Abstract: A crystal filter with continuously variable passband, and a constant input and output impedance using quartz crystals, fixed inductors and varactor diodes. The width of the filter passband is set by a DC voltage on a single control line through a resistor biasing network.

Abstract: The disclosed dual mode direct current power supply comprises an AC transformer, a pair of DC output terminals, and first and second rectifier-filter circuits connected in parallel between the transformer and such terminals; the first circuit comprising first rectifiers for producing first rectified pulses, a first capacitor adapted to be charged to the peak voltage of such pulses, and an electronic voltage regulator including a series transistor; the second rectifier-filter circuit including second rectifiers for producing second rectified pulses, a choke input filter including an inductance and a second capacitor adapted to be charged through the inductance to the average voltage of the second pulses, and a series diode; the regulator causing the transistor to be nonconductive at high DC output voltage and then conductive increasingly in response to decreasing output voltage whereby the first rectifier-filter circuit supplies an increasing share of the load current.