UBZR1 Voltage Controlled Oscillator Module

Chelys Lyra

Summary/Usage

The UBZR1 synthesizer has three voltage controlled oscillators. All three can feed the audio path. One of them can also be used as a low frequency oscillator for modulation purposes.

Functional Description

Each VCO has the following features

No physical knobs or switches. All functionality is voltage controlled
1 V/Octave exponential frequency control
Eight output wave forms
- Sine wave
- Triangle wave
- "Shark" wave (VCO3 inverted sawtooth instead)
- saw tooth wave
- Four rect-angle waves with different duty cycle. Duty cycle can be modulated
Two current outputs:
1. Current output for the audio path
2. Current output for modulation or ring modulation
Two built in exponential voltage controlled amplifiers

Features Specific to VCO1, VCO2 or VCO3

What	VCO1	VCO2	VCO3
Linear frequency modulation input	y	y
Hard synchronisation input		y	y
Hard synchronisation output	y
Ring Modulator circuit		y
Inverted Sawtooth wave form			y

How the VCOs are interconnected

Linear frequency modulation is common for VCO1 and VCO2
VCO1 can hard synchronise VCO2
VCO1 can ring modulate VCO2
VCO3 is being hard synchronised by the keyboard
VCO3 can be removed from keyboard pitch control
VCO3 can be set to low frequency

VCO Module Building Blocks

UBZR1 VCO Building Blocks

The VCO module consists of two parts:

The VCO saw tooth generator with 0 - 5 V saw tooth output
The VCO wave shaper part with a following wave form selector and two voltage controlled amplifiers (VCA).

VCO Saw Tooth Generator

UBZR1 VCO Saw Tooth Generator

The VCO is a classical transistor exponentiator, current controlled Miller integrator and a mono-flop controlled reset circuit.

VCO Wave Shaper

At the wave shaper's core is a 1-transistor sawtooth triangle converter. Sine waves are generated by over-loading the LM 13700 transconductance amplifier, which is used in the VCA sub-circuit.

Rectangle waves are generated using a standard comparator design.

The wave shaper provides eight different wave forms

VCO Wave Form Selector

An 8 to 1 analog multiplexer selects the chosen waveform. The multiplexer is controlled by a 3-bit digital logic signal ("switch controller triple").

VCO Built-In Voltage Controlled Amplifiers

The VCO has a final output stage as a voltage controlled amplifier (VCA). There are in fact two VCAs, which are feed with the same input by the wave selector, but their outputs can be used independently. This came in naturally as the LM 13700 contains two transconductance amplifiers. Hence, the VCO module has two current outputs. Current outputs allow for easy mixing into a simple resistor load.

The output currents depend exponentially on the control voltages.