Abstract: A linearized differential amplifier having two feedback paths. One of the feedback paths functions to linearize the output of the differential amplifier. The remaining feedback path functions to slightly limit the operating range of the differential amplifier to ensure the operational stability of the differential amplifier.

Abstract: A voltage controlled oscillator having a first feedback path and a second feedback path. The first feedback path may include a phase shift unit (13) as necessary to compensate for an inherent phase shift provided by the resonant circuit (11) used therewith. The second feedback path includes a steering network (17) that responds to an external control signal to cause the output signal to be selectively in either of two phase relations with respect to the incoming signal. The control signal can also control the magnitude of the output signal. By control of the phase relationship and the magnitude, the steering network (17) can effectively control the output frequency of the resonant circuit (11).

Abstract: In a radio having an in phase multiplier (A) for providing an in phase detected signal, a detection unit for receiving this signal and for determining whether this signal includes any negative polarity components which would indicate that a viable broadcast signal does not exist on the incoming frequency of interest. This detection unit can include a comparator (11) for comparing the incoming signal against a selected reference value. The invention can also include a signal quality evaluation unit (58) for providing either a digital or analog quality signal that can also effect the output and be used by appropriate processing circuitry to further refine the channel selection process.

Abstract: An adaptive phase locked loop being particularly useful with low frequency input signals, the adaptive phase locked loop including a phase comparison unit (12) for comparing a component of the input signal with the reference signal having an average phase, a polarity detection unit (14) for providing a polarity indication signal dependent upon the polarity of the output signal, and a reference signal unit (13) for providing a reference signal having an average phase to the phase comparison unit (12). In response to the polarity indication signal, the reference signal unit (13) can vary the rate at which the average phase of the reference signal can be changed at either of two rates, with the first rate representing a relatively slow rate and the second rate representing a relatively fast rate.

Abstract: A control circuit in an AM stereophonic receiver examines a detected signal for occurrences of overmodulation and, when a first given level of overmodulation is reached, begins to turn down the correction signal used in the demodulation and, if overmodulation continues, would eventually eliminate the correction signal. At a second given level of overmodulation, the circuit will begin to attenuate the stereo difference signal from the receiver output and eventually eliminate it. When the overmodulation is reduced, operating conditions return toward normal.

Abstract: A variable Q filter network in an AM stereophonic receiver provides elimination of any adjacent channel carrier interference while providing the function of a tone control, either automatic or manually controlled. Parts count and costs, IC pinouts and the possibility of intermodulation are all reduced.

Abstract: A correction control circuit for use in compatible AM stereophonic receivers of the type utilizing a cosine correction signal to remove inherent distortion from the received and detected signals. During periods of weak or noisy signals or when co-channel interference is present, excess cosine correction signal causes a reduction in the amount of stereo difference signal component in the output signals, and in the amount of correction provided. In a worst case signal situation, there will be no difference signal component in the output and the monophonic output not be corrected.

Abstract: The tone detector provides accurate detection of a single predetermined tone frequency within a mixture of frequencies, utilizing a carrier signal source at a relatively high frequency. The high frequency signal is divided down to two frequencies, one just above and one just below the tone. The divider output is then maintained at the frequency of the tone by continuously adjusting the divisor number between the two divisor values. Multiplying the received signal or a portion thereof by the divider output provides a detect signal when the tone is present in the received signal with sufficient amplitude.

Abstract: In a stereophonic broadcasting receiver, a single indicator with two levels of illumination serves to provide indication of both accurate tuning and the presence of stereophonic signals. One signal or a combination of signals related to the tuning circuits of the receiver cause the indicator to be illuminated to a perceptible level. When, by use of a pilot tone detector, it is certain that stereophonic signals are being received, additional current is caused to flow through the indicator, raising the illumination to the maximum allowable level. The circuit has particular application to manually tuned receivers.

Abstract: The AFC gain of the local oscillator of an AM stereophonic decoder is linearized by the use of difference channel (L-R) information for controlling the capacitance of two varactor-type devices coupled across and in series with the oscillator inductance coil. The improvement helps to prevent "falsing" of the pilot tone indicator and the mono/stereo mode circuitry, particularly at the ends of the tuning range of a manually tuned receiver.

Abstract: In an AM stereo system wherein a low frequency pilot tone is added to the difference channel signal before modulation of the carrier, "image shifting" and audio distortion in a receiver, due to residual pilot tone signal, are removed by separating out a portion of the pilot tone after it has been filtered and phase shifed for enabling the stereo mode of operation. The separated signal is then inverted and delayed appropriately for each of the PLL and audio circuits.

Abstract: In the field of AM stereo detection, co-channel or adjacent channel interference may cause some distortion and side-to-side movement of a stereo signal source as well as false "stereo detect" signals in AM stereo receivers having phase locked loop (PLL) systems. The invention provides a filter receiving the incoming signal and passing only the frequencies resulting from said interference; a sensor measures the output of the filters and if this signal is above a given level, the monophonic mode is enabled, i.e., the pilot detector, the stereo presence indicator and the stereo mode switch are disenabled for a timed interval. A higher given level of interference is required for stereo disenablement when stereo transmission with pilot tone is already being received.

Abstract: A low frequency tone detector filters out undesired frequencies in a received signal, than a latching circuit locks on when a given number of low frequency cycles are detected, having at least a predetermined peak amplitude. Thereafter, only a second given number of missing or low amplitude cycles will reset the latch. When the detector determines that the received signal is weak or of very poor quality, false detect signals are essentially prevented. The circuit is particularly suited to use with AM stereophonic signals where a low frequency tone is added to the difference channel for providing mono/stereo indication and mode switching.

Abstract: A low frequency tone detector filters out higher frequencies, then a latching circuit locks on when a given number of low frequency cycles are detected which have at least a predetermined peak amplitude. Thereafter, only a second given number of missing or low amplitude cycles will reset the latch. The circuit is particularly suited to use with AM stereophonic signals where a low frequency tone is added to the difference channel for providing mono/stereo indication and mode switching. The circuit can also provide an early detect for use by a scanning circuit in a receiver.

Abstract: A mechanical radio receiver tuner with microphonics elimination circuitry is disclosed. During a mechanical tuning mode, the mechanical position of a movable tuning element provides an associated adjustable reactance which determines a control voltage for controlling the output of a voltage controlled oscillator (VCO) that serves as the local oscillator for a superheterodyne radio receiver. After initially selecting a channel frequency by adjusting the output of the VCO with the mechanically movable tuning element in the tuning mode, a switch means disconnects the movable tuning element from operative control of the local oscillator frequency and implements a locked tuning mode by providing the VCO with a control voltage related to the difference between the frequency of the output signal of a mixer stage and the center frequency of a fixed tuned narrow passband IF stage that follows the mixer. Preferably, this control voltage for the locked tuning mode is provided by a phase locked loop.

Abstract: An AM stereophonic pilot tone is detected using two signals derived from a received broadcast signal. One signal is proportional to the phase deviation and one is a two-level signal which is a function of the magnitude of the phase deviation. The detector controls activation of the visual indicator, muting of the stereo channel of mono/stereo mode switching with almost complete elimination of false detections during monophonic transmission and poor stereo transmission.