One of the most powerful aspects of modular synthesis is the fact that almost everything can be modulated, making possible highly advanced sounds that change over time and can seem almost alive. Meet the LFO.
Low Frequency Oscillators, or LFOs, are quite simply oscillators with a slower range. Their only job is to constantly cycle between minimum voltage and maximum voltage at a speed set by the frequency parameter. Whereas typical VCOs often can’t go slower than 20Hz (20 cycles per second), LFOs can often go slower than several minutes between cycles. Because of this, LFOs get used for cyclic modulation that is impossible to do with standard oscillators.
One way to use an LFO on a standard keyboard patch is to create vibrato (pitch modulation) by slightly modulating a VCO. Patch from an LFO output into a VCO’s FM input, often this is accompanied by a CV amount knob that you can use to control the depth of the modulation. The higher you turn the LFO’s frequency knob, the faster the pitch fluctuations will occur.
In order to create tremolo (volume modulation), you could patch from the LFO output into another CV input on your VCA and that would achieve cyclic volume fluctuations but there would be one problem. When not pressing a key your VCA should be closed so that no sound comes out, but in this case the LFO will always be active so the VCA will leak sound. To get an ideal tremolo you should patch from the VCA output into the input of a second VCA. Now you can patch an LFO output into the CV modulation input of the second VCA to get silence between notes but volume modulation during pressed keys. The following photo illustrates a keyboard patch with tremolo and uses a MIDI to CV converter module so that a MIDI keyboard such as a KMI 12 Step can be used to control the synth.
Although triangle and sine waves are popular waveforms for LFOs, often saw and square waves are available as well. Square waves are very handy for outputting a regular stream of trigger signals, which are useful for many purposes including driving the clock input of a sequencer module.
Watch the following video for an example of a voltage controllable LFO by Pittsburgh Modular: