Trapezoid quadrature VCO

Trapezoid quadrature VCO front view
Trapezoid VCO front view

I always wanted a VCO with through zero capacity. Why not combining this with a unusual wave form and quadrature outputs? Usable as LFO as well? I found the original article and schematic about the Trapezoid VCO on Don Tillman’s site (Link to original article from 19 July 2003). The article consists off three parts with the core implementation in part 2. I kept the basic idea and changed nearly everything else. I use an other exponentiator scheme and temperature stabilization. The addressing hardware for the DG409 is changed. A LFO option is added. Another reference voltage device is used. And quadrature square outputs are implemented. The additional wave forms triangle, sine, ramp up, ramp down and pulse are covered in an extra module

Specs and features

  • Trapezoid quadrature output
  • Square quadrature output
  • Through zero modulation
  • V/Oct, FM log and trough zero CV input
  • Temperature compensated
  • Voltage controlled pulse width
  • LFO Range switch
  • Coarse and fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption around 65mA each rail

The documentation and the Gerber files for download can be found in my website.

Trapezoid VCO schematic
Trapezoid VCO schematic

J. Donald Tillman did an excellent job describing the core of his Trapezoid VCO. Please refer to the original article as linked above. The exponentiator I use is a well known and classical design. You can find many description of it out there. The rest is straight forward. The LFO option is implemented with the DG202. Here are just four capacitors added switchable in parallel to the audio frequency capacitors. The connectors shown are for adding the waveform module to generate triangle, sine, ramp up, ramp down and pulse.

Trapezoid VCO populated PCB
Trapezoid VCO populated PCB
Trapezoid VCO quadrature output
Trapezoid VCO quadrature trapezoid output
Trapezoid VCO quadrature square output
Trapezoid VCO quadrature square output
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation
Trapezoid VCO through zero modulation