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4.8 HOW TO MEASURE HUM

Hum is undesirable modulation of the television video carrier by

power line frequencies and harmonics (e.g., 60 or 120 Hz), or other

low frequency disturbances (FCC limit: < 3%). To measure Hum,

simply press the Hum key when tuned to any non-scrambled chan-

nel. In the Hum mode the hum modulation of the tuned channel or

frequency will be displayed in either % or dB as selected by the

operator. Soft keys allow the operator to select 60, 120 (50, 100 Hz),

or <1,000 Hz filters for this measurement to help in troubleshooting.

A 60 Hz modulation component suggests a possible corroded connec-

tor, a 120 Hz component tends to indicate a possible failure related to

the DC supply in the amplifier - possibly a capacitor going bad and

aggravating the ripple. A proprietary DSP technique enables Hum

measurements on modulated carriers (non-scrambled channels).

Stealth has been given the capability of measuring the 1Hz Hum

component. To activate this feature, set the Fundamental Hum

Frequency in the MEASUREMENTS setup screen to 1Hz. The

filter options on the Hum screen will then become; 1Hz, <50Hz, and,

<1kHz.

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Wavetek 3ST Specifiche Pagina 56