Motivation
I want to rotate sound through four channels of my modular synthesizer. Or move successive through different CV or audio sources. This is easily achieved with a quadrature LFO and four VCA. The core of this voltage controlled quadrature LFO was published in Electronotes EN#122 pg13ff 1981 and designed by Thomas Henry. I took the core and added the voltage control and the sine shapers.
Specs and features
- Four triangle quadrature outputs, 90° apart
- Four sine quadrature outputs, 90° apart
- Voltage controlled
- Hi-Low range switch
- Frequency from 30Hz down to some minutes
- FM lin
- FM log
- Runs on +/-15V and +/-12V
- Power consumption around 30mA each rail
Implementation
Schematic
Voltage controlled quadrature LFO schematic 01
Voltage controlled quadrature LFO schematic 02
Description:
The voltage control part and the sine shaper are well known designs. The triangle core is commonly used as well. The interesting part is how the 90° triangle is derived. The Electronotes EN#122 gives a very detailed description what is going on.
Quadrature LFO screenshot: Sine quadrature waveforms
Quadrature LFO screenshot: Triangle quadrature waveforms
Top
Calibration
- Triangle adjust
- Adjust TR4 for triangle symmetry of the 0° triangle
- Adjust TR5 for the offset of the 90° triangle.
- Sine adjust
- TR7 (TR9) adjusts the amplitude, TR6 (TR8) adjusts the waveform
- Start with TR7 (TR9) and then adjust TR6 (TR8).
- Those adjustments are not independent. Go back and forth until you are satisfied.
- Initial frequency
- Set coarse and fine pot to maximum.
- Adjust TR1 (TR2) for your max wanted frequency. I use 1Hz and 10Hz.
Building hints
- None
Special parts
- None
Download
Voltage controlled quadrature LFO documentation downloadVoltage controlled quadrature LFO Gerber files download
Voltage controlled quadrature LFO: Front view
Voltage controlled quadrature LFO: Populated PCB
