Frequency converting apparatus for converting an RF television signal to a video signal employing low IF techniques
AssigneeDeutsche ITT Industries GmbH
InventorsOtmar Kappeler, Dietmar Ehrhardt
Life of this patent
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Abstract
A frequency converter circuit for television signals is disclosed which contains a tuner and a low-IF converter which converts the television signal to a low IF signal to obtain adjacent-channel selectivity with low-pass filters. The low-IF converter further includes a phase-correcting stage and an amplitude-correcting stage for the two quadrature-signal paths. By digitizing the two signal paths after the first quadrature mixing process, their amplitude and phase stability is further improved.
Claims15
Frequency converting apparatus for converting an RF television signal to a video signal by employing a low IF processing apparatus which apparatus includes first and second signal paths each path containing a first multiplier for receiving at an input a television signal with each multiplier followed by a low pass filter having an output coupled to an input of a second multiplier in each path, with the first and second multiplier in each path receiving at other inputs respective quadrature sinusoidal signals as generated by a quadrature signal generator whereby said first path provides a first low IF quadrature signal at the output of said associated second multiplier and said second path provides a second low IF quadrature signal at the output of said associated second multiplier with the outputs of said second multipliers coupled to an adding circuit to provide said video signal at its output and including a phase and amplitude correcting means operative to provide properly phased quadrature signals of proper amplitude to assure the quadrature relationship in said first and second signal paths, in combination therewith of apparatus for improving said conversion to said video signal, comprising:
- a Nyquist filter positioned in each signal path so that the output of said low pass filter in said first path is coupled to the input of a first Nyquist filter, with the output of said low pass filter in said second path coupled to the input of a second Nyquist filter, with the output of said first Nyquist filter coupled to the input of a first variable gain amplifier having a control terminal in said first path having the output coupled to the input of said second multiplier in said first path and with the output of said second Nyquist filter coupled to the input of a second variable gain amplifier having a control terminal in said second path whose output is coupled to the input of said second multiplier in said second path, with the output of each variable gain amplifier in each path also coupled to the input of an associated comparator in each path, for comparing the output of said associated variable gain amplifier with a derived correction signal at another input, with each control terminal of said variable gain amplifier coupled to the output of said associated comparator to vary the inputs to said associated second multipliers according to said derived correction signal and means coupled to the outputs of said first and second variable amplifiers for providing said derived correction signal.
Apparatus for converting a R.F. television signal to a video signal said apparatus including a tuner for receiving said RF television signal and for providing at an output an intermediate frequence (IF) television signal, comprising:
- a clock source for generating at an output a given frequency signal;
- digital dividing means coupled to said clock source for providing at a first output a divided clock signal of a given phase and at a second output a divided clock signal of a 90.degree. phase;
- a first switching modulator having one input adapted to receive said IF signal and one input coupled to said first output of said dividing means;
- a second switching modulator having one input adapted to receive said IF signal and one input coupled to said second output of said switching means;
- a first low pass filter having an input coupled to the output of said first modulator and a second low pass having an input coupled to the output of said second demodulator;
- a first analog to digital converter having an input coupled to the output of said first low pass filter;
- a second analog to digital converter having an input coupled to the output of said second low pass filter;
- a filter digital Nyquist filtering means having an input coupled to the output of said first analog to digital converter;
- a second digital Nyquist filtering means having an input coupled to the output of said second analog to digital converter;
- a first squaring means coupled to the output of said first Nyquist filtering means and a second squaring means coupled to the output of said second Nyquist filtering means, with the outputs of said first and second squaring means coupled to respective inputs of a digital summing means for providing at an output a digital signal of a squared value indicative of a video signal;
- and
- a square root extractor means coupled to the output of said digital summing means to obtain a digitized video signal in the video band.
Dependencies are inferred from the claim text.