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Presented AES, Sept. 1998
EFFECT TRANSFORMERS TRANSMISSION DIGITAL AUDIO SIGNALS
Paul, Vice President Scientific Conversion, Inc. Novato, California Transformers used transmission digital audio signals affect signal fidelity, interference susceptibility, conducted emission. Commercially-available transformers specified digital audio exhibit vast differences performance. Transmission standards often specify only transformer ratio bandwidth, ignore many other parameters which affect critical applications. This paper reviews function, parameters performance digital audio transformers, presents data frequency response, pulse aberration, common mode rejection ratio, jitter. also discusses several applications compares number transformers detail. Transformers Digital Audio Transmission Systems Digital audio transmission systems transformer coupling provide balanced outputs, improve common mode noise rejection, match impedances, reduce conducted emission susceptibility. (Note: Throughout this paper, term "digital audio signal" refers Manchester encoded type modulation with embedded clock, etc. specified AES/EBU, SPDIF, AES-3 corresponding twice speed signals such DVD, etc.) Figure block diagram direct-coupled transmission system, without transformers. signal fidelity affected transmitter slew rate, cable, pickup common-mode interference along cable. receiver' differential input sees common-mode noise appearing between transmitter receiver grounds. performance receiver circuit depends levels common-mode interference signal. Interference sources include high speed microprocessor clocks, noise, switching power supplies, crosstalk from adjacent cables. receiver differential amplifier characterized common-mode rejection ratio (CMRR) which decreases with increasing frequency. Because high frequency noise capacitively coupled cable (crosstalk), direct-coupled input highly susceptible such noise. Another characteristic direct coupling that crosstalk cable shield enter contaminate transmitter' receiver' internal circuitry (e.g. power, clocks ground planes) connector shield cable return grounded circuit ground. Figure block diagram transmission system using transformers. transmitter' digital signal coupled output through transformer. (Note that figure shows balanced cable single-ended coax also used.) transformer output isolated from chassis ground. output connected balanced cable, cable connected transformercoupled receiver. resistors source termination impedance match characteristic impedance cable.
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Presented AES, Sept. 1998
insertion transformer receive circuit greatly improves high frequency CMRR, thus reducing recovered clock jitter. Even receivers with high jitter attenuation will benefit, since transformer also attenuates interference contamination common-mode noise. Both transmitter receiver sides make transformer coupling break ground loops, reduce conducted EMI, provide voltage impedance matching.
AES/EBU Signal Bandwidth Frequency components digital audio signal exist beyond minimum bandwidth often specified Figure spectrum AES/EBU signal, with sample rate (Fs), swept from with vertical scale dB/div. Note spectral content below kHz. Figure spectrum same signal from MHz, displaying substantial energy above MHz. These spectra show that digital audio signal bandwidth extends beyond minimum bandwidth usually specified. Although minimum bandwidth provides useable transmission, recovery low-jitter clocks accurate data transmission real-world noisy environments will benefit from greatly increased bandwidth. transformer often limiting factor endto-end bandwidth such system. extension beyond minimum times both upper lower bandwidths, (e.g. from FLOW FHIGH) recommended. twice speed applications, example, DVD, FHIGH increased MHz. Transformer Equivalent Circuit Figure generalized equivalent circuit transformer [4]. components except ideal transformer represent parasitic effects. resistance primary secondary windings. PLKG SLKG primary secondary leakage inductances caused separation space windings. Capacitance PSHUNT CSSHUNT primary secondary inter-winding shunt capacitance. Inductance PMAG primary magnetizing inductance, representing self-inductance primary winding when secondary current flows. PLOSS core loss component representing energy dissipated core magnetized de-magnetized. inter-winding capacitance from primary secondary P-S. These parasitic components, combination with ideal transformer impedance external circuit environment, form broad bandpass filter with typical HIGH FLOW ratios 1000 10,000. low-frequency corner, FLOW determined circuit formed source impedance plus combination with magnetizing inductance highPMAG frequency corner FHIGH defined circuit including shunt capacitances PSHUNT CSSHUNT series leakage inductances LPLKG LSLKG circuit environment's impedance. This lumped constant model second-order approximation does account pulse aberrations distributed nature windings.
