CV Input Signal Types #10 — LFO CV
| Property | Value |
|---|---|
| Category | Bipolar / Modulation |
| Voltage Range | -5V..+5V (10Vpp, modern modules), -2.5V..+2.5V (5Vpp, older Doepfer) |
| Polarity | Bipolar |
| Bandwidth | DC..~20Hz (typical), up to ~500Hz with fast LFOs (Maths, Tides) |
Description
LFO CV (Low Frequency Oscillator) is a slow, periodic modulation voltage. LFOs typically oscillate below 20Hz and generate waveforms like sine, triangle, sawtooth, square, or random. The bipolar nature enables symmetric modulation around a center point.
Some LFOs (e.g., Mutable Instruments Tides, Make Noise Maths) seamlessly transition into the audio range (up to ~500Hz). The boundary between LFO and audio oscillator is fluid in Eurorack.
Use Cases
- Vibrato (pitch modulation, typically 3–7Hz)
- Tremolo (VCA modulation)
- Filter sweep (cutoff modulation)
- Panning (stereo field movement)
- PWM (pulse width modulation)
- Phaser/flanger sweep
Eurorack Examples
- Mutable Instruments Tides: Function generator, LFO output -5V..+5V, seamlessly up to audio rate
- Doepfer A-145: Classic LFO, ±2.5V
Hardware Implementation
Input Stage: Inverting Summing Amplifier with -10V Offset
Standard stage as V/Octave (R_in=100k, R_ref=200k, R_fb=33k, -10V reference). Maps -5V..+5V to 3.3V..0V. The full ADC range is utilized.
Clipping behavior: Eurorack signals can reach ±12V. At -12V input: op-amp tries to output +5.6V, clips at 3.3V. At +12V: tries -0.66V, clips at 0V. The single-supply MCP6004 thus protects the ADC.
Firmware Requirements
- Sample rate: kHz range recommended (e.g., 2–4kHz), to capture even fast LFO edges (square wave at 100Hz) without jitter
- Smoothing: Light IIR low-pass against ADC noise (α ≈ 0.9). Oversampling + decimation filters ADC noise optimally.
- Bipolar value range (-1.0..+1.0) in software