Voltage Controlled Oscillator (Excellent 1V/Octave Tracking)
Article by Ray Wilson
FOR HISTORICAL REFERENCE ONLY
This is an intermediate to advanced project and I do not recommend it as a first project if you are just getting started in synths or electronics. Only the circuit and some explanation are shown here. A lot of project building, troubleshooting and electronics experience is assumed. Additionally, electronic equipment ownership (scope, meters, etc.) is taken for granted. If you are interested in building this project please read the entire page before ordering PC boards to ensure that the information provided is thorough enough for you to complete the project successfully.

Features

  • Years of MFOS VCO improvements all rolled into this design.
  • Good tracking over several octaves (6 to 8).
  • Can be temperature compensated with a metal film 2K Ohm 1/4W +/-2% T.C. +3300 PPM tempco.
  • Simultaneous Rectangle, Ramp, Triangle and Sine Outputs (Ramp core design)
  • Designed for 1V/Octave control voltage
  • 4 1V/Octave control voltage inputs.
  • 1 Linear control voltage input.
  • Hard sync input.
  • Rectangle wave duty cycle adjustable between 10% and 90%
  • Rectangle wave duty cycle voltage controllable
  • Frequency range is from sub audible (approximately 0.1 hertz) to ultra-sonic 20kHz.
  • Power supply can be +/-12 or +/-15 volts.
  • Easily obtainable parts.

MP3 Samples

Sine wave with sequencer and keyboard
Sine wave with sequencer and keyboard
Triangle wave with sequencer and keyboard
Ramp wave with sequencer and keyboard
Square/Pulse with sequencer and keyboard
Sync Effects
Frequency Range Demonstration
Modulation with keyboard and LFO
Messing about with ramp waves (3 VCOs)
Messing about with square waves (3 VCOs)
Messing about with the sequencer attached (4 VCOs)
Messing about with the sequencer attached (4 VCOs)

Links to previous versions for reference

Introduction

Oscillators are the main tone generating modules of your synthesizer. This oscillator produces sine, square, triangle and ramp waveforms and with careful adjustment tracks at 1V/octave over 6 to 8 octaves. The parts for it are easily obtained and not extremely expensive. The whole circuit is here so breadboard it and see if you like it. I like this oscillator but please recognize that it relies on a well matched pair of transistors. Its frequency range is from below audible to beyond audible. If this oscillator meets your needs I have high quality PC boards for sale for this design.

Voltage Controlled Oscillator Schematic Page 1 PDF

+/-15V Users Take Note

While helping diyer Doug Slocum improve the tracking on his MFOS VCOs I found a few things +/-15V users should change.

 
 
Page 1 shows the voltage to exponential current conversion and ramp generator circuitry. U1-A is the control voltage summer. It's output feeds the scale trimmer R16 whose wiper is connected to the base of the current reference NPN transistor in the feedback loop of U1-B. The CV summer (U1-A and associated components) with it's 2K feedback resistor and 100K input resistors has a gain of -1/50th or -.02. Its purpose is to accept control voltages fed to its inputs and scale them to 20mV per volt of input. The trimmer R16 permits adjustment to 18mV per volt which is what the base of the NPN in the SSM2210 at pins 8(c), 7(b) and 6(e) expects in order to convert the voltage in an exponential manner. Linear changes in voltage applied to the CV1 through CV4 inputs of U1-A are converted to exponential changes in current at the current sink (collector of NPN transistor at pins 1(c), 2(b) and 3(e) of the SSM2210 Super Matched NPN Pair IC). For each additional volt of CV there is a doubling of the current flowing into pin 1 of U3.
 
The high frequency compensation circuitry D1, R9 and R11 allow the output of U1-B to boost the current at the current sink at higher control voltage (and thus higher frequency) to make up for the finite time required to discharge the integration capacitor C6 during oscillation. There is some interaction between the scale trimmer and the high frequency compensation adjustment so expect to go back and forth a bit during calibration. I like to set R11 to minimum (wiper to pin 1) and first calibrate with just R16. I adjust R11 when I see the frequency start to go flat at higher (greater than 4kHz) frequencies.
 
The current flowing into pin 1 of U3 causes the integrator made up of U2-A and C6 to ramp from ground toward V+. The better the linearity of the op amp used for U2 the better the oscillator will track. The ramping voltage on the output U2-A is fed into the comparator made up of U2-B and associated components. As long as the ramping voltage is below the voltage on the non-inverting input of comparator U2-B (approximately 2V), U2-B's output stays at about -10.6 volts. When the ramp voltage gets to about 2.2 volts U2-B's output shoots from about -10.6V to about 8V and forward biases D8, charges C17 and turns on the PN4391 N channel JFET which resets the integrator (causes the output of U2-A to return to ground). The reset takes about 1uS after which the output of U2-A again begins to ramp up linearly. The output of U2-B when U2-A's output exceeds 2.2V is a very brief pulse (ALWAYS use the x10 setting on your scope probe to observe it) that goes from -10.6V (op amp negative saturation voltage) to about 8V and then back to -V. It is about 3 uS in duration.
 