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Presented AES, Sept. 1998
Figure computer simulation frequency response model environment. primary inductance, PMAG varied from 1400 (typical values) causing frequency 3-dB point FLOW reduced from kHz. Figure similar plot where series leakage inductance LPLKG changes from high-frequency corner, HIGH. reduced from maximum bandwidth, recommend choosing transformer with highest possible primary inductance PMAG lowest possible leakage inductance, PLKG. Since leakage inductance difficult measure specify, high-frequency corner, FIGH used specify that parameter. Frequency Response Saturation Effects Figure shows effect transformer's high-frequency corner pulse risetime. upper trace aberration pulse generator. middle trace transformer with FIGH equal lower trace transformer with FHIGH equal MHz. benefit wider bandwidth obvious. Figure shows effect transformer's low-frequency corner, FLOW, pattern received digital audio signal. upper trace taken with transformer with lower trace taken with transformer with kHz. Note closing pattern lower trace, which causes increase inter-symbol interference. This highlights importance using transformers with extended LOW. Figure illustrates effect saturation transformer core signal. This effect nonlinear; accounted model above, function voltage time duration each pulse. maximum flux density property core material, determines point which transformer action ceases. saturated core primary inductance, LPMAG near zero, thus shorting both primary secondary. transformer upper trace large flux capacity 300µVs, exhibits saturation. lower trace transformer with similar frequency response only flux capacity, showing severe saturation. flux capacity must maximized avoid such problems. Regardless transformer's flux capacity, capacitor required series with transformer's primary prevent bias from causing saturation. Pulse Aberration Nonlinear phase frequency response creates pulse aberration. This present even transformers with relatively wideband, flat magnitude frequency response. Figure obtained with 12.288 MHz, risetime pulse generator (12.288 fastest symbol rate digital audio signal). upper trace very linear phase response transformer with minimal aberration. lower trace transformer with similar bandwidth with substantial phase nonlinearity; note severe overshoot pulse aberration.
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Common-Mode Rejection Ratio Interference Suppression
Presented AES, Sept. 1998
Figure circuit test common-mode rejection ratio (CMRR) transformer. wideband leveled generator drives both sides primary winding. Both sides secondary winding attached resistive termination wideband millivoltmeter. output component common-mode signal which "leaks" through primary-to-secondary capacitance transformer. transformer interwinding shield, returned ground plane. Figure plot CMRR frequency obtained with this test, different transformer designs. Since improved CMRR prime motivation transformer, CMRR most significant parameter specify. same consideration suppression external interference received signals also apply reduction conducted interference emitted equipment. symmetrical nature (reversing input output) interference equivalent circuit means emitted interference such microprocessor clocks, high speed clocks, etcetra will reduced same ratio common-mode rejection demonstrated above. Regulatory compliance conducted digital audio cables connectors improved using low-capacitance shielded transformers. Common-Mode Interference: Induced Jitter Test Figure test jitter-induced high frequency common-mode asynchronous noise. Audio Precision System generates AES/EBU test signal output transmitted pairs long cable AES/EBU decoder circuit. cable specification Type Length Wire Inductance Capacitance Resistance Network wire, Belden 1538A twisted pair uH/conductor 1940 pF/each conductor others 2.67 /conductor
Three unused cable pairs connected parallel generator with termination wideband millivoltmeter monitor interference level. interfering signal applied three unused pairs, generating crosstalk that appears common-mode noise current coupled through capacitance between unused pairs active pair. receiver circuit couples digital signal through transformer under test decodes with Crystal Semiconductor CS8412 AES/EBU receiver. rising edges frame sync output (pin CS8412) decoder define output sample time. This clock analyzed Hewlett-Packard 5370B Time Interval counter, capable statistical analysis jitter measurement. time light cross pencil's diameter!)
