Abstract: In a system in which an intermediate frequency signal is synchronously detected using an intermediate frequency carrier signal derived from the intermediate frequency signal by means of a tuned circuit and a limiter circuit to remove amplitude variations therefrom, an automatic fine tuning signal is derived in a phase detector to which are fed the intermediate frequency carrier signal after its phase has been shifted 90 degrees in a phase shifter and either the intermediate frequency signal or the intermediate frequency carrier signal. When the intermediate frequency carrier signal is used in the phase detector, a second tuned circuit is used therein and buffering between the two tuned circuits prevents interaction therebetween.

Abstract: A television receiver is disclosed in which there are provided a television signal receiving circuit including RF and IF stages; a video detector connected to the television signal receiving circuits; an AGC circuit connected between the video detector and television signal receiving circuit for automatically controlling the gain of the latter, the AGC circuit having a predetermined time constant; and a ghost detector connected to the video detector for detecting a ghost signal during a certain period in a vertical blanking period of the video signal from the video detector. In this case, a switching device is provided for switching the AGC circuit at least during the certain period in the vertical blanking period such that the detected ghost level is not substantially influenced by the transient level change of the AGC circuit.

Abstract: An automatic fine tuning (AFT) circuit includes a frequency discriminator for discriminating an intermediate frequency signal, a capacitor supplied with the discriminated intermediate frequency signal to produce an AFT voltage thereacross, a comparator circuit for comparing the AFT voltage with a reference voltage, a first switching transistor controlled by the comparator circuit and which supplies and charges the capacitor with the reference voltage during a channel selection operation when the AFT voltage is less than the reference voltage, a second switching transistor controlled by the comparator circuit and which connects the capacitor to ground to discharge the same during a channel selection operation when the AFT voltage is greater than the reference voltage, and a third switching transistor for rendering the comparator circuit operative during a channel selection operation and inoperative during an AFT operation.

Abstract: An automatic fine tuning loop in a television receiver is controlled by phase comparison between a video intermediate frequency signal and a reference oscillator signal. The reference oscillator signal is used in synchronous detection of the video intermediate frequency signal. The frequency of the reference oscillator is controlled by a phase locked loop, of which it is a part, and which includes a low-pass filter having selectable high and low cut-off frequencies. A lock-out detector selects the high cut-off frequency for rapid correction when lock-out is detected and selects the low cut-off frequency for frequency stability when lock-in is detected. An AFT disable switch disables the automatic fine tuning loop during manual tuning. In one embodiment, a disable switch ganged with the AFT disable switch selects the high cut-off frequency in the low-pass filter to prevent hunting.

Abstract: A muting circuit for a television or AM signal receiver which uses a synchronous detector.The muting circuit includes an input terminal for receiving an amplitude modulated signal, a synchronous detector for detecting such AM signal, a circuit for supplying a reference signal having a predetermined frequency and phase to the synchronous detector, an amplifier, a circuit path for delivering to the amplifier the detected signal produced by the synchronous detector, a level detecting device connected to the output of the synchronous detector for producing an excess signal when the detected signal exceeds a predetermined level, an integrator for producing a muting signal in response to the production over an extended period of time of the excess signal, and a switching device connected to the amplifier for muting the signal produced by the amplifier in response to the production of the muting signal.

Abstract: A synchronous detector adapted to detect a modulated information signal, such as a video IF signal. The modulated information signal is provided in the form of a vestigial sideband signal. A band-pass filter including a tuned circuit tuned to the frequency of the carrier on which the information signal is modulated has a pass band which is less than the frequency spectrum of the vestigial sideband signal so as to limit the frequency spectrum of the signal passed by the filter to a double sideband signal. A limiter is coupled to the band-pass filter to receive the double sideband signal and for deriving a switching carrier therefrom, the frequency of the switching carrier being equal to the frequency of the carrier on which the information signal is modulated. An emitter-follower circuit is connected between the band-pass filter and the limiter so as to couple the double sideband signal from the filter to the limiter.

Abstract: An oscillating circuit provides first and second reference signals having a phase difference of 90.degree. with respect to each other. The oscillating circuit includes an oscillator providing the first reference signal at an output thereof, a voltage-to-current converting circuit, which may incorporate a common-base transistor, and a capacitor connected between the output of the oscillator and an input of the converting circuit. The output of the converting circuit then provides the second reference signal which differs in phase by 90.degree. from the first reference signal. The oscillator may include differentially connected transistors and an LC resonant circuit.

Abstract: Demodulating apparatus for a modulated signal comprised of a carrier deriving circuit which is supplied with the modulated signal and which derives an unmodulated carrier therefrom, the carrier deriving circuit including a tuned circuit having a center frequency substantially coincident with the carrier frequency of the modulated signal and a limiter circuit in which the limiter circuit imparts a phase shift to the signal supplied thereto in connection with its elimination of the amplitude fluctuations in the signal produced by the tuned circuit. A synchronous detector receives the modulated signal and the derived carrier so as to detect the modulated signal with the derived carrier.

Abstract: A television receiver has an automatic fine tuning circuit with a first phase-locked loop including a first low pass filter and a synchronous video detector with a second phase-locked loop including a second low pass filter. The cut off frequency of the first low pass filter is selected lower than the cut-off frequency of the second low pass filter, thereby enabling a stable operation of two phase-locked loops simultaneously.

Abstract: An amplifier provides a complete video and sound I.F. signal to a synchronous detector switching circuit, and the switching signal to be applied to the switching circuit is obtained from the same amplifier by connecting a grounded base stage, with its emitter impedance, in series with the amplifier load. The grounded base stage has a load tuned to the I.F. carrier frequency, and the filtered signal from the grounded base is the signal applied as the switching signal to the synchronous detector switching circuit.