, the heat dissipation in the external concern. added isofprimary thermalnoiseoftheattenuator have on measurements of distortion. At low test voltages, the compares the impact diﬀerent attenuators’ thermal noise will In this article, our author discusses external attenuators and with diﬀerent impedancesonhand. Table 1 Table og ⎝ ⎜ ⎛ V V output input represents the the represents ⎠ ⎟ ⎞ calculations used here can be appliedsame the toso is eitherThis case. values. Lpad the half as here conﬁgurations. The Upad resistor values are shown the and unbalanced in balanced to be both used Upad enables also point ground The systems. test useful to control common mode issues with ﬂoating be can that reference ground a provides separate be combined into a single resistor but keeping them (see Figure 1 signal balanced the of side one attenuating backtoback in a mirrored conﬁguration; each side using Ohm’s law: calculated be can attenuator an of voltage output a and The input. the audio analyzer across analyzer resistor shunt audio the and source signal the between attached resistor series a of consisting noise. than handling high voltages and minimizing added importance greater of are ﬂow signal bidirection and matching impedance where communications, in RF used primarily are variants their and Pipads output and input direction. specific Symmetric a topologies, such as Tpads have and they words, other In nature. in asymmetrical are attenuators These measurements. ampliﬁer power for choices The Upad is simply two identical Lpads is identical The Upad two simply placed An Lpad is a simple resistive divider network network divider resistive simple a is Lpad An L pad outputvol ). The two shunt resistors of a Upad can can Upad a of resistors shunt two The ). tage =× Vi n Rshunt Rshunt + Rserie s carboncomposition resistors have a higher noise noise higher a have and resistors thickﬁlm carboncomposition Carbonﬁlm, resistor. the across voltage the with varies and material resistive noise,ﬂow throughbythe theelectron is caused (Hz) Hertz in Bandwidth = BW Kelvin in degrees temperature T =Operating k = Boltzman’s constant, 1.38 × 10 1.38 constant, k =Boltzman’s where, R = Resistance value level of this noise is deﬁned as: RMS The voltage. noise the higher the resistance, the higher The value. of resistance the root square the to proportional is that resistor a by produced cost. as well as properties physical and electrical There are many stradeo ﬀ to be considered for both Choosing aResistor Type the design of higher impedance attenuators. discusses article this of remainder The accuracy. measurement impact may that noise excess add resistance values.higher However,using higherby resistance values reduced be can dissipation heat The overheat. to them causing exceeded, be could resistors these of rating power the voltages, high ohms and are comprised of 1/8 W or hundred 1/4 W Atresistors. few a just of impedance an have attenuators available attenuator’s commercially Most the of design. consideration careful more Load a Without Testing 12 dB, and 18 points (see dB attenuation dB, 6 to access still is there and Ω 8 is total resistance The resistors. Ω 0.5 eight having side each with Upad a using tested be should outputs pull resistors is attenuated 18 dB. Ampliﬁers with push eighth the and seventh the between node the and attenuation dB 12 provides resistors seventh the and sixth the between node The attenuation. 6 dB (between the fourth and the ﬁfth resistors) provides node center The value. attenuation differing a in the series. provides Each node resistors between for measurement. the is This recommended wayresistors. to attenuate an ampliﬁer output multiple using Upads or loads can These loads. also be conﬁgured as Lpads The Test Load asan Attenuator Current noise, sometimes referred to as 1/ƒ 1/ƒ as to referred sometimes noise, Current Thermal (Johnson) noise is the voltage noise noise voltage the is noise (Johnson) Thermal resistor. perfect a as thing such no is There requires load a without amplifiers Testing An 8Ω load can be made with eight 1 Ω resistors with resistive tested are often Power ampliﬁers VR =× 4 ×× kT
