Trapezoid quadrature through zero VCO (Euro version) with waveshapers

Trapezoid quadrature through zero VCO with waveshapers: Front
Trapezoid quadrature through zero VCO with waveshapers: Front

This is my third take on the Trapezoid VCO core designed by Don Tillman. My first implementation for a 15V banana system with separate waveshaper can be found here. My second implementation for a 15V banana system with integrated waveshaper can be found here.This time I moved on to the 12V Eurorack format. The core is still based on the original design from Don (used with permission). 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. Another reference voltage device is used. A octave switch is added. And quadrature square outputs are implemented. As well as the additional waveforms triangle, sine, ramp up, ramp down and pulse.

Specs and features

  • Trapezoid quadrature output
  • Square quadrature output
  • Triangle quadrature output
  • Sine quadrature output
  • Pulse output, 0deg, 90deg
  • Ramp up output 0deg, 90deg
  • Ramp down output 0deg, 90deg
  • Octave switch
  • Through zero modulation
  • PWM input
  • V/Oct, FM log and trough zero CV input
  • Temperature compensated
  • Fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption around 110mA each rail

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

Trapezoid quadrature through zero VCO with waveshapers: Schematic control PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic control PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB
Trapezoid quadrature through zero VCO with waveshapers: Schematic main PCB

J. Donald Tillman did an excellent job describing the core of his Trapezoid VCO. Please refer to the original article as linked above. Don Tillman gave me the advice to use only two capacitors in the core. The exponentiator I use is a well known and a classical design. You can find many description of it out there. The rest is straight forward.

Trapezoid quadrature through zero VCO with waveshapers: Populated control PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated control PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated main PCB
Trapezoid quadrature through zero VCO with waveshapers: Populated main PCB
Trapezoid quadrature through zero VCO with waveshapers: Back view
Trapezoid quadrature through zero VCO with waveshapers: Back view
Trapezoid quadrature through zero VCO with waveshapers: Side view
Trapezoid quadrature through zero VCO with waveshapers: Side view
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot square wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot square wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot sine wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot sine wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot pulse wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot pulse wave out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot triangle through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out
Trapezoid quadrature through zero VCO with waveshapers: Screenshot trapezoid through zero out

Trapezoid quadrature VCO through zero (flat version) with waveshapers

Trapezoid quadrature VCO through zero (flat version) with waveshapers
Trapezoid quadrature VCO through zero (flat version) with waveshapers: Front view

This is my second take on the Trapezoid VCO designed by Don Tillman. My first implementation can be found here. This time I mounted the PCB’s parallel to the front to save space and integrated the wave shapers. The core is still based on the original design from Don (used with permission). 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. Another reference voltage device is used. And quadrature square outputs are implemented. As well as the additional waveforms triangle, sine, ramp up, ramp down and pulse.

Specs and features

  • Trapezoid quadrature output
  • Square quadrature output
  • Triangle quadrature output
  • Sine quadrature output
  • Pulse output, 0deg, 90deg
  • Ramp up output 0deg, 90deg
  • Ramp down output 0deg, 90deg
  • Through zero modulation
  • PWM input
  • V/Oct, FM log and trough zero CV input
  • Temperature compensated
  • Coarse and fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption around 135mA each rail

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

Trapezoid quadrature VCO through zero (flat version) with waveshapers: Back view
Trapezoid quadrature VCO through zero (flat version) with waveshapers: Back view

Quad waveshaper for trapezoid quadrature thru zero VCO

Quad waveshaper for trapezoid quadrature VCO
Quad waveshaper for trapezoid quadrature VCO

This is the waveshaper for my Trapezoid quadrature through zero VCO. It gives the quadrature outputs for triangle, sine and the outputs for saw (ramp up, ramp down) and pulse. To use it you need my Trapezoid quadrature VCO. The waveshaper has no external input for waves, it is internally connected with the Trapezoid quadrature VCO

Specs and features

  • Four triangle quadrature outputs
  • Four sine quadrature outputs
  • Two saw (ramp up) outputs 90° apart
  • Two saw (ramp down) outputs 90° apart
  • Two pulse outputs 90° apart
  • Voltage controlled pulse width
  • Runs on +/-15V and +/-12V
  • Power consumption around 50mA each rail

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

Schematic 01 quad waveshaper for quadrature thru zero VCO
Schematic 01 quad waveshaper for quadrature thru zero VCO
PCB quad waveshaper for quadrature thru zero VCO
Schematic 02 quad waveshaper for quadrature thru zero VCO

The triangle waves are created by algebraically averaging two trapezoid waves 90° apart. This is done here with IC3A for 270° and 0°. IC3C adds and averages 90° and 0°. You can use other combinations as well. IC3B and IC3D gives the inverse triangle waves. The sines are derived from the triangle with well known “old style” circuitry. The ramp outputs are build from two triangles 180° apart, level shifted and switched between them with the square wave. IC5A takes the 90° triangle, shift it up to 0..10V and lowers the amplitude to 0..5V. IC5B takes the 270° triangle shift it down to 0..-10V and lowers the amplitude to 0..-5V. IC6 (DG409) switches between this two triangles with means of the 270° square. Switching in the right moment put the needed parts of the triangle back together to the saw. The pulse outputs are done with the usual technique moving the switching point of a comparator around with the ramp wave.

