Abstract: A radio receiver comprises a frequency conversion circuit connected between a frequency-variable local oscillator and a divide-by-N programmable counter incorporated in a phase locked loop frequency synthesizer and adapted to continuously change the frequency to be divided by the programmable counter. Owing to the frequency conversion circuit, the local oscillator frequency is permitted to continuously change in the locked state of the phase locked loop so that the intermediate frequency offset adjustment of receivers, that is, the adjustment of the intermediate frequency of each receiver to the center frequency of an intermediate frequency filter used in the receiver can be made easily, and that a reception of such a broadcast station as having a carrier frequency which is not an integral multiple of a channel spacing frequency also can be made.

Abstract: A radio receiver such as an FM stereophonic receiver includes a detector circuit connected to an intermediate frequency amplifying stage in the receiver for delivering a signal responsive to the signal strength and the degree of multipath interference, and a light emission section controlled in its average brightness and flicker in accordance with the detected signal so as to visually indicate the signal strength and the degree of multipath interference, respectively. The light emission section preferably is used also as another indicating device such as a dial pointer or a stereophonic indicator.

Abstract: A noise eliminator for a radio receiver comprises an automatic gain control amplifier for amplifying a noise signal extracted from an intermediate-frequency signal or a detector output signal of the receiver, a circuit for producing a faithfully pulsating noise signal and an average noise signal from the output of the amplifier circuit, an operational circuit for detecting an impulse noise component through a comparative operation of the pulsating noise signal and the average noise signal, and a gate circuit for controlling a signal transmission in the receiver in accordance with the detected impulse noise component.

Abstract: A low-impedance loop antenna for receiving medium-frequency-band radio waves has an inductance of 1 microhenry up to 100 microhenries and an effective loop area larger than 20 cm.sup.2, whereby it is capable of receiving radio waves with a markedly improved rejection of undersirable radiations in the form of electric field from external noise sources. Such low-impendance loop antenna is coupled to a radio receiver via an input transformer whereat the impedance of this antenna is stepped up for the purpose of impedance matching. The input transformer has a primary winding provided with a tap adapted to be connected to the antenna and whose ends are connected to a variable capacitor constituting a tuning circuit jointly with the input transformer.

Abstract: A receiver for selectively receiving either an FM-FM type television sound multiplex broadcasting or an FM-AM type FM stereo multiplex broadcasting includes a PLL (phase-locked loop) circuit for forming a 38 kHz switching carrier for demodulating an FM stereo multiplex signal during reception of the FM stereo multiplex broadcasting, which PLL circuit is concurrently used to attain various purposes in receiving the television sound multiplex broadcasting. In a first embodiment, the PLL circuit is used for FM-demodulating a subchannel signal during reception of the television sound multiplex broadcasting. In a second embodiment, the PLL circuit is used to detect a state that the television sound multiplex broadcasting wave is received during reception of the television sound broadcasting. In a third embodiment, the PLL circuit is used to detect whether the television sound multiplex broadcasting being received is a stereo broadcasting or a dual sound broadcasting.

Abstract: In a superheterodyne radio receiver, by observing the RF and IF signal levels is detected an occurrence of an interference caused by an interfering strong signal of an adjacent station at a frequency close to that of a desired station signal to be received by the receiver. In case such interference is detected, a cancelling signal is fed to the RF amplifier stage to cancel out this interfering station signal, or the selectivity of the RF amplifier stage is set to be sharper in order to reject the interfering station signal.

Abstract: A radio receiver includes a phase-locked loop which is locked with the tuned frequency of a broadcast radio wave applied thereto, so that the output of the phase-locked loop is displayed by a display unit to display the tuned frequency. The displayed frequency is free from possible deviation of a local oscillation frequency from the tuned frequency.

Abstract: In a station signal frequency indicating system for a superheterodyne radio receiver, a station-signal frequency received by the receiver is evaluated on the basis of the oscillating frequency of a local oscillator in the receiver, the evaluation being compensated for a difference between the tuning frequency of the receiver and the station-signal frequency received. This evaluation is performed by, for instance, counting a signal generated at the local oscillator by a counter, and the compensation of evaluation is accomplished by, for example, varying a preset number for the counter in accordance with the dc-voltage level of the output of a demodulator in the receiver. Such system is capable of indicating a correct station-signal frequency so long as the frequency difference is less than a predetermined value, even when the receiver is not tuned exactly to the station-signal frequency.

Abstract: The front end circuit is constituted by first and second signal paths for transmitting a high frequency signal received by an antenna, and a transfer switch for selecting either one of the signal paths depending upon the state of the received signal, and a circuit for converting the frequency of either one of the selected signals to an intermediate frequency signal. Two signal paths have different filtering attenuating and amplifying characteristics, thereby providing an optimum condition for either a local or distant reception of radio wave signals.

Abstract: An FM radio receiver in which the output of an FM detector is direct-coupled with the input of a stereo multiplex demodulator. The stereo multiplex demodulator circuit includes an operational amplifier having first and second inputs, a switch circuit having first and second outputs at which separate left and right audio signal components appear, and negative feedback paths having feedback resistors and led from the first and second outputs of the switch circuit to the first input of the operational amplifier. The second input of the operational amplifier is kept at a given DC potential so that the first input of the operational amplifier is biased to the given DC potential. A reference potential point of the FM detector is supplied with the given DC potential so that a DC potential at the output of the FM detector is made equal to the given DC potential, permitting direct-coupling of the output of the FM detector to the input of the stereo multiplex demodulator.