× BW 23
Figure Figure 2
). rated power insures the operating temperature staystemperature closer to the ambient temperature.operating the insures maximum power its rated below well resistor However, the failing. operating without operate can resistors as power transformers. sensitivefrom circuitsaway or strong placed magnetic be sources such should they used, If ﬁelds. can both emit and pick wound resistors up magnet tested. being device speciﬁc the for considered be should inductance acceptable maximum The value. and but vary from 0.1 to 400 Hµ depending on wattage is inductance actual a function of the method The construction types. resistor other than inductance heat tremendous handle loads, but they also exhibit can higher resistors Wirewound minimized. be in large arrays to minimizeused heating. unless recommended not are and signal in changes temperature within a nontrivial single cycle of a exhibit lowfrequency can mass thermal resistors. Small surfacemount resistors with low multiple from element series the element, constructing series or the in resistors wattage higher with using ppm), (50 ≤ cients ﬃ resistors coe using temperature by lower mitigated be can This measurements. ﬀ o throw can that attenuation in and shunt series resistors inthe an Lpad between can produce changes erences ﬀ di temperature to Heating changes. value resistor’s a of sensitivity purposes. measurement for avoided be should and types other than index with resistor type. Metalfoil resistors approach approach resistors Metalfoil type. resistor with to 6dB, 12dB, points. and18dBattenuation side having eight 0.5Ωresistors. resistance Thetotal Ω andthere isstill8 isstillaccess Figure shouldbetestedusingaUpadwitheach withpushpulloutputs 2:Ampliﬁers Power ratings are an indication of how hot hot how of indication an are ratings Power wire because occurs susceptibility Magnetic generally should attenuators in Inductance the indicates coefficient temperature The Cost and availability can vary significantly significantly vary can availability and Cost audioxpress.com the balancedsignal. onesideof attenuating conﬁguration—each side toback inamirrored Lpads placedback identical is simplytwo Figure 1:TheUpad 
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TEST & MEASUREMENT FOCUS TEST & MEASUREMENT FOCUS 44 a 141 V dissipation at40Vand Table 3:Lpad power combinations for20dB Table 2:Lpad resistor  0.18µV/135 dBv 0.057 µV/145 dBv 99 Ω 9.1Ω 20.08dB 20.08 dB 110Ω 11Ω 1,000Ω 100Ω 2 Case 1 Case 0.81µV/122 dBv 0.57µV/125 dBv 0.41 µV/128 dBv 1,982Ω 991Ω 504Ω 20.08dB 20.08dB 19.94dB 2,200Ω 20,000Ω 1,100Ω 560 Ω 5 Case 10,000Ω 4 Case 5,000 Ω 3 Case 1.23µV/118 dBv 4,601Ω 20.19dB 5,100Ω 47,000Ω 6 Case x
March 2019
Resistor Choices Resistor eisShunt Series

Practical Test & Measurement audioxpress.com can produce ~20 dB attenuation as well as the over the combination as that by well produced noise as thermal attenuation dB ~20 produce can that combinations resistor common some shows series attenuator.the the of of resistors shunt and combination parallel the simply is circuit equivalent Thevenin the impedance, output a negligible has test under ampliﬁer the Assuming resistance. equivalent of Thevenin its as a function levels. signal lower at particularly (THD+N), noise + distortion such as signaltonoise ratio (SNR) or total harmonic will be most evident in noiserelated measurements This the impact readings of measurements. certain Impact ofNoiseonaMeasurement cost. reasonable a at dissipation to power higher combinations handle parallel and series in wired be can These quantities. individual in each cents 30 to 15 approximately cost ppm/°C cient ﬃ 50 coe a temperature and resistors tolerance 1% W, Thinfilm 0.6 at rated attenuators. constructing for compromise available readily inexpensive, an can also be Availability a problem. Leaded thinﬁlm erresistors oﬀ types. times resistor 100 other to than up more many cost in can they resistor However, ideal ways. an of performance the of the product being tested. Suppose the desired desired the