To modulate sound signals by rhythmical amplitude fluctuations was a technique that has already been used in the 1950s and became famous again through grunge music.
Besides the first DeArmond tremolo effect, about which I already wrote in this article, there are many other possibilities to produce a tremolo effect.
The harmonic tremolo is a special kind of tremolo that has been used in the Fender amps of the 1960s.
After spitting the signal into a high pass and a low pass one of these signals is modulated by LFO, the other one by a reversed LFO.
By this you can emphasize either the bass or the treble.
This sound almost reminds of the sound of a vibe.
And for years this kind of effect is available in pedals.
The Strymon Flint for example has got a harmonic tremolo mode, as well as the Earthquaker Devices Night Wire and the new Supro Tremolo.
The Chase Bliss Audio Gravitas is at least able to create a similar sound.
Ring modulator as a tremolo
To create a tremolo with a ring modulator can also produce an interesting sound.
A very low trigger frequency will cause phase cancellation.
A nice example for this is the Moog MF-102.
But some other ring modulators also are able to create tremolo effects.
The classical tremolo sound of amplifiers is created by a LED or a power tube bias fluctuation.
While the smaller Fender amps (Vibro Champ, Princeton Reverb) use the bias tremolo, which creates a particularly harmonic tremolo effect, the bigger amps (like Deluxe Reverb and Twin Reverb) use a LED tremolo.
The Fulltone Supa Trem and the Demeter Tremulator serve a pretty good emulation of this sound.
Like all modulations the tremolo effect can vary depending on the type of waveform.
Most common is the sinusoidal modulation which causes a soft tremolo while the rectangular modulation rather chops the signal up.
Some pedals make it possible to choose a certain wave form and also provide sawtooth wave or random modulation.
The right place in the effect chain
If the tremolo has got tap tempo or can be synchronized via midi you can create marvelous sounds by using the tremolo behind a delay.