Thus the output of U2-A (point RAW) is a ramp wave that goes from ground to +2.2 volts. The ramp's frequency is determined by the current flowing into pin 1 of U3. Since the current doubles for every increase in 1V applied to the CV summer the frequency doubles in kind. When the current doubles the rate of the ramp generator doubles and thus the frequency of the ramp wave doubles.
 
This is the heart of the VCO (literally). Applying a control voltage to the LIN input causes a linear shift in frequency as this input affects the reference current in the exponentiator. Applying a square wave from another oscillator to the sync input causes the comparator to reset the integrator (on the rising edges of the square wave) which produces interesting timbres when the outputs of the oscillator providing the sync signal and the oscillator being synced are mixed. Panel mounted pots R2 and R3 provide the initial frequency (tuning) adjustment. Detailed calibration instructions are in a section below. R16 and R11 should definitely be multi-turn cermet type trim pots so you have fine resolution when adjusting the V/Oct scale and high frequency compensation.
 
These ideas were pioneered by the Alan Pearlmans, Bernie Hutchins, and Robert Moogs of the world I am merely a student of their landmark work.
 
For best performance and least temperature drift use: 1% resistors throughout this portion of the circuitry, a metal film 2K Ohm 1/4W +/-2% T.C. +3300 PPM tempco for R24, and a silver mica or polystyrene capacitor for C6. If you use the tempco for R24 you need to put it in direct thermal contact with the matched transistor pair used in the linear voltage to exponential current convertor. The board is designed to accomodate a wide range of transistors from matched pairs in 8 pin pacakages to two separate transistors. You can substitute some of the parts as listed at the beginning of the parts list section below but the PN4391 and OP275 are highly recommended.
 
Approx. Current Consumption
+12V28mA
-12V26mA
Assume slightly higher for +/-15V
 

Voltage Controlled Oscillator Schematic Page 2 PDF

Wider Pulsewidth Adjustment Range

While putting together a few oscillators recently I changed R68 to 1M and got a much wider range of pulsewidth adjustment. However, in the extreme settings (all the way up or all the way down) it could cause the pulse wave to cease output. I don't mind this as it lets me set very narrow pulse widths.

 
 
Page two shows the circuitry that converts the raw ramp wave into ramp, triangle, sine, and rectangle waveforms. RAW is applied to inverting buffer U4-A where gain and offset are applied to the signal. The original 2.2 volt signal is boosted to about 10 volts P-to-P. Adjust R5 so that the sawtooth waveform at pin 1 of U4-A oscillates evenly about ground.
 
Click for PDF
 
U4-A's output is fed to inverting unity gain buffer U4-B. The positive excursions of the outputs of U4-A and U4-B are rectified by D2 and D3 and dropped across R21 (20K resistor to -12V) and fed to U4-C inverting buffer with a gain of 2. This results in a triangular waveform at the output of U4-C after the Saw Offset and Tri Offset trims have been properly adjusted. As explained in the drawing this design produces inaudible glitches at the time the integrator is reset by the comparator. This is because of the finite fall time of the integrator's output. Even though the integrator's fall time is about 1 uS the output of U4-A after inversion and gain takes about 3 uS (due to slew rate limiting of the op amp) to go from low to high (remember its inverted). U4-B takes about 3 uS to go low (its the original with x2 gain, again due to slew rate limitations). Thus a glitch of about 6 uS takes place at the peak of the triangle wave. This glitch is so fast that in practice it contains no audible information. C14 reduces the amplitude of the glitch by filtering the majority of it to -V.
 
Click for PDF
 
The triangle waveform is fed into the circuit which uses the U5 (LM13700 or equivalent) to apply non-linear distortion which effectively approximates a sine wave. R60 (Sine Bias Trim), R64 (Sine Shape Trim), and R59 (Sine Level Trim) are all used to get the best sine shape possible. The figure below illustrates the effects of these pots on the waveform. Shaping circuits are never perfect and in the end if you achieve 1% distortion figure you will be very happy. If you don't have a distortion analyzer use your scope and your ears to determine the best sine shape. It is interesting to adjust the waveform to where you believe it sounds best and then tweak a little. You will be suprised to see how little distortion it takes to start adding overtones to the fundemental frequency. Adjust until you hear the purest tone with the least overtones. Listen while sweeping the frequency with the coarse adjust knob as well. Again, try to achieve the purest tone throughout the oscillators musical range.
 
