The timing capacitor shown is actually for about 10 seconds of sweep time. There is no particular reason to use the crystal frequency shown - the HC family is functional at much higher frequencies. A very long time ago, the programmable inputs of the lower counter would have been driven by the parallel port of a computer to create a rudimentary musical synthesiser. If only one frequency is wanted at a time, then the inputs can be driven by dual-in-line switches, as shown in the insert. However, if a sweep is needed, the top counter must be included to drive the programmable inputs. An output mark to space ratio of 1 is yielded by the toggle flip flop.
About one second of sweep time is convenient to watch on the oscilloscope (or to photograph), but not much longer. As you will probably want several cycles at the lowest frequency as well, there is also a minimum for the scan time.
If you have access to a programming board for microcontrollers, matters are much simpler, as usual. It is prudent to draw the output through a 560 Ω resistor, but otherwise no external circuit is needed (other than a crystal): These pages may be of interest:
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