This is the receiver for the previous circuit, also found in 90s Radio Electronics. The circuit operates at 2.25 MHz, detecting variations in signal frequency due to the Doppler effect.
This circuit is an improvement of the previous circuit. An input resistor must be included...
This circuit uses a capacitor that depends on the frequency of the crystal. We found it in a 1967...
We found this circuit in a 1994 publication. It uses common components and can measure frequencies from 0...