Click for PDF
 
In order to provide bias levels for the rectangle wave comparator I use the ramp wave which appears at the output of U4-B. Note that the positive excursions forward bias D4 and charge C13. The negative excursions forward bias D7, D6 and D5 and charge C12 negatively. We end up with ramp positive peak minus one diode drop on C13 and ramp negative peak - 3 diode drops on C12. We buffer these voltages with U6-A and U6-B respectively and apply them to the ends of R39 panel mounted Pulse Width Adjustment pot. The wiper of R39 provides continuous adjustment between these two voltages and is connected to the inverting input of U7 (which is used as a comparator). When the voltage of the ramp waveform applied to the non-inverting input via R29 goes above the threshold set by the pot the output of U7 goes high. When the voltage of the ramp waveform applied to the non-inverting input via R29 goes below the threshold set by the pot the output of U7 goes low. Thus you have pulse width adjustment of between approximately 10% to 90% duty cycle for the rectangle wave's output. Voltage applied to the PWM input changes the threshold and thus provides a means to "voltage control" the pulse width.
 

Voltage Controlled Oscillator Scale Calibration

After getting the waveforms adjusted properly you need to set the oscillator scale factor and high frequency compensation trim pots. You will need a stable adjustable voltage source that can provide 0 through 10 volts, an accurate DVM, and a frequency counter. You can use an oscilloscope instead of the frequency counter but the frequency counter is more accurate. Without a frequency counter or scope you will need to use your ears. The following process is iterative and can be tedious. If you have a keyboard controller or MIDI to CV convertot that puts out 1V/octave I find it is easier to simply play octaves on the keyboard and trim the scale factor and high frequency compensator until I get the correct 1V/octave response.
 

Voltage Controlled Oscillator PC Board Drawings and Photos

Alternate placement of R24 When using a PT094 (or equivalent) tempco
 
Notice that there is a legend for R24 but there are also two alternate mounting holes on the board specifically for mounting a metal film 2K Ohm 1/4W +/-2% T.C. +3300 PPM tempco as R24 so that it is in thermal contact with U3 (or matched discrete transistor pair). Use thermal grease or epoxy to make the thermal contact. The tempco can go above a dual transistor package or 2 discrete transistors.
 
IMPORTANT INFORMATION! You should only install one of the resistors. If you use the tempco for R24 in the alternate mounting then DO NOT MOUNT THE REGULAR R24 RESISTOR. Otherwise your oscillator scaling will be off by a factor of 2 (i.e. an octave will span 2 keyboard octaves).

 
Populated VCO board. (Click for a larger view.)
 
Notice that R24 (regular resistor or tempco) is not mounted yet. The two holes on either side of the SSM2210 are for mounting the tempco in contact with the SSM2210. Do not mount the regular R24 if you use it. Either use one or the other never both.

!!! NOTICE !!! In my opinion multi-turn trimmers are absolutely necessary for R11 and R16. However if you use side adjust single turn trim pots for any of the other trimmers (R5, R7, R59, R60 and/or R64) make sure you orient them so that you can adjust them without difficulty.
 
 
 
Voltage Controlled Oscillator Component Designator View
 
 
 
Voltage Controlled Oscillator Component Value View
 
 
 
Voltage Controlled Oscillator PCB Bottom Copper (Parts Side View Shown)
 
 
 
Voltage Controlled Oscillator PCB Top Copper(Parts Side View Shown)
 
 
 
Voltage Controlled Oscillator PCB Silk Screen(Parts Side View Shown)
 
 

Voltage Controlled Oscillator Front Panel Drawings and Wiring

Voltage Controlled Oscillator Front Panel PDF
 
This overlay fits the UNIVERSAL panels I sell. I used a UNIVERSAL panel and two brackets when constructing my VCO.
 
 
Dave Kronemeyer took the time to make a nice Front Panel Express design and share it with all of us. If you use it make sure you take into account any differences related to the wiring diagram.
Click for Front Panel Express File
Click image to download .FPD File (Front Panel Express) File
David Kronemeyer did a nice Front Panel Express layout.
Visit Front Panel Express
Click the image to download the .FPD File.
 

Voltage Controlled Oscillator Back Panel PDF
 
 
 


Voltage Controlled Oscillator Project Parts List

Order enough DIP sockets for ALL chips. Sockets are your friends!

Capacitor voltage rating should be 25V or greater for all capacitors.
Non-electrolytic capacitors can be film or ceramic type.
The integrator cap (C6 .005uF) needs to be temperature stable so buy silvered mica, polystyrene or polycarbonate.