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Presented AES, Sept. 1998
counter measures each frame sync (Fs) period, collects sample 10,000 periods calculates standard deviation that set, direct measure wideband jitter. impact jitter audio signal function frequency amplitude signal, architecture converter.References [5-8] derive relationship between jitter recovered audio's dynamic range. Common-Mode Interference: Induced Jitter Results Good recovered clock quality essential bit-error-rate data recovery. addition, recovered clock-jitter essential minimize audio signal degradation those systems relying recovered clock operating converters. Figure plot jitter nanoseconds common-mode interference level (ref. taken with setup described above. interfering signal 6.98 sinewave. upper curve unshielded transformer with lower curve CP-S shielded transformer. This data illustrates dramatic effect that high CMRR transformer have jitter. optimum performance high noise environments jitter applications, capacitance, high CMRR, shielded transformer required. Transmission System with Shielded Transformers Common-mode noise transformer primary induces current through transformer primary-to-secondary capacitance (typically then receiver input. Adding interwinding shield provides substantial fold) improvement CMRR. Figure shows shield introduces capacitance (from primary shield, P-Shield which shunts most common-mode current away from secondary winding diverts ground. Some capacitance remains from primary secondary leads, traces, other parasitic capacitance. possible realize good design! common-mode current divides proportion capacitance ratio, P-Shield. receiver input sees only small current coupled through CP-S. general, addition shield transformer design leads tradeoff, increasing leakage inductance reducing bandwidth. Typical Digital Audio Transformer Applications 10.0 Balanced System with Shielded Transformers Figure balanced system using shielded transformers both transmit receive sides. shield each transformer connected ground plane associated connection from shield ground plane must have short, inductance path, maintain shield's effectiveness.
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10.1 Unbalanced Interface with Ratio Transformers
Presented AES, Sept. 1998
Figure unbalanced system. Since transmit output, transformer step-down ratio matches primary impedance cable impedance reduce output voltage level series 0.1µF capacitor blocks bias output prevent transformer core saturation. transformer shield returns transmit ground ground plane secondary goes connector, with side secondary connected connector shell. shell unbalanced connector either float chassis (earth) ground. floating connector shell used (e.g. break ground loops) then small high-frequency return capacitor example added from connector shell chassis (earth) ground. digital audio receiver finite dynamic range CMRR. level, unbalanced signals, (e.g. SPDIF from weak source lossy cable) will increase error rate jitter since receiver less margin interference rejection. step-up transformer increase signal voltage level, improving both receiver performance common-mode rejection. receive side Figure connector feeds transformer with step-up. secondary terminated with transformer shield attached through short direct path ground receive (e.g. local bypass capacitor, ground plane). same considerations mentioned above transmit side floating connectors also apply receiver. 10.2 Bridging Unbalanced Input Floating Coax Connector Figure shows high impedance bridging receiver using step-up transformer. resistor terminates transformer maintain proper frequency response. That resistor reflected primary presenting light load line. connector shown floating, with bypass capacitor chassis ground. 10.3 Phantom Power Remote Digital Device Low-power converters make remotely powered devices such digital microphones reality. Figure phantom power circuit where signal cable carries power. power source local side system connected center output transformer. remote device gets power from input transformer's center tap. Decoupling filters should used both ends. 10.4 Dual output transmitter, Balanced Unbalanced Some applications require both balanced unbalanced outputs. Figure uses center tapped transformer. primary resistor reflects center secondary series resistor matches unbalanced output impedance. balanced output obtained across entire secondary. Note that only outputs connected time.
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Comparison Typical Transformers
Presented AES, Sept. 1998
Figure compares parameters performance seven types commercially-available digital audio transformers. Note substantial differences most parameters. higher cost best available transformer easily justifiable high-quality equipment designs, since that cost negligible fraction total material cost, benefits substantial. worst transformers degrade performance specifications equipment impair regulatory compliance. Layout Considerations transformer isolates relatively noisy external connections from (hopefully!) clean internal circuitry equipment. capacitance shield transformer provide rejection high-frequency common-mode interference present external connection. Good printed circuit layout practice further improve noise rejection finished design, example, adding ground planes shielding minimizing primary secondary capacitance. typical layout transformer circuit (either input output) shown Figure external coaxial connector connected with short direct traces transformer. traces longer, they should kept away from connectors. ground planes used, which split under transformer. ground plane facing connector goes chassis (earth) ground. ground plane facing transformer shield present) attached ground return associated with short direct trace. CONCLUSIONS Transmission circuits digital audio signals improved transformers both transmit receive sides. Measurements show that transformer substantial impact digital audio signal waveform, rejection common mode noise interference recovered clock jitter. Good transformer implementation application improve signal waveform fidelity, increase rejection reduce conducted emission. benefits more than compensate small cost increment high-performance transformers. ACKNOWLEDGMENTS author wishes thank Steve Harris Crystal Semiconductor, Richard Redl ELFI S.A., their very kind assistance suggestions.
