This circuit is from a Radio Electronics from March 1970. The crystal depends on the frequency at which the signal is to be captured and the resonant circuit determines the tuned band.
This circuit was obtained from an American 80's documentation. The balanced mixer can be shown in...
This circuit was found in a 90s Electronic Engineering. The quadruple operational amplifier can be...
I found this circuit in an English publication from 1989. This configuration can be mounted with...