Substitutions


OP275 - LT1112, AD706, LF442, TL072
AD SSM-2210 - LM394 DIP or Two matched transistors (specs similar to 2N3904)
R24 (metal film 2K Ohm 1/4W +/-2% T.C. +3300 PPM tempco) - 2K Carbon Comp Resistor (provides no temperature compensation)
LM13700 - LM13600, NE5517, AU5517, NTE870, NJM13600D
TL074 - Any quad JFET op amp
PN4391 - 2N5457, J210, NTE457

PN4391 can be replaced with 2N5457 N-Channel JFET
The PN4391 and 2N5457 N-Channel JFETs went out of production in 2012 so here are some sources for the NTE457, PN4391 and 2N5457 all of which are good replacements.

  • Allied Electronics NTE457 (direct replacement for 2N5457)
  • Mouser Electronics NTE457 (direct replacement for 2N5457)
  • Wholesale Electronics Inc. NTE457 (direct replacement for 2N5457)
  • Parts Express. NTE457 (direct replacement for 2N5457)
  • online components.com NTE457 (direct replacement for 2N5457)

    MFOS sells a pack of 5 brand new Fairchild 2N5457 N-Channel JFETs and a pack of 5 brand new Fairchild PN4391 N-Channel JFETs

  • Resistor Types & Tolerances

    I suggest using 1% metal film for all resistors shown on page 1 of the schematic. Using 1% metal film for all resistors shown on page 2 of the schematic except for R68 (10M) and R22, R26, R34, R49, R51 (1Ks) will give greater uniformity of operation among multiple VCOs and better temperature tolerance but 5% carbon film resistors can be used as well.

    Qty. Description Value Designators
    1  LM13700 Dual gm OpAmp  LM13700  U5  
    1  OP275 Dual BiFET Op Amp  OP275  U1  
    1  SSM2210 Matched NPN Pair  SSM2210  U3  
    1  TL071 Op Amp  TL071  U7  
    2  TL072 Dual Op Amp  TL072  U2, U6  
    1  TL074 Quad Op Amp  TL074  U4  
    1  PN4391 N Channel JFET  PN4391  Q1  
    8  High Speed Switching Diode  1N914  D1, D2, D3, D4, D5, D6, D7, D8  
    3  Linear Taper Potentiometer  100K  R2, R3, R39  
    2  Metal Film 1/4 Watt 1% Resistor  2K  R24, R46  
    2  Metal Film 1/4 Watt 1% Resistor  47K  R20, R65  
    1  Metal Film 1/4 Watt 1% Resistor  475 ohm  R19  
    1  Metal Film 1/4 Watt 1% Resistor  470K  R61  
    3  Metal Film 1/4 Watt 1% Resistor  4.7K  R9, R36, R52  
    1  Metal Film 1/4 Watt 1% Resistor  3K  R41  
    12  Metal Film 1/4 Watt 1% Resistor  100K  R6, R10, R15, R18, R23, R27, R29, R31, R44, R50, R66, R67  
    11  Metal Film 1/4 Watt 1% Resistor  10K  R1, R12, R13, R17, R25, R32, R35, R40, R42, R47, R71  
    1  Carbon Film 1/4 Watt 5% Resistor  10M  R68  
    1  Metal Film 1/4 Watt 1% Resistor  130K  R53  
    1  Metal Film 1/4 Watt 1% Resistor  18K  R70  
    5  Carbon Film 1/4 Watt 5% Resistor  1K  R22, R26, R34, R49, R51  
    5  Metal Film 1/4 Watt 1% Resistor  1M  R8, R28, R30, R45, R69  
    2  Metal Film 1/4 Watt 1% Resistor  200K  R56, R57  
    3  Metal Film 1/4 Watt 1% Resistor  20K  R14, R21, R48  
    1  Trim Pot 10 Turn  20K  R11  
    1  Trim Pot 10 Turn  100 ohm  R16  
    2  Trim Pot 10 Turn  100K  R60, R64  
    2  Trim Pot 10 Turn  10K  R5, R7  
    1  Trim Pot 10 Turn  1M  R59  
    1  Ceramic Capacitor  10pF  C4  
    3  Ceramic Capacitor  100pF  C3, C17, C19  
    5  Ceramic Capacitor  .1uF  C1, C2, C8, C15, C16  
    1  Ceramic Capacitor  .002uF  C14  
    1  Ceramic Capacitor  4.7pF  C18  
    1  Ceramic Capacitor  .001uF  C5  
    2  Electrolytic Capacitor  10uF 25V  C7, C10  
    1  Silvered Mica Cap  .005uF  C6  
    2  Tantalum Capacitor  1uF 25V  C12, C13  

     
    Miscellaneous