AM distortion measurement method and apparatus usable on active audio carriers
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United States of America Patent
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Nov 16, 1993
Grant Date -
N/A
app pub date -
Dec 10, 1990
filing date -
Dec 10, 1990
priority date (Note) -
Expired
status (Latency Note)
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Abstract
A test instrument for measuring hum and other AM distortion components on active audio carriers such as in a cable television distribution system. The instrument has an RF tuner capable of tuning to the audio carrier frequency for a selected channel, and also has three IF amplifier stages including a third stage with a 280 KHz bandwidth, which is sufficiently narrow to reject the video signal when the tuner is tuned to the audio carrier frequency. An AM detector detects amplitude variations in the FM signal, and the AM detector output signal is filtered alternatively in a 400 Hz lowpass filter or a bandpass filter to facilitate measurement of the level of different forms of distortion. The center frequency of the bandpass filter is capable of being set to the AC power line frequency or twice the line frequency. The bandwidth of the bandpass filter is limited to about 2-3 Hz to minimize the effect of amplitude variations other than hum in the FM audio signal. The filter output signal is rectified and filtered in an AC signal level detector, producing a signal representative of the hum level for display on an analog meter.
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