Television – Receiver circuitry – Demodulator
Utility Patent
1999-02-23
2001-01-02
Lee, Michael (Department: 2714)
Television
Receiver circuitry
Demodulator
C348S725000, C348S731000, C348S727000, C455S265000, C455S180300, C329S347000, C329S349000, C329S350000, C329S351000, C329S360000, C333S017300, C333S016000, C333S020000, C333S018000, C333S018000, C375S324000, C375S340000
Utility Patent
active
06169585
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a circuit arrangement for demodulating an intermediate-frequency video signal generated while using a Nyquist edge, comprising a phase-locked loop including a phase detector, a loop filter and a voltage-controlled oscillator, and a video demodulator, the intermediate-frequency video signal being applied to the phase detector and the output signal of the phase-locked loop being applied to the video demodulator which converts the intermediate-frequency video signal into a baseband video signal.
2. Description of the Related Art
Circuit arrangements of this type are generally used for demodulating an IF video signal, i.e., for converting a video signal from the IF range, in which it is modulated in an amplitude-modulated form on an IF carrier, to the baseband. The IF video signal was previously filtered from a high-frequency mixture, using a Nyquist edge, Which has the result that the side bands of the video signal are amplified to differently strong extents in their range. Dependent on the amplitude of the video signal and hence, also on the carrier, this effect causes a phase modulation. Thus, a kind of conversion of an amplitude modulation to a phase modulation takes place. The phase modulation means that the IF carrier is phase modulated in the IF video signal. The phase-locked loop follows this phase modulation, so that the signal applied for demodulation to the video demodulator is also phase modulated. This, in turn, leads to the linearity of the video signal being influenced during demodulation.
SUMMARY OF THE INVENTION
It is an object of the invention to improve the circuit arrangement of the type described in the opening paragraph to such an extent that a phase modulation in the output signal of the phase-locked loop is suppressed.
According to the invention, this object is solved in that the phase comparator operates, by approximation, independently of modulation in the control range of the intermediate-frequency video signal, in that the baseband video signal is present in an inverted form with respect to the intermediate-frequency video signal, and in that a correction signal is derived from the baseband video signal by means of at least a feedback capacitor, this correction signal comprising signal components of opposite amplitude and phase position than those contained in the intermediate-frequency video signal and caused by the Nyquist edge, and is coupled to the loop filter.
The phase comparator in the PLL receives the IF video signal and the output signal from the voltage-controlled oscillator of the PLL. The output of the phase comparator supplies a control signal which is applied, via the loop filter, to a control input of the voltage-controlled oscillator. A condition for the faultless operation of the circuit arrangement according to the invention is that the phase comparator operates independent of modulation in the control range of the IF video signal, i.e., its output signal is actually dependent only on the phase relations of the two input signals and is not dependent on their modulation or amplitude.
The output of the voltage-controlled oscillator hence, the PLL supplies a signal which, in the ideal case, is sine-shaped, and is used for the video demodulator for demodulating the IF video signal applied thereto. The output of the video demodulator supplies the baseband video signal. In many known video demodulators, this signal is present in an inverted form with respect to the IF video signal. In the circuit arrangement according to the invention, this inverted baseband video signal must be generated either by a video demodulator constructed in this manner or by an inverter arranged subsequent to the video demodulator.
The baseband video signal generated in this way is fed back to the loop filter by means of at least one feedback capacitor. By means of this feedback capacitor, a correction signal is formed which compensates signal components which are comprised in the IF video signal and have been produced by generating the IF video signal by means of a Nyquist edge. These signal components are comprised in the correction signal with opposite amplitude and phase position so that they compensate each other in the loop filter with the corresponding disturbances.
In spite of the relatively simple construction, this compensation, by means of feedback via the feedback capacitor, is successful because the above-mentioned disturbances in the baseband video signal are present in the appropriate phase position which, in turn, is shifted by the feedback capacitor in such a way that the corresponding signal components are compensated.
An example will elucidate that this compensation actually occurs.
If the circuit arrangement receives, for example, an IF picture signal whose picture contents have the form of a cosine within which the contrast of the picture contents thus changes cosinusoidally via a picture line, then the carrier in the IF video signal is cosine-modulated, accordingly. Since this IF video signal was filtered from a high-frequency mixture by means of a Nyquist edge, the carrier comprises, however, a phase modulation. This has been produced by the fact that, in the range of the Nyquist edge, the side bands of the video signal are amplified to a differently strong extent. If the amplitude of the carrier fluctuates, a phase modulation is produced by this differently strong amplification of the side bands. Thus, a quasi-conversion from an amplitude modulation to a phase modulation takes place.
In the above-mentioned example, in which the picture signal is cosinusoidally modulated, the fundamental wave of this phase modulation is sinusoidal (90° phase-shifted). In addition to this fundamental wave, further phase modulation components are present which, however, are not important for the further processing of the signal. The IF signal with the sinusoidal phase modulation of the carrier is applied to an input of the phase detector. Since the output signal of the phase detector is, by definition, approximately proportional to the phase modulation of the input signal, the control signal at the output of the phase detector is also phase shifted by 90° with respect to the cosine-shaped picture signal. It is, however, decisive that the output signal of the phase detector comprises phase modulation disturbances which are produced by the above-described phase modulation of the IF picture carrier. However, due to the phase shift, these interference components are now sinusoidal in the control signal of the phase detector i.e., compared with the cosine-shaped phase modulation of the fundamental wave, they are phase-shifted.
Due to these phase disturbances, the phase detector thus supplies a signal fluctuating at a sinusoidal fundamental wave, with which signal, the VCO is corrected. This is undesirable because the VCO should operate at a constant frequency in order that the video signal can be linearly demodulated during the video demodulation. The video demodulator demodulates the IF video signal which, in this example, has the cosine-shaped picture contents. Many known video demodulators supply this signal as a baseband signal in an inverted form with respect to the IF video signal so that the picture contents are minus cosine-shaped in this case. If this were not the case, the demodulator should be followed by an inverter. In any case, a picture signal having a minus cosine-shaped variation is now available in the baseband position. Due to the phase modulation of the control signal of the VCO and hence also its output signal, each signal comprises harmonics which are, however, not disturbing for the further feedback processing operation in accordance with the invention.
According to the invention, the inverted baseband signal is fed back via a capacitor to the loop filter of the PLL. Due to this capacitor, there is a 90° phase shift so that the signal at the coupling point in the loop filter has a minus sine-shaped variation and thus has exactly the opposite phase position as compar
Biren Steven R.
Lee Michael
Natnael Paulos
U.S. Philips Corporation
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