This circuit is from a 1924 book by Gernsback. The receiver uses two tubes and listens to headphones. Power is supplied by battery with the indicated voltages.
This low-noise, low-drift amplifier was found in a 90's EDN. It can be implemented with modern...
Using a CMOS 4001, this circuit provides frequency readings on an analog instrument. The idea of...
This analog frequency meter for low frequencies was found in Radio Electronics magazine from July...