Suppose tested. being product the of level of the test and the residual noise signal and distortionthe accuracy, measurement needed the on at 27°C. bandwidth a 20 kHz Attenuation The thermal noise of an Lpad is determined determined is Lpad an of noise thermal The can attenuator an from noise thermal The The signiﬁcance of this thermal noise depends depends noise thermal this of signiﬁcance The .2 .0 .5W0.04W 0.09W 0.18W 0.35 W 0.81W 1.62 W 0.003W 0.007 W 0.014 W 0.028 W 0.065 W 0.130 W 5,100Ω 2,200Ω 1,100Ω 47,000 Ω 20,000Ω 10,000Ω 6 Case 5 Case 4 Case .5 .2 .3W0.36W 3.23 W 0.029 W 0.259 W 560 Ω 5,000Ω 3 Case .0W013W1. 1.8W 16.2W 0.143W 1.30 W 110 Ω 1,000Ω 2 Case ae1100Ω 1 Case
Actual
Resistance eisSutSre hn eisShunt Series Shunt Series Shunt Series Thevenin eitrCocsDsiaina 0VDissipationat141V Dissipationat40V Resistor Choices Thermal Noise 32 .2 6 16.5W 165W 1.322W 13.22W 10 Ω Table 2 Table
unbalanced connections. AverLAB has provisions provisions has AverLAB connections. unbalanced those limits to 3000 W for balanced and 750 W for it increases attenuation 20 dB of external ampliﬁers, tenuator, at an inputs or 7.5 W at the unbalanced inputs. For larger Without can test up to 30 W into (unbalanced). and an 8 Ω load at theVrms balanced(balanced) 7.7 Vrms 15.5 to limited was input maximum its highpowered small, AverLAB keep To measure ampliﬁers. to Avermetrics, from an designing when attenuator. I the new AverLAB, used audio analyzer needed tradeoffs of types ThistoWorkPutting determined. Since noise sources are uncorrelated uncorrelated are sources noise Since be determined. can attenuator the from noise allowable RMS the residual, this of V µ 40 producing is ampliﬁer the that Given V. µ 40.46 exceed cannot analyzer the by seen residual total the show to dB 99.9 for solved is 2 Equation dB. ≤99.9 be must reading distortion the dB, 0.1 is noise added to due error can be tolerated.noise attenuator added this of much how is question of own. The its noise much too added has a reading THD+N of 100 dB the unless attenuator show still will analyzer the V.µ The 40 just to reduce residual also will and range, input AverLAB’s the will signal reduce output to 4 V,Lpad which is dB well within 20 A distortion. and noise residual of V µ 400 is there that shows 4 and Equation power solving rated the at V 40 is output ampliﬁer The (0.001%). dB 100 of THD+N a with Ω 8 into Ampliﬁer W 200 a Measuring easy. measurements these making makes which attenuation external for measurements set ﬀ o to equation for THD+N: THD+N: for equation the by deﬁned is This distortion. is measurement T HD+N (indB)=20L The following two examples demonstrate the the demonstrate examples two following The If the maximum acceptable measurement measurement acceptable maximum the If W 200 at rated is ampliﬁer hypothetical This og ⎝ ⎜ ⎛ residual noiseanddistortion tota l si g nnal ⎠ ⎟ ⎞
Total noise Total equation: the summation, RMS in place. The same equations as above are used used again toﬁnd that allowable attenuatorare noise for above as equations same The place. in analyzer sees only 0.283 V with the 20 dB attenuator be tested. being level must lowest the at considered attenuator the of noise the Therefore, sweep. level automated and an in during needed as switched out similarly be cannot attenuator external a ﬁxed However, accuracy. maximum measurement input ensure to its changed level signal autorange the as normally audio would An power. analyzer maximum to amplitude low a V. 40 at measurements for true only is this However, ampliﬁer. the measure the of Any V. to µ cient ﬃ su in Table thus 2 are shown 6.1 attenuators is accuracy dB 0.1 for noise This shows that the maximum allowable attenuator is used to determine the allowable attenuator noise. attenuator noise attenuator ) from the ampliﬁer, the the ampliﬁer, the 8 Ω ) from V into 1 W (2.83 At from sweeping by tested often are Ampliﬁers 2 = amplifier residual amplifier = 2
2 +
