Characterizing Digital Audio Transformers with Induced Jitter Histograms Presented at the Audio Engineering Society Convention, New York December 2001 By Jon D. Paul, Vice President Scientific Conversion, Inc.
	ABSTRACT Transformers are employed in digital audio systems primarily to reject common mode noise interference. A new test characterizes the interference rejection of a practical transmission system with a transformer at the receiver input. A sample set of the decoded frame sync clocks are accumulated by a statistical time interval analyzer. The analyzer calculates the mean value of the periods, the standard deviation (jitter), and provides a period histogram. The histogram and standard deviation establish a basis for comparing the high frequency interference rejection of various transformers and for quantifying the nature of the induced jitter. Test data are presented for 7 different types of transformers.
	Figures Fig. 1  Transformer Coupled Digital Audio Transmission System Fig. 2  Common-Mode Noise Rejection Fig. 3 Effect of Different transformers on CM noise Fig. 4  test fixture block Fig 5 Test Fixture Photo Fig 6 Time Interval Analyzer Fig. 7 Relationship of observed jitter to system component jitter Fig. 8 Induced Common-Mode Noise Jitter Comparison Fig. 9 Typical AES/EBU transformers Fig. 10 -  Fig. 18 Jitter Histograms of Various Transformers Fig. 19 Correlation of Various Transformer Parameters vs Jitter   Fig. 20 Jt Vs Cps Fig. 21  Comparison of 7 AES Transformers Summary and Conclusions •      Induced Interference Testing simulates field conditions •      Commercial transformers exhibit tremendous differences in interference rejection •      AES/EBU receivers exhibit jitter that is function of the transformer CMRR and capacitance •      Professional, broadcast and high resolution applications need maximum CMRR to minimize recovered clock jitter in the presence of noise.
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