Found in a 90s Popular Electronics, this transistorized circuit generates pulses from 2 Hz to 50 kHz in three selectable ranges. The circuit has both AC and DC outputs.
This application is not modern since few computers have parallel ports. But you can use this...
Using a 741 op-amp this circuit high-pass to 10 kHz was found in the February 1995 issue of...
Found in a 1969 EEE, this circuit is based on an operational amplifier of the time and on old type...