Abstract: A mixer is disclosed for use in a dual conversion AM-FM radio receiver in which AM and FM broadcast RF signals from a first IF amplifier are converted by the mixer from a first intermediate frequency to a second intermediate frequency for amplification in a second IF amplifier. A double balanced mixer circuit receives a signal at the first intermediate frequency and a local oscillator signal to generate signals at the second intermediate frequency on its two complementary outputs. One of the outputs is connected to the second IF stage through a narrow bandpass ceramic filter at the second intermediate frequency and a transistor switch. The other output is connected to the second IF stage through another transistor switch. The transistor switches are responsive to an external signal to alternatively connect the output through the ceramic filter for AM signals and the other output for FM signals.

Abstract: A noise blanking signal generator for an AM radio receiver includes a product detector adapted to receive an IF signal modulated by an AF signal and synchronously detect the AF signal, a first low pass filter adapted to derive from the product detector a filtered voltage indicating the average IF signal strength, a second low pass filter adapted to derive from the product detector an AF signal voltage and a reference voltage generating circuit ratiometrically related to the product detector through a common DC power supply and effective to generate a reference voltage substantially equal to the filtered voltage in the absence of an IF signal.

Abstract: An AGC voltage generating circuit for an AM radio comprises a product detector adapted to receive an IF signal modulated by an AF signal and synchronously detect the AF signal, a low pass filter adapted to derive from the product detector a filtered voltage indicating the average IF signal strength, and a reference voltage generating circuit ratiometrically related to the low pass filter through a common DC power supply and effective to generate high, medium and low reference voltages ratiometrically related to each other with the high and low reference voltages defining a voltage window about the medium reference voltage. Comparator apparatus is effective to compare the filtered voltage from the low pass filter with the high, medium and low reference voltages from the reference voltage generating circuit. Current source apparatus is responsive to the comparator apparatus to control the voltage on a capacitor to maintain the filtered voltage equal to the medium reference voltage.

Abstract: An automatic gain control circuit is disclosed which is direct coupled to phase opposed outputs of an IF amplifier and provides a DC control voltage for controlling the gain of the amplifier. The AGC circuit includes a peak-to-peak threshold detector and an integrating capacitor. The quiescent voltage across the capacitor is accurately established by a voltage divider network and a feedback network. Separate current sources are provided to establish dual time constants in the circuit so that distortion is minimized while accommodating a stop on station feature.

Abstract: A protection circuit for a bridge audio power amplifier having two pairs of output transistors coupled between positive and negative terminals of a direct current supply and providing a pair of output terminals for connection across a load. The bias of the output transistors are controlled in response to excessive current flow, a short of the output terminals to ground, excessive DC supply voltage, a short of the output terminals to the positive terminal of the DC supply, and excessive temperature to prevent permanent damage to the bridge audio power amplifier.