This circuit was found in documentation from the 1970s. However, it can be implemented with modern transistors such as 2N2222 for Q1 and BF494 for Q2. The crystal depends on the desired frequency.
This circuit source a voltage proportional to the number of the input pulses. The circuit has a...
This circuit is from a 90's Radio Electronics. The circuit operates accurately up to about 15 kHz...
This is a circuit suggested by Linear Technology in a 1980s documentation. The integrated circuit is from...