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REFERENCES
Presented AES, Sept. 1998
IEC-958, Digital Audio Interface, International Electrotechnical Commission, Geneva, (1989). AES3-1992, Recommended practice digital audio engineering Serial transmission format two-channel linearly represented digital audio data, Audio Engineering Society, York, (1992). AES3-1995, Information document digital audio engineering Transmission AES3 formatted data unbalanced coaxial cable, Audio Engineering Society, York, (1995). Nathan Grossner, Transformers Electronic Circuits, McGraw-Hill, (1983). 275, 385. Steve Harris, "The Effects Sampling Clock Jitter Nyquist Sampling Analog-to-Digital Converters, Oversampling Delta-Sigma ADCs," Journal Audio Eng. Soc., vol. 7/8, 1990 July/August. Patrick Trischitta, Jitter digital Transmission Systems, Artech House, Inc. (1989), ISBN 0-89006-248-X. Yoshitaka Takasaki, Digital Transmission Design Jitter Analysis, Artech House, Inc. (1991). Richard Cabot, "Digital Audio Transmission Jitter Important," Audio Precision Newsletter, vol. (1996).
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Effect Transformers Transmission Digital Audio Signals
Paul, Vice Pres. Scientific Conversion, Inc. Novato, California,
Highlights
transformers digital transmission? Spectrum digital audio signal Transformer effects waveform Transformer effects noise rejection Common-mode rejection: relationship jitter Jitter Histograms various transformers Transformer shielding noise rejection Applications: balanced system with shielded transformers, unbalanced interface, high-impedance bridging input, phantom power, dual balanced unbalanced output Comparison available transformers
Fig. Direct Coupled Transmission System
TRANSMITTER RSOURCE COAX CABLE TERM DIFF RECEIVER
TRANSMITTER GROUND
RECEIVER GROUND
Fig. Transformer Coupled Digital Audio Transmission System
TRANSMITTER
OUTPUT TRANSFORMER INPUT TRANSFORMER
RECEIVER
RSOURCE DIFF BALANCED CABLE RTERM
Reasons Using Transformers Digital Audio Transmission
Greatly improve common-mode noise rejection Reduce conducted emission ensure regulatory compliance Reduce transmit receive circuit susceptibility interference Break ground loops Balance input output Match impedance levels Achieve lowest possible recovered clock jitter
Fig. Spectrum Digital Audio Signal
SPDIF 48kHz, 10kHz/div., dB/div.
dB/div.
Fig. Spectrum Digital Audio Signal
SPDIF 48kHz, 5MHz/div., dB/div.
dB/div.
Fig. Equivalent Circuit Transformer
CP-S LPLKG
LSLKG
CPSHUNT ideal CSSHUNT
LPMAG RPLOSS
Fig. Frequency Response Transformer
Parameter: primary inductance, LPMAG
Attenuation [dB]
1400
[Hz]
Fig. Frequency Response Transformer
Parameter: leakage inductance, LPLKG
Attenuation [dB]
[Hz]
Fig. Effect Transformer HighFrequency Bandwidth Pulse Response
12.288 Pulse Generator
FHIGH Transformer
FHIGH Transformer V/div
ns/div.
Fig. Effect Transformer LowFrequency Bandwidth Pattern
FLOW Transformer
FLOW Transformer V/div ns/div
Fig. Effect Core Saturation Waveform
Transf.
µVs, Transf. V/div µs/div
Fig. Comparison Pulse Aberration
Squarewave Generator 12.288 MHz, in/out
Type very aberration Type excessive aberration V/div ns/div.
