3340 VCO Operation

The CEM3340 VCO integrated circuit was used in the Sequential Circuits Pro One and rev. 3 of the Prophet 5 as well as a number of synthesisers from manufacturers such as Moog and Oberheim. Within the Pro One the chip provided both the voice raw waveforms as well as formed the basis of the LFO. Chips like this helped simplify and shrink the footprint of VCOs by including the exponential current generator, triangle to saw converter and other circuits.

CEM3340 Circuit Overview

The chip has two main secions, to the left of the circuit diagram below is the exponential converter. This section takes the control voltage in 1 Volt per Octave at pin 15 and accurately convert it into a current with an exponential relationship to the control voltage at the collector of transistors $Q_{2}$ and $Q_{3}$.
The second section generates the waveforms (Triangle, Saw & Square) to be output from the chip. This is achieved by first generating a triangle wave by using the precision current from the exponential converter to charge and discharge $C_{F}$. This triangle wave is then passed to a triangle to saw converter to generate the sawtooth wave, and the sawtooth wave is then sent to a comparitor to generate a pulse/square wave.

Exponential Current Generator

The input for the control voltage at pin 15 forms the current summing node of an op amp summing amplifier circuit. This summing amplifier allows for multiple voltage control signals to be connected to the pin and summed allowing for a convenient way to mix tuning, modulation and high frequency trim signals. The output of the summing amplifier is then multiplied with a temperature compensation signal to yield a voltage which is then fed to the base of transistor $Q_{1}$.
To generate a current with an exponential relationship to an input voltage is acheived using $Q_{1}$ taking advantage of the following relationship: $$i_{CE} = e^{\alpha V_{BE}+\beta}$$ The op amp, $A_{2}$ maintains the collector voltage of $Q_{1}$ at $0 V$ by controlling the voltage at the emmiter of $Q_{1}$. A reference resistor $R_{R}$ is also connected to the collector of $Q_{1}$ and the supply rail. This resistor is appropriately sized to conduct a $10 \mu A$ current when the positive supply voltage is accross it. As the voltage at the base of $Q_{1}$ decreases, which would correspond with an increase in control voltage at pin 15, the op amp $A_{2}$ must make its output a lower voltage in order to maintain $0 V$ at pin 13. As the bases of both $Q_{2}$ and $Q_{3}$ are tied to $0 V$, and their emmiters are connected to the output of op amp $A_{2}$ and the emmiter of $Q_{1}$, then $V_{BE}$ for those transistors increases which results in an exponential increase in flow of current through them hence fufilling the function of exponential generator. This design as $Q_{2}$ and $Q_{3}$ help address the non-linearity of transisters by creating a $0 V$ reference. This effecively removes the $\beta$ component of the equation above. The final non-linear component of the voltage exponent in the above equation, $\alpha$, is handled by the tempco generator. As the exponential generator's output current increases, so does the junction temperature of transistors $Q_{1-3}$. This increase in temperature affects the relationship of current to voltage at the input and output of the exponential generator and makes the $\alpha$ component in the above equation variable. The tempco generator nulls out this variance by feeding the precision multiplier, hence multiplying the control voltage at the input, by a value that is the inverse of $\alpha$, ie $V_{BE} = \frac{1}{\alpha} \cdot V_{control}$. This design results in a highly accurate and frequency stable voltage controlled oscillator that can function over a range of temperatures.

Waveform Generation

The CEM3340 generates a triangle wave by charging or discharging a capacitor, in this case $C_{F}$ at a fixed current results in a linear voltage ramp up towards the voltage of the positive power supply, or down to that of the ground reference in the case of discharging. A buffer is used to isolate the signal at $C_{F}$ from the triangle output pin and the other waveform generators. The sawtooth and square waves are derived from the triangle wave. A comparitor is used to determine when the triangle wave is at its peak by comparing the current voltage of the triangle wave with a voltage reference generated by a voltage divider from $V+$ or ground. On a positive ramp when the reference is exceeded the comparitor activates which activates the two analogue switches. The left switch switches $C_{F}$ from the current mirror $CM$ output to the collector of $Q_{2}$ and dicharges the capacitor at a constant current to generate the negative ramp. The right analogue switch changes the comparitor reference to ground. $C_{F}$ then starts a discharge cycle and once the voltage is below $0 V$ the comparitor output returns to 0 and the analogue switchs switch back to charging the capacitor using the current mirror CM and setting the voltage reference back to being derived from the voltage divider. Soft sync is achieved via a decoupling capacitor onto the voltage divider reference to manipulate the reference to prematurly set the circuit to discharge mode. Hard sync is acheived by forcing a reset of the analogue switches directly. Details of the triangle to saw converter do not exist. The subsystem takes the output from the comparitor to determine if the system is in a positive or negative ramp and the triangle input. In all likelyhood this is acheived by using the positive ramp directly then inverting the signal and applying a positive voltage offset to the negative ramp portion of the triangle wave to generate the sawtooth. The PWM square wave is generated using a comparitor which compares the current voltage of the sawtooth wave with a variable voltage reference to modulate the pulse width of the square wave. With the sawtooth wave starting at zero volts and ramping to $10 V$ a voltage reference of $5 V$ would flip the output of the comparitor from $12 V$ to $0 V$ at 50% of the waves period producing a perfect square wave. At lower reference voltages the output will activate early lowering the duty cycle and higher will extend the duty cycle.