Abstract: A heterodyne stage of a receiver which includes a conventional mixer circuit and local oscillator circuit further includes a differential amplifier circuit which measures the amplitude of the injection signal generated by the local oscillator circuit and controls it precisely to a desired reference level by adjusting the current bias supply to the local oscillator circuit within a banded range to ensure start-up of the local oscillator upon energization and to protect against loss of injection signal under all operating conditions. The differential amplifier circuit provides for a precise amplitude reference setting and sufficient closed-loop gain for controlling the amplitude of the injection signal with a minimum of error between the measured and reference values.

Abstract: An LED and battery powered driver circuit provide efficient operation to conserve battery energy and maintain the intensity of light emitted from the LED at a substantially constant level despite changes in the battery voltage due to charging and discharging. A pulsed voltage output of a switching power supply is capacitively coupled to the LED. The voltage amplitude of the output pulses is maintained substantially constant by a duty cycle modulator which is connected in a feedback path of the switching power supply. A diode or a transistor circuit is provided to discharge the capacitor between voltage pulses.