Fig. Comparison Aberration
Received AES/EBU 96kHz in/out
Type
Type
Fig. Common-Mode Noise Rejection Test Circuit
Wideband trms
Transformer under test
interference source
RTERM
Fig. Common-Mode Noise Rejection
Type Type
100k 100M
[Hz]
Fig. AP-1/CS8412/48kHz Common-Mode Noise Induced Jitter Test Setup
AES/EBU output Transformer under test CS8412
SYSTEM
pair cable interference source
5370B counter
5370B Time Interval Counter
Fig. Induced Common-Mode Noise Jitter Comparison
Transformer
Transformer Interference level [dBm]
Fig. 96kHz Common-Mode Noise CS8404A/CS8414/96kHz Jitter
Crystal CS8404A
Transformer Under Crystal CS8414 Test pair cable
#24UTP
5370B counter
Yokogawa TA320 Time Interval Analyzer
Interference Source
11.269MHz
Yokogawa TA320
Time Interval Analyzer MS/s resolution statistics: P-P, dev, jitter histograms
Fig. trsf. jitter 3401ps
Fig. type" jitter 1540ps
Fig. type jitter 906ps
Fig. type" jitter 396ps
Fig. Shielded Transformer CommonMode Noise Equivalent Circuit
Cp-sh TRANSMITTER Balanced Cable Csh-s RECEIVER
Shielded Transformer
Termination
common mode
Fig. Balanced Transmission System Using Shielded Transformers
TRANSMITTER SHIELDED TRANSFORMER SHIELDED TRANSFORMER BALANCED CABLE 110S RECEIVER
mono PLANE TRANSMITTER CHASSIS GROUND RECEIVER CHASSIS GROUND
110S PLANE
Fig. Application Transformers Unbalanced System
TRANSMITTER RECEIVER
OUTPUT TRANSF. COAX CABLE
INPUT TRSF.
mono PLANE CHASSIS EARTH
PLANE CHASSIS EARTH
Fig. Hi-Z Bridging Unbalanced Input with Floating Coax Connector
RECEIVER INPUT OPTIONAL BYPASS CHASSIS
Fig. Phantom Power Remote Digital Device
LOCAL CENTER TRANSFORMER FILTER REMOTE POWERED
CABLE CENTER TRANSFORMER PHANTOM POWER SOURCE
Fig. Dual Output Transmitter Balanced Unbalanced
TRANSMITTER CENTER TRANSFORMER MONO UNBAL
Fig. Comparison Transformers
Parameter Primary Inductance Capacitance Shielded Cap. (NA= none) Flux Capacity Adv. cutoff FLOW Calc. cutoff FLOW Meas. cutoff FLOW Cutoff FHIGH Jitter 11.2896MHz/2.25V CMRR Pulse Aberration Size WxLxH Relative Cost Leadtime week 12-16 12-16 6-10 ratio 32.5 21.5 23.5 50.5 52.0 3.10 Unit 2500 100.0 1950 85.0 70.1 26.0 1260 2000 100.0 1900 2200 24.3 1060 3500 20.0 2400 20.0 19.5 1320 SC91601 5000 SC93702 20.0 20.6 13.0 SC97903 32.0 35.0 32.0
12.5 1.00 1.36 1.53
1.55 2.40 3.00 3.10 3.10
Fig. Suggested Layout Transformer Input Circuit
CIRCUIT GROUND PLANE CHASSIS GROUND PLANE
bypass Term.
INPUT CONNECTOR
TRANSFORMER TRSF. SHIELD RECEIVER
Summary Conclusions
Commercial transformers exhibit tremendous differences parameters performance Digital audio transformers substantially improve common mode noise rejection emission Second order effects such pulse aberration saturation greatly influence waveform fidelity. AES/EBU receivers exhibit jitter that function transformer CMRR capacitance Professional, broadcast high resolution applications need maximum CMRR minimize recovered clock jitter presence noise. High quality transformers offer cost effective improvements product design.
Recommendations
transformers with lowest possible PrimarySecondary capacitances leakage inductances best CMRR low-capacitance transformer, shields careful layout Transformer Frequency bandwidth depends minimum Frame Sync frequency. 1/20 AES/EBU min. spec Transformer bandwidth should AES/EBU spec 8MHz depending max. Minimize pulse aberration: under
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