# Design Process of a simple Sawtooth VCO

#### Jack Tranckle

Joined Jan 20, 2016
73
I am wanting to design a simple but stable sawtooth VCO with a 5 octave range, the dual power supply can be any value that is best suited, with reason. How would I begin to tackle the design process? is there steps for me to follow? would I begin by choosing the IC first? and what IC parameters are the most essential when choosing, such as slew rate, temperature ? I have already simulated a circuit in which I will base my design from but have used values from other designs. However I would like to find the relevant equations to calculate the required component values for my specific design. The plan is to design a MIDI to CV controller using a PIC, which is connect to an exponential converter to then supply a current to vary the frequency of this circuit. I understand the purpose of each component (except Rs3) but do not understand the correct way of calculating the values.

#### AnalogKid

Joined Aug 1, 2013
10,949
Please identify where each of the two scope traces are connected in the circuit.

ak

#### Jack Tranckle

Joined Jan 20, 2016
73
Please identify where each of the two scope traces are connected in the circuit.

ak
The blue displays the voltage output and the red the current output

#### Alec_t

Joined Sep 17, 2013
14,228
The charging current Ich for C1 is V6/Rs2. The cap charges to a peak voltage Vpk = V3*Rs5/(Rs4+Rs5). The charge q = C1*Vpk. The sawtooth period P = the time to reach the peak = q/Ich.

#### Jack Tranckle

Joined Jan 20, 2016
73
Hello
I have build this VCO, which was detailed here in AAC. It works well over 5 octaves.
It is described here:
www.allabaoutcircuits.com/projects/diy-synth-series-vco
In my LtSpice simulation I took the LT1022 because I don't have a model for TL084.

View attachment 146249

Amazing thank you, I can't see, to open the link that you sent. I'll try and search for it. curious, what kind of input have you got going into the exponential converter for simulation (PWL)? From my understanding, the CV will be supplying AC? I'm using multisim and am unsure what to input. Thanks

#### Jack Tranckle

Joined Jan 20, 2016
73
The charging current Ich for C1 is V6/Rs2. The cap charges to a peak voltage Vpk = V3*Rs5/(Rs4+Rs5). The charge q = C1*Vpk. The sawtooth period P = the time to reach the peak = q/Ich.

#### uwesynth

Joined Feb 17, 2018
7
Amazing thank you, I can't see, to open the link that you sent. I'll try and search for it. curious, what kind of input have you got going into the exponential converter for simulation (PWL)? From my understanding, the CV will be supplying AC? I'm using multisim and am unsure what to input. Thanks
Amazing thank you, I can't see, to open the link that you sent. I'll try and search for it. curious, what kind of input have you got going into the exponential converter for simulation (PWL)? From my understanding, the CV will be supplying AC? I'm using multisim and am unsure what to input. Thanks
I used the pwl source in LtSpice as CV input to the VCO. the pwl only generates 0V, 1V, 2V,3V,4V,5V,6V for a fixed duration. So you can see the frequency very well at the output over 5 octaves.

#### uwesynth

Joined Feb 17, 2018
7
Here you can see the pwl as CV-Input, the green signal, and the output. The schematic is a little bit different, then the one in the AAC link, but it behaves more or less the same:

#### Jack Tranckle

Joined Jan 20, 2016
73
I'v built the exponential converter but seem to be getting a very messy sawtooth output waveform. When I input a -1V current sink into the VCO, it is a very clear sawtooth. So it must be due to the exponential converter. Any ideas what might be causing this? I have attached a photo of the exponential converter and the waveform being generated. Thanks

#### Jack Tranckle

Joined Jan 20, 2016
73
Here you can see the pwl as CV-Input, the green signal, and the output. The schematic is a little bit different, then the one in the AAC link, but it behaves more or less the same:
View attachment 146253

Ahh ok i see, thats makes a lot of sense. You seem to have been able to produce a very clean signal, I tried your expo circuit but still had the same problem, a messy saw-tooth. I also noticed that your integrator is pulled to 58K instead of ground, what is the prupose of this ?

#### uwesynth

Joined Feb 17, 2018
7
you exponential converter looks nearly the same as mine. Take care, that the voltage at the gate of Q2 is only in the range of 0V...-100mV.
Thats why in my circuit there is a op-amp which converts the 0...6V of the CV-input into 0V...-100mV.
What is your voltage at the base of Q2?

#### uwesynth

Joined Feb 17, 2018
7
without the 56K in the integrator, means hard discharge, I got a more triangle output instead of saw. But both is possible, and makes no difference for the function

#### Jack Tranckle

Joined Jan 20, 2016
73
you exponential converter looks nearly the same as mine. Take care, that the voltage at the gate of Q2 is only in the range of 0V...-100mV.
Thats why in my circuit there is a op-amp which converts the 0...6V of the CV-input into 0V...-100mV.
What is your voltage at the base of Q2?
Ok I'm going to try and see if I can get your design to work, is it the op amp U4 that converts 0 - 6V to 0V to -100mV? since there is no converter, with a 5V AC input as CV, I am getting 5V at the gate of Q2. Also, I noticed you have an additional op amp U3, what is this used for?

Kind regards

Jack

#### uwesynth

Joined Feb 17, 2018
7
Ok I'm going to try and see if I can get your design to work, is it the op amp U4 that converts 0 - 6V to 0V to -100mV? since there is no converter, with a 5V AC input as CV, I am getting 5V at the gate of Q2. Also, I noticed you have an additional op amp U3, what is this used for?

Kind regards

Jack
U4 is an inverting OP with a gain of 1.4K/100K, so the voltage at the base of Q1 is about -70mV if CV is 5V. There is no AC at the CV input,
it is DC.
U3 is used as current to voltage converter, since out of the collector of Q2 there is a current. Q1 and Q2 are used as current mirror.

#### Jack Tranckle

Joined Jan 20, 2016
73
Excellent, thank you so much. I have finally got it working!! I tried with and without, but for some reason it only works without the current to voltage converter. I know that the VCO design requires a current sink to operate so maybe this is why? I have attached both to show how this is so. what is the purpose of having a potentiometer at the non inverting and converter op amps? Thanks again

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#### uwesynth

Joined Feb 17, 2018
7
Excellent, thank you so much. I have finally got it working!! I tried with and without, but for some reason it only works without the current to voltage converter. I know that the VCO design requires a current sink to operate so maybe this is why? I have attached both to show how this is so. what is the purpose of having a potentiometer at the non inverting and converter op amps? Thanks again
Great to hear, that it works now. The poti is used to trim the 1V/octave characteristic. It is not so easy to double the frequency for every 1V offset. But the potis helps for that. I will try too, to eliminate the I/V-converter and will see what happens.
What are you using the VCO for? I have 3 of them in my modular music snthesizer.