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 (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 trade-o several There use low-voltage for designed typically are they as application this in choice good a not attenuators usually are off-the-shelf analyzer. audio commercial an However, of voltage input the maximum above signals measure to way ective ff e cost This makes the L-pad and U-pad topologies ideal ideal topologies U-pad and L-pad 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 . 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. L-pad the half as here configurations. The U-pad resistor values are shown the and unbalanced in balanced to be both used U-pad 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 back-to-back 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 bi-direction and matching impedance where communications, in RF used primarily are variants their and Pi-pads output and input direction. specific Symmetric a topologies, such as T-pads have and they words, other In nature. in asymmetrical are attenuators These measurements. amplifier power for choices The U-pad is simply two identical L-pads is identical The U-pad two simply placed An L-pad is a simple resistive divider network network divider resistive simple a is L-pad An L- pad outputvol ). The two shunt resistors of a U-pad can can U-pad a of resistors shunt two The ). tage =× Vi n Rshunt Rshunt + Rserie s carbon-composition resistors have a higher noise noise higher a have and resistors thick-film carbon-composition Carbon-film, 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 strade-o 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 U-pad 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 U-pads or loads can These loads. also be configured as L-pads 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 Testing An 8-Ω 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 Wire-wound 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 low-frequency can mass thermal resistors. Small surface-mount 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 L-pad 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. Metal-foil resistors approach approach resistors Metal-foil type. resistor with to 6dB, 12dB, points. and18dBattenuation side having eight 0.5Ωresistors. resistance Thetotal Ω andthere isstill8 isstillaccess Figure shouldbetestedusingaU-padwitheach withpush-pulloutputs 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 to-back inamirrored L-pads placedback- identical is simplytwo Figure 1:TheU-pad |

March 2019

| 43

TEST & MEASUREMENT FOCUS TEST & MEASUREMENT FOCUS 44 a 141 V dissipation at40Vand Table 3:L-pad power combinations for20dB Table 2:L-pad 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 signal-to-noise ratio (SNR) or total harmonic will be most evident in noise-related 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, Thin-film 0.6 at rated attenuators. constructing for compromise available readily inexpensive, an can also be Availability a problem. Leaded thin-film 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 L-pad is determined determined is L-pad 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


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 high-powered small, AverLAB keep To measure amplifiers. to Avermetrics, from an designing when attenuator. I the new AverLAB, used audio analyzer needed trade-offs 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,L-pad 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 auto-range 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 = 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 20-dB 1500 A range. 141 approximately amplifier produces V kilowatt into an 8 Ω the in outputs have systems. Professional live-sound 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

www.iSEMcon.com [email protected] MEASUREMENT MICROPHONES

SATISFACTION GUARANTEED CUSTOMER Table 3 Table shows the the shows


reduce heating. tofurther elements constructed from multiple of value Rmay alsobe Figure 3:Any resistor |

March 2019

| 45


To download a spreadsheet with all the equations to to audioXpress-Supplementary-Material.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 L-pad accuracyspecification thatmeets L-pad 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 anL-pad 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 inseries-pairs 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

offering anextensiverangeofready-to-go toroidal transformers 47 SouthEndPlaza,NewMilford, CT06776 p: 860.355.4711/f:860.354.8597 Avel LindbergInc.

makes this a very practical and cost-effective cost-effective and solution. practical very a this makes resistors 1%thin-film 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 thin-film shunt resistor will rise almost 60°C whereas by the L-pad. 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 co-chairs 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, easyandlow-cost! Measure soundlike apro, 5-star AppStore rating! www.studiosixdigital.com Figure 3 Figure ). audioxpress.com AudioTools |

March 2019

| 47