Simple broadband RF generator
This is called a marker generator.

You generate an extremely narrow, extremely fast rise and fall pulse at the desired spacing.  The harmonics of the pulse will appear at all multiples of the spacing frequency.

If, for example, you generate very narrow fast pulses at 100 kHz, you will generate harmonic signals at 100 kHz, 200 kHz, 300 kHz, ...

The rise and fall times determine how high the harmonic series goes before decaying to nothing.  Faster edges give more higher harmonics.