Dual conversion settop terminal including a system for avoiding oscillator difference beats
AssigneeGeneral Instrument Corporation
InventorMatthew Waight
Life of this patent
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Abstract
An improved multiple conversion CATV settop terminal eliminates the adverse effects of oscillator difference beats by selectively controlling local oscillators and the use of a wideband bandpass filter. The wideband bandpass filter is coupled between the output of an up converter mixer and the input of a down converter mixer. Each mixer receives a second input from its corresponding local oscillator. A system microprocessor is programmed to control each local oscillator such that the oscillator difference beat frequencies are translated out of a following IF filter passband. Once translated, the oscillator difference beat frequencies no longer present a source of distortion to the received information signal.
Claims8
A CATV settop terminal which includes a receiver for receiving a plurality of CATV channel signals, the settop terminal comprising:
- a CATV channel selector for selecting a CATV channel signal from said plurality of CATV channel signals;
- a first frequency agile local oscillator for generating a first frequency for mixing with said selected CATV channel signal to generate a fixed high intermediate frequency signal;
- a filter for filtering said fixed high intermediate frequency signal;
- a second frequency agile local oscillator for generating a second frequency, at a predetermined value, for mixing with said filtered fixed high intermediate frequency signal to generate a fixed low intermediate frequency signal;
- a controller, responsive to said CATV channel selector, said controller including:
- means for determining said first frequency based on said selected CATV channel signal;
- means for comparing said first frequency with said second frequency to determine interfering oscillator difference beat (ODB) frequencies and for calculating a delta value based upon said ODB frequencies;
- and
- means for adjusting both said first and second frequencies by said delta value.
Dependencies are inferred from the claim text.