PCB quad waveshaper for quadrature thru zero VCO
Triangle screenshot from quad waveshaper for quadrature thru zero VCO
Triangle screenshot from quad waveshaper for quadrature thru zero VCO
Sine screenshot from quad waveshaper for auadrature thru zero VCO
Sine screenshot from quad waveshaper for auadrature thru zero VCO
Saw screenshot from quad waveshaper for auadrature thru zero VCO
Saw screenshot from quad waveshaper for auadrature thru zero VCO
Pulse screenshot from quad waveshaper for auadrature thru zero VCO
Pulse screenshot from quad waveshaper for auadrature thru zero VCO
Quad waveshaper for auadrature thru zero VCO back view
Quad waveshaper for auadrature thru zero VCO back view

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

NGF Project: Waveshaper

This is my waveshaper version for the NGF Project. It is basically the same schematic as for my waveshaper version 0.9.5 with an added output circuitry and wiring option to be used with a rotary switch or some SPST switches as in the original. I kept all the waveforms of the original Elektor Formant, but changed the circuitry significantly. The only thing i added is a symmetrical square out, that i found useful for sync. For converting the saw wave to triangle a precision full wave rectifier is used, followed by a level shifter. The originally used matched Germanium diodes are hard to obtain these days. Here you only have to match two resistors. The switching glitch is not audible and can be suppressed with means of a capacitor of some 100pF if wanted. Don’t choose the capacitance to high, it rounds the edge of the triangle. For triangle to sine conversion i used a well known circuit that works better then the two diodes approach from the original Elektor Formant. This circuitry can be found at various places in Electronotes and others. The circuitry is very sensible to the used resistor values and input voltage. The resistor values are done for a BF245B. BF245A or C will not work. If you use another input voltage you probably have to change type of FET and resistor values. For wave shaping from saw to spaced saw I kept the original circuitry from the Elektor Formant. I only have to change some resistor values due to the different signal level. New added is the saw to square waveshaper because I found it useful to have a symmetrical square output. The saw to pulse shaping is quite different to the original but quite standard.

The documentation for download can be found on my website.

NGF Project: Waveshaper stuffed PCB

NGF Project: Waveshaper stuffed PCB

NGF Project: Waveshaper schematic

NGF Project: Waveshaper schematic

NGF Project: Waveshaper output saw, square and spaced saw

NGF Project: Waveshaper output saw, square and spaced saw

NGF Project: Waveshaper output saw, triangle and sine

NGF Project: Waveshaper output saw, triangle and sine

NGF Project: NGF VCO Core two

NGF VCO core two faceplate

NGF VCO core two faceplate

I have build my first original Elektor Formant VCO’s in the 70’s. They worked quite well and do so until today. Over the years i did implement some changes and extensions. I have developed a PCB Version with my changes and extensions published as VCO Core one. But only for those few lucky ones with some spare uA726. Here is the second VCO for my Next Generation Formant project. As in the NGF VCO core one I tried to keep the basic architecture of the original Elektor Formant. With the goal using only today available parts. This means finding a heated transistor pair for the expo converter and a Schmitt Trigger with hysteresis for the switching. Because there are no heated transistor pairs obtainable in the market today I was on my own. There are some solutions for a heated expo pair out there. Most of them build around no longer available parts or quite expensive. A still easy to get and cheap part is the LM3046. The SMD version is still in production today (2017 March) and the DIL version is easy to source as well. Though not longer in production. The PCB for the NGF VCO core two holds both versions. You can stuff it with the SMD- or DIL version. The 4093 is used as Schmitt Trigger for switching. Using the 4093 makes some level shifting necessary for proper switching. The switching capacitor is reduced to 470pF which give an even more better volt per octave characteristic then the original over the audio range. A LFO option is implemented as well. In conjunction with the waveshaper you have now the same waveforms available as voltage controlled LFO.The uA726 is replaced with LM3046 and the 7413 with 4093. All internal control voltages are stabilized. I removed the need for the +5V current supply, added an octave switch, FM lin and sync in. A new feature is the LFO switch. The signal level is raised to 10Vpp to get a better SNR throughout the system.

The documentation is available for download on my website.

VCO Core two: stuffed PCB

VCO Core two: stuffed PCB

VCO core two saw out

VCO core two saw out

All changes I have made to my original Elektor Formant VCO are implemented: No extra 5V source is needed, all control voltages are stabilized, Octave switch, FM lin, 10Vpp output. New LFO option.

NGF VCO core two schematic

NGF VCO core two schematic

To keep the characteristic of the original Elektor Formant I kept the heated exponential converter, here build around the LM3046. The PCB is developed to hold the DIL and the SOIC version as well. The Schmitt Trigger used is the 4093. Due to different switching levels in the original some transistors are added for level shifting. The octave switch is build with the OPA2277 and the voltage reference REF102. Switch SW1 turns the LFO option on and off. The level for the Sync input is adjusted with P1. Fine tune of the pitch is done with P2. If you need a wider range for the tuning you can change it with changing the potentiometer or the resistors values here. The basic pitch is set with TR4. Output voltage is set with TR3 and the DC offset with TR1.