Abstract: An antenna input circuit for an AM/FM radio receiver comprises an input transformer for FM broadcast reception, an input transformer for AM broadcast reception, at least one antenna terminal for the connection of the transmission line leading from an FM-receiving antenna, and a filter network connecting the antenna terminal to each primary winding of those input transformers. The filter network is such that from the antenna terminal, it feeds, with little loss, FM broadcast signals to the FM input transformer, while it feeds, with an allowable amount of loss, AM broadcast signals to the AM input transformer. Thus, the antenna input circuit allows an AM/FM receiver to receive either an FM broadcast or an AM broadcast by using only a single antenna and without changing the connection of the antenna transmission line.

Abstract: A plurality of parallel branch paths are connected in an intermediate frequency or high frequency amplifying circuit of a radio receiver for the purpose of switching the bandwidth. Between an intermediate point of each branch path and the ground is connected output electrodes of a transistor, and the base electrode of the transistor is selectively connected to a source of control voltage via a transfer switch to render the selected transistor conductive.

Abstract: A demodulation circuit for an FM stereo receiver is constructed to invert the FM stereo composite signal by an inverting amplifier and then to supply the inverted signal to a switching circuit for producing demodulated left and right signals. The composite signal is applied to a phase-locked loop circuit for applying a 38 kHz switching signal to the switching circuit. A 19 kHz signal produced by the phase-locked loop circuit is converted into a sine wave signal by a waveform converting circuit. The sine wave signal is converted into a direct current signal which is applied to one input of a level comparator having the other input connected to receive the 19 kHz pilot signal extracted from the composite signal. The output of the comparator is sent to a level control circuit which controls the level of the signal supplied to the input of the inverting amplifier. Accordingly, the pilot signal is canceled in the signal sent to the switching circuit.

Abstract: In a radio receiver circuit provided with an automatic gain control circuit, a tuning indicator driving circuit is connected to a point in the signal transmission line of the receiver circuit at which the ac voltage level is zero and the dc voltage level is controlled by the output of the automatic gain control circuit. The change of the dc voltage level is utilized to indicate the input signal level in an indicator.

Abstract: An FM stereophonic receiver includes a system for producing a test signal having an audio frequency and a constant amplitude, permitting an easy setting of the input level of a tape-recorder which will record an FM program through the receiver. The system is so arranged that the oscillation frequency of a voltage-controlled oscillator involved in a phase-locked loop circuitry of a multiplex decoder in the receiver can be changed so as to make it possible to derive the test signal from a certain point in the circuitry. No separate audio signal generator is required for producing the test signal.

Abstract: In an FM receiver, the noise component is extracted from the output of the FM detector of the receiver, and the extracted noise component is rectified by a rectifier circuit. The output of this rectifier circuit serves as a driving signal for the tuning indicator or the muting circuit of the receiver.

Abstract: An FM multiplex stereo demodulating circuit for obtaining separated left and right channel signals from a composite signal containing a stereophonic signal and a pilot signal, wherein the pilot signal component in the composite signal to be applied to a multiplex demodulator is cancelled by an inverted pilot signal, thereby expanding the frequency range and decreasing the carrier leak. The inverted pilot signal is produced by a sub-carrier generating circuit comprising a phase locked loop circuit and a phase shifter.

Abstract: A composite signal consisting of a plurality of signals of different frequencies is applied via a cancelling circuit to a phase-locked loop circuit which is adapted to be locked with a certain signal contained in the composite signal to generate a signal synchronous with this certain signal. A cancelling signal generated from a cancelling signal generating circuit is applied, in addition to the composite signal, to the cancelling circuit when the phase-locked loop circuit is in the locked state, thereby cancelling, by this cancelling signal, those signals contained in the composite signal other than the certain signal. Thus, only the certain signal is inputted from the cancelling circuit to the phase-locked loop circuit when the latter is in the locked state.

Abstract: A noise-cancelling apparatus for use in an FM stereo receiver. This apparatus blends the signals of the left and the right channels of the receiver with each other substantially evenly throughout the entire audio frequency range of such signals so that noise components are cancelled with each other and S/N ratio is improved. This blending is accomplished either by connecting the output terminal of the left channel to the output terminal of the right channel by a common resistor, or by causing a decay, through a filter, of those frequency components higher than the pilot signal frequency among the input signals of the multiplex demodulator provided in the FM stereo receiver.

Abstract: An FM multiplex receiver comprises an averaging detection type demodulator circuitry. Two negative feedback paths lead from the L and R outputs of the switching circuit to the input of the amplifier, the output of which is connected to the input of the switching circuit. Output signals from the switching circuit are time-shared alternative ones in the stereophonic reception and continuous ones in the monophonic reception. The amplification factor observed during monophonic reception is substantially one half of that observed during stereophonic reception. With a very simple circuitry, the occurrence of undesired difference in sound levels is prevented between the monophonic listening and the stereophonic listening.