only attenuator that could be made using 0.6 W 0.6 using made be could that attenuator only the is 6 141 at V. Case attenuator an in dissipated both at resistor W. 1,500 each and W 200 by dissipated power the with along 1 Table as combinations resistor same a to signiﬁcant consider.factor but becomes heat dissipation within the attenuator range) input balanced ~14AverLAB’s to (within Vs voltage this reduce would load. W A attenuator 20dB 1500 A range. 141 approximately ampliﬁer produces V kilowatt into an 8 Ω the in outputs have systems. Professional livesound ampliﬁerssound home frequentlyin found those as amplifier, such Measuring a1,500Watt Ampliﬁer combination that would satisfy that accuracy target. resistor impedance highest the ers ﬀ o 3 Case that 0.43 µV. just is now Table 2 shows 0.1 accuracy dB It is easy to see that signiﬁcant power can be be can power signiﬁcant that see to easy is It W 200 a for only was example previous The
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SATISFACTION GUARANTEED CUSTOMER Table 3 Table shows the the shows
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reduce heating. tofurther elements constructed from multiple of value Rmay alsobe Figure 3:Any resistor 
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TEST & MEASUREMENT FOCUS TEST & MEASUREMENT FOCUS 46 a  x
To download a spreadsheet with all the equations to to audioXpressSupplementaryMaterial.html equations the all with calculate the attenuators, visitspreadsheet www.audioxpress.com/page/a download To Project Files at www.avermetrics.com. download for available is a spreadsheet Such reference. handy a as spreadsheet a into entered conveniently be can These variables. different for solved earlier shown • • attenuators: and ofuse external More HelpfulInformation March 2019
Vn Variable R Atten and distortion reading reading distortion level and test values, resistor actual on based accuracy Eq values onbased test level and desired accuracy Eq Dist Acc ThevActual Vn Vn Vn Vn These equations are simply variations of equations equations of variations simply are equations These The following calculations are helpful in the design R V DUTActual P R P R Vn n uations for use: Determine the measurement resistor ideal the Determine design: for uations series series shunt shunt Actual Thev Atten meas Meas Actual Actual DUT Att Actual

Practical Test & Measurement audioxpress.com Vn Actual Atte RV nL =× Thev Equations forUse Actual Vn P R Vn P series Ac Thev shunt Dist Equations forDesign 41 =× Meas DUTActual nV Vn Vn Vn Vn Vn V cD =× Actual n Actual Actual nT Att = = 20 meas Atte DUT Te RR Atte =× shunt ⎝ ⎜ ⎛ ⎝ ⎜ ⎛ RR =Vn nm n = LevelActual mp series 22 =× RR RR =Level ×10 =− =− =Vn ⎝ ⎜ ⎛ /. =− =− series series 20 ( og 11 41 = ×× // Level Level Thev DUT () RR is nVn Vn BW RR Thev Lo + + eculshActual seActual Att / em tD hculsAta seActual shActual shActual ( ×10 Meas 11 Meas g ea / ×10 pB 10 () shunt shunt Vn sA ×× 22 DUT 0 Atte + 22 1 / Atte Atten/20 . culActual Actual n WE ⎠ ⎟ ⎞ ⎠ ⎟ ⎞ 10 38 Dist/20 / n Acc/20 20 / 2 2 20 is / Dist Level × × ER ) t Actual tt Actual − R R 38 Meas 23 series + shunt × R 20/ − ⎠ ⎟ ⎞ 3 ) ThevActual eprtr Temperature indegrees Kelvin Temperature Vn teuto h au fteetra teuto decibels(dB) Thevalue attenuation oftheexternal Attenuation Variable Bandwidth Bandwidth of measurement Hertz (Hz) Hertz Bandwidthofmeasurement Bandwidth R Atten Level itrinTertddsoto ee fteDTdecibels(dB) level Therateddistortion oftheDUT Distortion Dist cuayTlrbemaueeto cuaydecibels(dB) Tolerable measurement ofaccuracy Accuracy Acc R R Dist ThevActual Vn DUTActuual Vn Vn Vn R P R Vn Vn P shActual seActual R Level series series shunt shunt Actual Thev Atten meas Meas Actual Actual otg iie ai fagvnatnao k:1 Voltage dividerratioofagiven attenuator k post DUT Meas Actual Actual Power dissipatedintheshuntresistor at The outputlevel ofthedevice undertest os rdcdb h culatnao volts (V) Noise produced by theactualattenuator Residual seenby theanalyzeratlimit of distortion andleveldistortion (at27°Cand20kHz Lpad accuracyspeciﬁcation thatmeets Lpad accuracyspeciﬁcation thatmeets Residual measured noiseanddistortion Highest value ofaseriesresistor foran The residual ofthe noiseanddistortion Highest value ofashuntresistor foran