Test Procedure for Hum Modulation - SCTE

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Appendix 2 – Derivation of the 100mVDC Delta Calibration The 100mVDC delta hum modulation reference calibration procedure establishes, through a –1 dB RF level change, a repeatable envelope change that can be compared to the actual peak-topeak hum voltage. From this a hum modulation ratio can be determined. Consider the signal: v(t) = B 1 [ − v (t)] cosω t (A - 5) hum Where B is the peak amplitude of the RF carrier, v hum (t) is the peak-to-peak hum voltage, and B in dB is defined as B(dB) = 20Log(B/B o ). If B is attenuated by 1 dB (which corresponds to a 100 mVDC delta on the voltmeter), then: − dB = 20Log B @ −1dB So, ∆B Time domain hum modulation can be defined as: c ( ( )/ B ) (A - 6) 1 o ( − ) ( ) (@ 1dB,100mV ) B B 10 1/ 20 − 20 − = − • = B 1−10 1/ p−p O O O ( ) V (A - 7) p−p Env Hum(dB) = 20Log ⎡ ⎢⎣ max − Env min Env max ⎤ ⎥⎦ (A - 8) Where Env max is the –1 dB delta in B (envelope change) that can be compared to the actual peakto-peak hum voltage (Env max -Env min ) to give a hum modulation ratio in dB. [ ∆B (@ −1dB, 0.1Vp −p )] [ ] (A - 9) Env max = ( −1/ 20 1−10 ) Env max − Env min = Hum p−p (A -10) At the 100mVDC calibration reference (which is a 100 mV p-p delta envelope reference), the equation for hum modulation is: ( −1/ 20) ⎡Hum Hum(dB) = 20Log(1 −10 ) + 20Log⎢ ⎣ mVp−p 100mV p−p ⎤ ref ⎥ ⎦ (A -11) Where Hum p-p is given in millivolts, and hum modulation is given in dB. Hum(dB) = −19.27 − 20Log 100mV ( ref ) + 20Log(Hum ) = −59.27 20Log( Hum ) (A -12) p− p mVp−p + mVp−p 12
Appendix 3 – Conversion of Hum Modulation from dB to percent (%) The Federal Communications Commission (FCC) defines hum in Technical Standards part 76.605(a) as: The peak to peak variation in visual signal level caused by undesired low frequency disturbances (hum or repetitive transients) generated within the system, or by inadequate low frequency response, shall not exceed 3 percent of the visual signal level. This appendix will detail the derivation of hum modulation in percent, and will provide a table with conversions of hum modulation from mV p-p to dB and percent hum. Hum Modulation may be expressed as a percentage of the peak to peak variation of the carrier level to the peak voltage amplitude of the carrier. From equations A-9 and A-10, located in Appendix 2, Env max is defined as the –1 dB delta in B (envelope change) that can be compared to the actual peak to peak hum voltage (Env max - Env min , or Hum p-p ) to give a hum modulation ratio. To express hum modulation as a percent: Hum(%) ⎛ HummVp−p ⎞ ⎛ HummVp−p ⎞ = ⎜ ⎟ *100% ⎜ ⎟ *100% = .109* HummVp−p Env = (A-13) max 919.5mV ⎝ ⎠ ⎝ p−p ⎠ where, Env max 100mV 100mV = (from A-9) p−p p−p ( 20) p p [ ] 1/ = = 919.5mV − − 1−10 .1087 The 100mV delta is shown as the standard reference. However, for hum measurements, the actual delta of voltages between the 1 dB change should be used for improved accuracy. The conversion of hum in mV p-p to dB and percent is shown in Table 1. 13
Page 1 and 2: ENGINEERING COMMITTEE Interface Pra
Page 3 and 4: Table of Contents 1.0 Scope 1 2.0 D
Page 5 and 6: 4.0 EQUIPMENT 4.1. The general equi
Page 7 and 8: 5.3. Set the oscilloscope to the fo
Page 9 and 10: Note: The mV p-p value must be divi
Page 11 and 12: Note: Reliable triggering of the os
Page 13: Appendix 1 - Hum Modulation Derivat
Page 17 and 18: Appendix 4 - Time domain vs. Freque
Page 19 and 20: The relationship of the peak to pea
Page 21: Appendix 7 - Special Test Condition

Test Procedure for Hum Modulation - SCTE

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