Oscillator Cores & Exponential Converters

A bit about Oscillator Cores.... they are one of the building blocks of VCOs

VCOs have 3 main parts :
1.The core/cores
2.The waveshapers
3.The exponential converter

This page is mostly about the core (though I'll touch briefly on the other two).
When I first started out, I though that the waveforms in most analog modular VCOs were produced independently. This however is very far from the truth.

Most VCOs derive their multiple waveforms from just 1 main waveform known as the oscillator core.
The other waveforms such as sine and square are usually produced using waveshapers.
Whenever you buy a VCO, most manufacturers will describe their oscillator as having one core or another.

There are 2 main waveforms used: triangle and sawtooth.
They are produced differently and both have their strengths and weaknesses.

They  both use what are called integrating capacitors.

The simpliest waveform core is the sawtooth.... and thus seems to be more common.
It works on the principle that capacitors store charge. They fill up with charge until a reset voltage is reached. The rate at which the capacitor charges up is determined by the input voltage. (ie it's voltage controlled) ... this rate of charge/discharge is the frequency of the oscillator.

The triangle waveform is a bit more difficult to make as a core.
Instead of the charging/discharging of the capacitor, we have a change in direction of the current.

Both cores have a timing mechanism that resets the waveform back to its starting point by discharging the capacitor. (Then the cycle starts again).

The timing mechanism is usually a comparator. When the waveform exceeds a reference voltage the comparator triggers and the waveform resets.
The retrigger mechanism can be something like a transistor. If it is a digital timer then the oscillator is what is referred to as a digitally controlled oscillator... DCO,  instead of a VCO (Voltage controlled oscillator)

VCOs that have two cores are very rare. The most obvious is that of the ARP 2500

The 1004 has both triangle and sawtooth cores.
Both cores use a single reset mechanism.

The later ARP 2600 used 4027 & 4027-1 VCO submodules. These had a 10 Vpp sawtooth output.

The Buchla 158 Dual Sine-Sawtooth Generator  and the 144 Dual Square Wave Generator both use a sawtooth core. 
The later Buchla 258 was a triangle core dual oscillator. The 259 is also a triangle as far as I know.
The modern Buchla 261e breaks the mold ...it has a digital sine wave core. ( but the timbre section is analog).
The Roland SH-101, Oberheim Ob-Xa and Moog Memorymoog all use a Curtis CEM3340 chip.
This is a triangular core.
The Yamaha CS series VCOs use a sawtooth core oscillator.
The very early descrete component (no ICs) RA Minimoog VCOs (of which I understand only 300 exist)
used a sawtooth core.


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The exponential converter converts a linear control voltage into a exponential current.
Why is this needed at all ??? 
It's needed because most human senses (including hearing) are logarithmic

The exponential converter helps the oscillator core create a waveform that has a frequency logarithmically proportional to the input current. These converters are very very temperature sensitive.

This is a picture of the exponential converters of the 1004 VCO. It's part of an ARP 2500.
The 4001was an encapsulated NPN based current generator. 
The 4002 was PNP based.
The circuit boards were enclosed inside a plastic case which was filled with an epoxy potting compound.
This case helped stablise the VCO tracking of the 2500 but also made any future repairs very difficult.

The most basic type of exponential converter uses a bipolar junction transistor. (BJT).
and maybe some voltage dividers and a tempco.

The NPN type seems most popular. (though you can use a PNP).
For a single transistor, there is a  exponential relationship between the Vbe and Ic
(voltage between the base/emitter & collector current).

Here is the equation:

 

 This all looks pretty straightforward except for the problem of temperature.
The collector leakage current is influenced by temperature so in order to maintain accuracy we must keep the transistor at a constant temperature.

If your VCO uses a 1V/oct tuning, then any 1V increase, must double the VCO's frequency.

Of course not all synths use logarithmic voltage control.
Korg and Yamaha use linear voltage control (often called Hz/volt).
Here, the frequency is directly proportional to the input voltage and there is less of a need for exponential converters.

Links
+ A bit about Transistors
+ Waveshapers