NGF VCO core two schematic heater

NGF VCO core two schematic heater

This circuitry is part of my NGF VCO Core 2. It make use of two transistors to heat the LM3046. It works with the SMD and DIL Version as well. The given resistor values keep the current values and the power dissipation below the maximum ratings. T3 is used to measure the actual temperature. The voltage drop over T3 is direct proportional to the chip temperature. It is compared to the voltage at pin 2 of the LM3046 which is derived from TR6. The temperature is easily adjusted with TR6. Between heating off and maximum temperature. The testing bridge is driven with a stabilized 10V voltage source. T4 and T5 are used as heaters. R41 and R43 limit the maximum current. The different values are selected with purpose to keep pin 13 of the LM3046 (the substrate) the lowest negative point at the chip.

 

Building Block: 3046 Heater

For my Next Generation Formant project i needed a replacement for the obsolete uA726, because i wanted to stay as close as possible to the original Elektor Formant VCO with my NGF VCO Core 2. This means a heated exponential matched transistor pair. There are a variety of possibilities to achieve this goal. Most of those solutions uses now obsolete parts or are hard to build. But there is still on well known solution with with the 3046 transistor array. And best, it is still available. At least the SMD Version is still in production. The DIL version can be found as well, but is no longer in production.

The circuitry I found on the net did only use one transistor for the heating. This leaves one transistor unused. My thought is using two transistors makes for a faster heating up and more stability against environmental change. To my surprise I can not find a solution which uses two transistors instead of one. Here is my take on the circuitry.

3046 heater schematic

3046 heater schematic

This circuitry is part of my NGF VCO Core 2. It make use of two transistors to heat the 3046. It works with the SMD and DIL Version as well. The given resistor values keep the current values and the power dissipation below the maximum ratings. T3 is used to measure the actual temperature. The voltage drop over T3 is direct proportional to the chip temperature. It is compared to the voltage at pin 2 of the 3046 which is derived from TR6. The temperature is easily adjusted with TR6. Between heating off and maximum temperature. The testing bridge is driven with a stabilized 10V voltage source. T4 and T5 are used as heaters. R41 and R43 limit the maximum current. The different values are selected with purpose to keep pin 13 of the 3046 (the substrate) the lowest negative point at the chip.

The below picture shows the graph temperature in deg. Celsius vs. Voltage in mV at pin2. It is quite linear. You can easily derive the needed voltage for your preferred temperature from the graph. The data were taken from a LM3046 SMD mounted on a PCB. The figures for the DIL Version are slightly different. They will show up on my website as well as some more details ASAP. Temperature was measured with Fluke 63 IR thermometer.

3046 heater: temperature vs. voltage plot

3046 heater: temperature vs. voltage plot

Tool: Transistor Matcher

Matched transistor-pairs are often needed while building modules for synthesizers. Especially for exponentiators and differential amplifiers. Not to forget the Moog style ladder filters. I build an transistor matcher for me many years ago and still use it. I know that there are easier matching circuits out there today but mine works great for me so there is no need to change. Mine is based on the original transistor matcher found in the “Technical Service Manual for Moog Modular Systems” on page 53. I added some circuitry for stabilizing the used voltages. That was done to be able to repeat the measurement without bothering about the power supply adjustment. We are dealing with 1mV – 2mV here!

Transistor Matcher schematic

Transistor Matcher schematic

 

There was no need for a PCB. I just put it on perfboard and used IC sockets for the DUT.

Transistor Matcher

Transistor Matcher

 

The usage is easy but you have to consider some precautions. The measurement is very temperature sensitive. You need to keep your environment stable. Use pliers to mount the DUT. Place the transistors into the socket. Measure base to emitter voltage. Don’t touch the transistors with fingers. The finger heat will cause the readings vary. Mark down the Vbe and find two transistors that the Vbe matches within 2mV or better.

Transistor Matcher usage

Transistor Matcher usage

 

With today standards of fabrication transistors it is not unusual to find nearly every transistor within the 2mV Vbe range in a batch. You can easily match your pairs to better standards. Depending on your equipment 0.5mV Vbe match should be easily to reach. But always remember: The measurement is very sensitive. Be careful with your setup!

Waveshaper: Saw to spaced, square and pulse

For wave shaping from saw to spaced saw I kept the original circuitry from the Elektor Formant. I only have to change some part values due to the different signal level.

 

Waveshaper: Saw to spaced saw

Waveshaper: Saw to spaced saw

New added is the saw to square waveshaper because I found it usefull to have a symmetrical square output.

Waveshaper: Saw to Square

Waveshaper: Saw to Square

The saw to pulse shaping is quite different to the original but quite standard.

Waveshaper: Saw to Pulse

Waveshaper: Saw to Pulse

Oscilloscope screenshots:

Screenshot: Saw to square and spaced saw

Screenshot: Saw to square and spaced saw

Screenshot: Saw to pulse

Screenshot: Saw to pulse