Oscillators

LFO

The low-frequency oscillator — the modulation source that makes a patch breathe, sweep, and groove.

What is a LFO?

A low-frequency oscillator — LFO for short — is an oscillator that runs *below* the range of human hearing. Where a VCO cycles hundreds or thousands of times per second to produce a pitch, an LFO might cycle once every four seconds, or once every four minutes. It is not a sound source; it is a modulation source. Its job is to produce a slow, repeating control voltage that *moves* something else.

Almost everything in a synthesizer that *moves on its own* is being driven by an LFO. The slow swell of a filter cutoff. The gentle wobble of vibrato. A tremolo on a VCA. A panning sweep, a pulse-width wobble, a slow drift in detuning between two oscillators. All of those are an LFO patched into a CV input. Where a VCO is the *voice*, the LFO is the *gesture* — the part of the sound that feels alive because something is moving it.

The four classic LFO shapes — sine, triangle, sawtooth, square — are the same four a VCO outputs, but read at human-perceivable speeds they feel completely different. A sine becomes a *swell*. A triangle becomes a *symmetric ramp up and down*. A sawtooth becomes a *slow rise and a sudden snap back*. A square becomes a *hard alternation between two states*. Each shape has a feel; choosing the right one is half of what makes modulation musical.

Tempo-locked LFOs — ones that follow an incoming clock — are how movement gets into the rhythmic grid of a track. Without sync, an LFO drifts out of time with the music; with sync, every wobble lands on the beat. This is why a clock input is non-negotiable on any modulation source meant for performance.

Our LFO

Like our VCO, our LFO outputs all four classic shapes — sine, triangle, sawtooth, square — simultaneously, so you can route different shapes to different destinations from a single instance. Send sine to a filter for smooth sweeping, square to a gate input for rhythmic stabbing, sawtooth to pitch for slowly-rising sweeps — all at once, all locked to the same period.

The frequency range is intentionally wide: from 0.004 Hz (one cycle every four minutes, useful for ambient patches that evolve over the length of a piece) up to 1 kHz — well into audio rate. That last bit matters: at audio rate the LFO becomes a second VCO and can be used as an FM modulator, so the same module covers slow modulation, tempo-rate modulation, and audio-rate sound design.

Three features make it patch-friendly. The bipolar ↔ unipolar switch lets you swap between ±5V (centered on zero) and 0 to +5V (always positive) — important when modulating things like a VCA where you don't want negative voltage. The clock sync input measures the period between incoming pulses and locks the LFO to it, so movement is always in time with the rest of the patch. And the reset input snaps the phase to zero on a gate edge, so every note starts from the same modulation position.

In a patch

An LFO almost never makes sound on its own — it goes *into* something else. The most common destinations are: a VCO FM input (vibrato), a VCF cutoff CV (filter sweep), a VCA CV (tremolo), and a panner CV (auto-pan). Run a triangle LFO at 5 Hz into a VCO FM input through an attenuverter set low, and you have a perfectly classic vibrato.

Pick your LFO speed by what it's modulating. 0.02-0.1 Hz for glacial drift in ambient patches. 0.2-1 Hz for gentle movement that you notice as motion but not as rhythm. 2-8 Hz for traditional vibrato/tremolo. Above ~20 Hz an LFO crosses into audio rate and starts producing audible sidebands — at that point you've left modulation territory and entered FM synthesis.

Patch a clock into the LFO sync input to lock its period to the project tempo. Patch a gate from an envelope or sequencer into the reset input to re-zero the LFO phase on every note, so the modulation always starts at the same point in its cycle. These two inputs are what turn an LFO from a free-running drift into something composable.

Inputs

V/Oct (cv) — Pitch tracking input. 1V per octave. Lets you scale the LFO rate with a pitch CV so it tracks a keyboard or sequencer.
FM (cv) — Frequency modulation input. Modulates the LFO rate. Amount is set by the FM attenuverter knob. Useful for creating complex, evolving modulation patterns.
RST (gate) — Reset input. A rising edge (gate going high) resets the LFO phase to zero. Use to sync the LFO to note triggers or other events.
CLK (gate) — Clock sync input. The LFO measures the time between clock pulses and locks its frequency to match. Overrides the Rate knob when patched.

Outputs

SIN (cv) — Sine wave output. The smoothest waveform - great for vibrato and gentle filter sweeps where you want no abrupt transitions.
TRI (cv) — Triangle wave output. Linear ramps up and down. Good for tremolo and pitch sweeps. Slightly brighter-sounding modulation than sine.
SAW (cv) — Sawtooth wave output. Ramps up linearly then drops sharply. Creates asymmetric modulation - a slow rise followed by a quick snap back.
SQR (cv) — Square wave output. Switches abruptly between high and low. Perfect for trill effects, hard-panning, or as a clock/gate source for other modules.

Controls

Rate — LFO frequency from 0.004Hz (one cycle every 4 minutes) to 1000Hz (audio rate). Most modulation uses live in the 0.1Hz to 10Hz range.
PW — Pulse width of the square output, from 5% to 95%. At 50% it is a true square wave. Narrow pulses create short triggers; wide pulses stay high most of the time.
Offset — Bipolar/Unipolar toggle. Bipolar (default) outputs +/-5V centered on zero. Unipolar outputs 0 to +5V - useful when modulating a parameter that should only go in one direction (like filter cutoff).
Invert — Inverts all outputs. A rising ramp becomes a falling ramp, positive becomes negative. Handy for creating opposite-direction modulation on two targets.
FM Atten — FM attenuverter. Scales the FM input from -1 to +1. Negative values invert the modulation direction. At zero, the FM input has no effect.
PW Atten — Pulse width modulation attenuverter. Scales external PW modulation from -1 to +1.

Inspired by

A clean, full-range LFO topology with simultaneous sine/triangle/saw/square outputs, clock sync, and reset. The wide range (0.004 Hz to 1 kHz) and V/Oct tracking are conveniences contemporary LFOs converge on; the architecture itself is canonical.

Doepfer A-145
Make Noise MATHS
Buchla 281 Quad Function Generator

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