Abstract: An AM Stereo receiver is described which includes an AGC having an output which indicates whether the received signal has significantly decreased in level and having a clamping circuit for controlling the voltage at the input of a VCO. The clamping circuit is further defined as having a window detector for sensing a change in the voltage at the input of the VCO according to a predetermined threshold; a selectable controller, responsive to the window detector, for adjusting the voltage at the input of the VCO; and a window reduction circuit, responsive to the output signal of the AGC, for altering the predetermined threshold to provide the controller with a more sensitive indication of a drift in the voltage at the input of the VCO. The receiver additionally includes a conventional locked loop driver for steering the VCO according to the phase of the received input signal.

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: An attenuator for use in attenuating an incoming signal. The attenuator includes a primary attenuation unit (12) and a dynamic reference signal unit (13) that provides a reference signal that substantially tracks a DC component associated with the attenuator. A comparison unit (14) subtracts the reference signal from the attenuated signal to provide an attenuated signal substantially free of unwanted signal components. In one embodiment of the invention, a variable impedance unit (101) can be provided to supplement the attenuation action of the attenuator (12) and effectively expand the attenuation range of the circuit and thereby provide greater freedom from distortion.

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: An analog divider circuit as for use in AM stereophonic receivers provides minimum phase variation with control voltage level change, by means of a feedback circuit. The division process is performed by subtracting logarithms of currents produced by the dividend and divisor voltages, then deriving the antilog of the difference or remainder. At the critical point in the process, where junction capacitances do not accurately track the change in the control level, cross-coupled capacitors stabilize the circuit by feeding back a signal from the circuit output to minimize any undesired phase changes.

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: A function generator as for use in a system of compatible quadrature AM stereo approximates the tangent function curve, allowing for the provision of essentially undistorted intelligence signals. A form of differential amplifier, including additional multiple threshold transistor circuits with common collectors and common bases, has at its input terminals a signal related to an angle of modulation and provides an output signal approaching a signal related to the tangent of the angle of modulation, the accuracy of the approximation depending on the number of circuits included. The function generator has the capability of providing other similar functions and of use in other applications. As used for AM stereo decoding, a tangent function output is derived from the phase modulation components of the stereo signal and, when multiplied by the amplitude modulated signal, the stereo difference signal can be derived.

Abstract: An automatic chrominance gain control circuit for a color television receiver distinguishes between lower frequency chrominance variations (large color objects) and higher frequency chrominance variations (small objects) to minimize gain control for large color objects and to emphasize gain control for highly saturated contrasting colors in small objects across the picture field.