MGB Voltage Controlled Oscillators
12 January, 2004
The MGB VCO design traces from two sources;
Although, this specific design varies in components and wave shapers used,
the PAIA 9720 VCO core design appears to work well.
As tedious as proto-assembly can be, the kit is probably well advised, alternatively, if some of the following modifications are of interest, a PAIA board (substituting those parts attributable to the specific feature of interest) would save a lot of time.
The VCO core depicted can be understood as embodying four circuit configurations;
- Input Control Voltage Summing,
- Exponential Generator,
- Sawtooth Flyback oscillator,
- Positive and Negative Voltage References.
The VCO input summing is a simple extrapolation from the National Data Sheet.
The exponential generator is the National log circuit redrawn here
, as an aid for understanding. As the Sawtooth oscillator of the following stage will increase in frequency with increasingly positive input voltages, we exploit the log relationship in reverse or anti-log direction.
The Sawtooth Oscillator holds true to convention and implements a comparitor which quickly discharges a cap when it's threshold is exceed. This threshold ensures a 10 Volt Ramp emits.
The Voltage References provide a stable source for both the exponential converter and waveshaper circuits, and front panel controls as well.
The MGB VCO emit's the standard 4 raw waveshapes typical of synthesizers; a Sine, Triangle, Sawtooth, and duty cycle agile Pulse waveforms.
Amplifier U3-B ensures a +/-5VDC level and allows the sawtooth waveform to be centered around 0VDC. U4-B is a comparitor which allows the Sawtooth waveform to be compared against an external or front panel control derived reference voltage so that varying widths of Pulse waveforms can be generated.
U5-A & B compose a full-wave rectifier which converts the sawtooth waveform into a uni-polar triangle wave. U6-B allows the triangle wave to be centered around 0VDC and provides gain to ensure a +/-5VDC signal swing.
The centered triangle waveform is routed to an LM13700 "trimmed" to sufficient amplitude to overdrive the input and produce an approximation of a sinesoidal waveform. The resulting waveform has approximately 2% distortion mainly due to "cusps" on the signal +/- peaks. These cusps are cancelled in the later Wave Select Circuitry resulting in a sine waveform of less than 1% distortion.
MGB VCO waveform selection is controlled by the MGB front panel "electronically" (vs. mechanically using rotary switches).
SSM2404 analog switches are used to route any of the 4 VCO waveforms to following audio stages (VCA's). The Wave Select electronics also perform compensitory functions such as subtracting a weighted inversion of the triangle waveform from the raw VCO sine wave to remove "cusps" demonstrable on the peak sinewave signal excursions. The output of the Wave Select switch array is also inverted to compensate for the inversion resulting from the following VCA stages which set's the "audio level" of the selected waveform(s) of the VCA.
MG-B Oscillator Circuit Variation
The MGB VCO implementation differs from the PAIA 9720 design as follows:
- Precise Stable Voltage sources provide reference for the exponential generator, the waveshapers, and the front panel controls.
- Dual Op-Amps replace "Quad's" to allowed easier hand wire layout (and DC/AC circuit partitioning) the sawtooth integrator is provided a LF412, the quad comparitor replaced with the dual LM393, and all other op-amps with LF353's.
- "DC" and "AC" grounds meet at the power supply, "star grounding" is used by the "point to point" wiring of components and signals. The individual oscillators have separate "DC/AC" grounds which also meet only at the Power Supply.
- Analog Devices MAT03 dual transistors where used for the exponential anti-logging transistors thus avoiding the requirement for hand matching,
- Mechanically "lined up" and "leveled" the LM13700 and MAT03 packages to enable the use of a common draft-proof "box".
- Wave Shapers are characterised for +/- 5 Volts and implement differently than the PAIA waveshapers.
The picture on the left depicts the two MGB oscillators as constructed on .1" centered perf board. All i.c.'s are socketed but the MAT-03 and various transistors are not. The layout of the oscillators approximates a mirror image and each oscillator has a seperate 30 conductor IDC ribbon cable for interconnect to pots, switches and jacks.