This circuit can be used to test crystals from 1 to 20 MHz and also has an output to measure their frequency. We found it in the November 1998 Electronics Now magazine.
This circuit identifies the polarity of DC signals and if the signal is AC. The circuit is from...
Found in American documentation from the 1990s, this circuit generates signals in the 350 to 500...
This circuit was obtained from HAM Radio magazine in an 80's issue. The FETs are types of the time...