Highest Thevenin equivalent resistance inllvlse yteadoaaye volts (V) Signal level seenby theaudioanalyzer Noise contribution of the attenuator at Noise contributionoftheattenuator the accuracyspeciﬁcationatrated Power dissipatedintheseriesresistor based onthetestlevel andattenuator for anLpad thatwillmeet attenuator Thevenin equivalent resistance ofthe Precise given attenuation thechosen Accuracy ofthemeasured distortion Measured reported distortion by the Value ofN Residual noisefrom DUT (without culdsoto fteDTdecibels(dB oftheDUT Actual distortion DUT atLevel andDistortion Chosen seriesresistor accuracy speciﬁcation Chosen shuntresistor attenuator noise) attenuator actual attenuator limit ofaccuracy DUT by theanalyzer Description audio analyzer attenuator following the external followingtheexternal resistors at Level (DUT) Level used BW) decibels (dB) decibels (dB) decibels (dB) ohms (Ω) ohms (Ω) ohms (Ω) ohms (Ω) ohms (Ω) ohms (Ω) ohms (Ω) ohms (Ω) volts (V) volts (V) volts (V) volts (V) volts (V) volts (V) volts (V) Units K load in the attenuator. attenuator. the in load heat the handle to easier it make solutions other These dB. 0.4 a than less cause dB would 5 0.2 Case a and than less by reading measurement the tolerable. be not may which rise, temperature 70°C a almost see will resistors in to each resistor ~0.4 W. However, at 0.4 W, these 5000 Ω series resistance. This brings the dissipation wired as four inseriespairs to parallel the produce be could resistors Ω k 10 eight instance, For load. heat the handle to resistors W 0.6 multiple using reading would be 99.25 dB, a diTHD+N erence ﬀ of 0.75new dB. the noise attenuator of V µ dB. 0.1 1.23 by With measurement the change to noise of 0.43 µV only However, W,0.1dB. takes 1 it by at measurement the change to noise of V µ 21.6 take would it fact, In the signiﬁcantly. cause change to not reading would attenuator the from noise for a distortion of 100 dB. The 1.23 µV of additional signal and the ampliﬁer’s residual would be 0.14 µV V.µ 1.23 be will noise the that shows 2 Table However, resistors. to pleasetheear, butwon’t takeyouforaride. The resistor combination in Case 4 changes changes 4 Case in combination resistor The built be could 3 Case in resistor series The At full power, the analyzer will be seeing a 14.1 V
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makes this a very practical and costeffective costeffective and solution. practical very a this makes resistors 1%thinﬁlm W 0.6 of use The hand. on impedances erent ﬀ di with but attenuation similar of attenuators multiple have to handy be would it reason, this For concern. primary of is attenuator high At external the in dissipation concern. heat the voltages, primary test of is attenuator of the noise thermal added the voltages, test low At thermal noise will have on distortion measurements. Conclusions elements to further reduce heating (see heating reduce further to elements multiple from constructed be also may R value of resistor Any 20°C. than less rise will it 4 Case in thinﬁlm shunt resistor will rise almost 60°C whereas by the Lpad. In 3, of Case a the temperature 0.6 W
Technology Associations and contributes on other working groups as well. as groups working other on contributes and Associations Technology Consumer the of committee standards audio R3WG2 the cochairs Jonathan Society. Engineering Audio the of governor and president vice former a also is He Keysight). Technologies (now Agilent at Manager Product a Senior and Precision Audio for Sales and test of of Director a was he that to LLC. Prior Avermetrics, for years Marketing and Sales of VP 25 as served recently than most He more audio. in with exclusively 15 last the engineer experience, measurement electrical an is Novick Jonathan About theAuthor This article discusses the impact an attenuator’s The shunt resistor dominates the noise produced a x modern, easyandlowcost! Measure soundlike apro, 5star AppStore rating! www.studiosixdigital.com Figure 3 Figure ). audioxpress.com AudioTools 
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TEST & MEASUREMENT FOCUS