Trapezoid quadrature VCO through zero

Motivation

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 waveforms triangle, sine, ramp up, ramp down and pulse are covered in an extra module.


Update:Working on a new version. Check back later


Specs and features


Implementation

Schematic

Trapezoid VCO scheamtic

Trapezoid VCO schematic

Description:

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 screenshot trapezoid waveforms

Trapezoid VCO screenshot: Trapezoid quadrature waveforms


Trapezoid VCO screenshot square waveforms

Trapezoid VCO screenshot: Square quadrature waveforms


Trapezoid VCO screenshot thru zero modulation

Trapezoid VCO screenshot: Through zero modulation


Trapezoid VCO screenshot thru zero modulation

Trapezoid VCO screenshot: Through zero modulation


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Calibration

  • V/Oct adjust
    • Low frequency: Set the frequency to about 100Hz
    • Apply +1V to the V/OCT input (Use Scaled Voltage Reference or keyboard). The frequency should double
    • Adjust Trimmer TR_2 to until it works.
    • High frequency: Set the frequency to about 2500Hz
    • Apply +1V to the V/OCT input (Use precision voltage source or keyboard). The frequency should double
    • Adjust Trimmer TR_1 to until it works.
    • Repeat low and high frequency trim.
    • A more elaborate procedure is found here: NGF: VCO Core two
  • OTA offset voltage
    • Remove DG409
    • Ground the V/Oct input
    • Disconnect P4 or set it to center. Ground the wiper connector.
    • Set P1 (coarse frequency) slightly below 12:00.
    • Set P2 (fine frequency) to center.
    • Set P3 (FM log) to zero
    • Set P5 (TZ mod) to zero
    • Measure the DC offset at pin 13 of the OTA. Adjust trimmer TR1 to zero.
    • Measure the DC offset at pin 12 of the OTA. Adjust trimmer TR2 to zero.
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Building hints

  • R7 (2k tempco 3300ppm) is mounted beneath the SSM2220
  • Match C8, C9, C10 and C11
  • Match C4, C5, C6 and C7
  • Match R29, R31
  • Match R10, R11

Special parts

  • The SSM2220 could be replaced with two (hand matched) transistors
  • The OTA CA3280 could be replaced with the AS3280 (not tested though). The AS3280 comes in SOIC, so you have to use an adapter.

Download

Update: Docs withdrawn. Working on a new version. Check back later


Trapezoid VCO front view

Trapezoid VCO: Front view


Trapezoid VCO populated PCB

Trapezoid VCO: Populated PCB


Trapezoid VCO side view

Trapezoid VCO: Back view


2019 August

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