Understanding Oscillators: The Heart of Your Synthesizer

🌊 What is an oscillator?

An oscillator is a circuit that produces a repetitive, periodic signal—a waveform that cycles over and over. In synthesizers, oscillators generate the raw audio signal that gets shaped by filters, amplifiers, and effects. Unlike a recording that plays back a fixed sound, an oscillator continuously creates a fresh signal based on its circuit parameters. The rate at which the waveform repeats determines its frequency (pitch), measured in Hertz (Hz)—440 Hz is the musical note A4, cycling 440 times per second.

Oscillators existed long before synthesizers: radio transmitters, clock circuits in computers, and electronic test equipment all use them. But synthesizer oscillators are special because they're designed to be voltage-controlled (VCOs)—their frequency responds to an input control voltage, typically 1 volt per octave. This voltage control is what lets you play melodies from a keyboard: each key sends a different voltage to the oscillator, changing its pitch in musically useful intervals. Understanding this voltage-to-frequency relationship is key to grasping how modular synthesis works.

Modern digital oscillators (DCOs) generate waveforms using microcontrollers or dedicated chips, offering perfect tuning stability and complex waveforms. But analog VCOs have unique character: slight pitch drift, temperature sensitivity, and interaction between oscillators create organic movement that many musicians prefer. Both types have their place, and DIY builders often start with analog circuits because they're more intuitive to understand and modify. Moritz Klein's VCO tutorial series on YouTube provides excellent visual explanations of these concepts.

The four classic waveforms

Sine wave: The purest tone, containing only the fundamental frequency with no harmonics. Sounds smooth and flute-like. Triangle wave: Soft and mellow, contains only odd harmonics that decrease rapidly in amplitude. Sawtooth wave: Bright and buzzy, contains all harmonics (both odd and even) decreasing in amplitude. The workhorse waveform for subtractive synthesis. Square wave: Hollow and reed-like, contains only odd harmonics at full strength, creating a distinctive "pulse" character.

These waveforms differ in their harmonic content—the additional frequencies present above the fundamental. A sawtooth at 100 Hz contains 200 Hz, 300 Hz, 400 Hz... extending into ultrasonic ranges. Filters sculpt this harmonic content to create different timbres. You can visualize waveforms using free software like Audacity or an online oscilloscope simulator to see how they look and sound.

⚙️ Simple oscillator circuits you can build

The humble 555 timer IC is the easiest entry point for understanding oscillators. In astable mode, it generates a square wave whose frequency is set by two resistors and a capacitor. The formula is approximately f = 1.44 / ((R1 + 2×R2) × C). With R1 = 10kΩ, R2 = 10kΩ, and C = 100nF, you get roughly 480 Hz—close to B4 on the musical scale. Change the capacitor to 47nF and the frequency doubles to about 1020 Hz (C6). This direct relationship between component values and pitch is what makes analog circuits so intuitive.

For a more synthesizer-appropriate VCO, the LM13700 operational transconductance amplifier (OTA) is popular in DIY designs. Thomas Henry's designs and the Electric Druid Eurorack VCO guide show how to build exponential converters (the circuit that translates 1V/octave control voltage into frequency changes) and core oscillators. These circuits are more complex than a 555 but produce proper sawtooth and triangle waves with good tracking across multiple octaves.

Modern DIY oscillators often use the AS3340 or CEM3340 (or their clones like V3340) integrated VCO chips. These contain the entire exponential converter and waveform generator in one 16-pin IC. You add external resistors and capacitors to set the frequency range, then patch in control voltages. The Erica Synths EDU DIY VCO is an excellent learning platform built around this chip, with clear documentation explaining each stage of the circuit.

🎚️ Voltage control: how keyboards talk to oscillators

The 1V/octave standard means each 1-volt increase in control voltage doubles the oscillator frequency (raises pitch by one octave). If 0V produces 100 Hz, then 1V produces 200 Hz, 2V produces 400 Hz, 3V produces 800 Hz, and so on. This exponential relationship matches how we perceive musical pitch: equal voltage steps produce equal musical intervals. A 5-octave keyboard outputs 0V to 5V (sometimes with an offset like 0V = C1, 5V = C6).

Temperature affects VCO tuning because transistor behavior changes with heat. Professional oscillators use tempco resistors (temperature-compensated) and matched transistor pairs to minimize drift. Cheaper designs live with some drift—you retune between songs. Digital oscillators eliminate this problem entirely but lose the subtle analog character. The choice depends on your priorities: studio-stable tuning versus organic, breathing tone.

Modulation inputs let you add vibrato (slow LFO), create FM synthesis (audio-rate modulation), or implement pitch-tracking filters. Each modulation source adds or multiplies with the main control voltage before reaching the exponential converter. Understanding this signal flow—keyboard CV + LFO CV + envelope CV = final pitch—is essential for patching complex sounds in modular systems.

🔬 Measuring and testing oscillators

An oscilloscope shows waveform shape and lets you measure frequency by counting cycles per screen division. But you don't need an expensive scope for hobby work—a DSO138 pocket oscilloscope ($30) or smartphone audio interface with Android oscilloscope app (Keuwlsoft) works fine for audio frequencies. Test your oscillator by connecting it to an amplifier or mixer and listening—your ears are the final judge of whether it sounds good.

To verify 1V/octave tracking, apply known voltages from a precision voltage source (or calibrated modular power supply) and measure output frequency with an online chromatic tuner. Apply 0V → note the pitch. Apply 1V → pitch should be exactly one octave higher. Apply 2V → two octaves above 0V. If tracking drifts (1.02 octaves at 1V, 2.05 octaves at 2V), adjust the exponential converter trim pot. Most VCO designs include a "scale" or "tracking" trimmer for this adjustment.

Common oscillator problems: no output (check power supply, verify core oscillator is running with scope), frequency too high or too low (wrong capacitor value), poor tracking (exponential converter needs calibration), distorted waveforms (overloading amplifier stages or incorrect bias voltages). Systematic troubleshooting—checking each stage in order—isolates problems faster than randomly swapping components.

🎵 Beyond basic oscillators

Once you understand simple VCOs, explore advanced concepts: Pulse-width modulation (PWM) varies the duty cycle of a square wave, creating sweeping timbral changes. Sync forces one oscillator to reset when another completes a cycle, generating aggressive harmonic sounds. Through-zero FM allows the carrier frequency to go negative, producing bell-like metallic tones impossible with standard FM. Each technique has dedicated circuit implementations covered in synth DIY forums and design guides.

Wavetable oscillators store multiple waveform cycles in memory and smoothly morph between them—popular in modern Eurorack modules but requiring microcontroller programming. Additive synthesis combines multiple sine waves at different frequencies and amplitudes to build complex timbres from first principles. Both approaches offer different creative possibilities than subtractive synthesis but require more complex circuits or programming knowledge.

The best way to understand oscillators is to build several different designs and compare them. A 555 square wave generator, an LM13700-based triangle core, and an AS3340 chip-based VCO each teach different lessons about frequency control, waveform generation, and circuit design. Document what you learn: keep a notebook with schematics, component values, and tuning procedures. This becomes your reference library for future projects and modifications.