Wavetek 147 Dokumentacja Pobierz pdf (Strona 148)

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Measuring System Performance: Hum Measurements

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marker so that it is more convenient for the user to select the

desired (example: worst case) noise floor level.

The reset soft key will cause the measurement to recycle. This

action is desired if the same input has been monitored for a

long period of time, and the user wants to monitor the input to

catch ingress.

Hum is undesirable modulation of the television video carrier by power line

frequencies and harmonics (50, 60, 100, 120 Hz), or other low frequency

disturbances (FCC limit: < 1kHz). When you’re using Hum measurement

mode, the icons at the left of the display screen show the hum test

frequencies (see Fig. 6-8). When you press the related hum frequency soft

key, the left side of the icon darkens. The dB/% icon toggles the displayed

value between dB and percentage.

When there is an undetectable level of hum the following message appears

on the screen:

ERROR

INSUFFICIENT SIGNAL LEVEL TO

PERFORM THE MEASUREMENT

To measure hum, simply press the HUM measurement mode key when

you’re tuned to any non-scrambled channel.

Soft keys allow you to select 50, 60, 100, 120 Hz, or <1,000 Hz filters for this

measurement. A 60 Hz modulation component suggests a possible

corroded connector, a 120 Hz component tends to indicate a possible failure

related to the DC supply in the amplifier—possibly a capacitor failing and

thus causing ripple to increase.

HUM MEASUREMENTS

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