Signal Generator Symbol
Definition: The Signal Generator symbol represents a test-equipment source that produces a defined periodic waveform — sine, square, triangle, or arbitrary — at a settable frequency and amplitude for stimulating and testing circuits, drawn per IEC 60617 and ANSI Y32.2 / IEEE 315 convention as a circle containing a sine-wave curve, with Output and GND terminals.
Also known as: function generator, waveform generator, sine wave generator, test oscillator, audio generator, signal source, sweep generator, AWG.
What the Signal Generator symbol means
The Signal Generator symbol denotes a stimulus source in a test or measurement setup: an instrument (or an on-schematic idealization of one) that injects a known, adjustable waveform into the circuit under test. Unlike the independent AC voltage source symbol used in pure circuit theory, the signal-generator symbol implies a piece of bench equipment with a defined output impedance — almost universally 50 ohms — settable frequency, amplitude, DC offset, and often modulation and sweep capability. In block diagrams it marks where excitation enters: the input of an amplifier being characterized, the clock of a logic prototype, or the swept source feeding a filter whose response is being plotted.
Schematics in service manuals, lab procedures, and application notes use this symbol to say 'connect a generator here and set it to the stated frequency and level.' Modern instruments span classic analog function generators, direct digital synthesis (DDS) generators with microhertz resolution, arbitrary waveform generators (AWGs) that replay sampled or computed waveforms, and RF signal generators with calibrated output levels down to −140 dBm.
How to identify the Signal Generator symbol
The symbol is a circle with one cycle of a sine wave drawn inside, with two terminals: Output (signal) and GND (return/chassis). Variants replace or supplement the sine with a square or triangle wave to indicate a function generator's multiple waveforms, and an arrow through the circle can indicate adjustability (a swept or variable source). A small 'G' or the legend 'SIG GEN' sometimes accompanies the circle in service literature.
IEC 60617 renders an AC source as a circle with a tilde or sine curve; the generator usage is the same drawing applied as test equipment, and IEC also provides a generator symbol as a circle containing the letter G. ANSI Y32.2 / IEEE 315 likewise uses a circle enclosing the waveform. Distinguish it from the independent AC voltage source of circuit theory mainly by context and labeling: a generator symbol in a test setup carries instrument settings (frequency, level, source impedance) rather than an idealized phasor value.
Function in a circuit
A signal generator synthesizes its waveform — by analog oscillator loop (Wien bridge, function-generator triangle/square core), by DDS (a phase accumulator stepping through a sine lookup table clocked at a fixed rate), or by DAC playback in an AWG — then scales it through an output amplifier and attenuator to the requested amplitude behind a series 50 Ω source resistance. The 50 Ω convention matters practically: the front-panel amplitude is usually calibrated assuming a 50 Ω load, so driving a high-impedance input delivers TWICE the displayed voltage unless the instrument is told the load is high-Z.
In use, the generator stimulates a circuit while an oscilloscope, meter, or analyzer observes the response: sweeping frequency traces a filter or amplifier's frequency response; a square wave reveals ringing and slew limits; a burst tests transient behavior; amplitude steps establish gain linearity and clipping onset.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 provides the AC source/generator symbols — a circle containing a sine curve (alternating source) or the letter G (generator) — used for signal sources on schematics and test diagrams. Instrument performance itself is covered by product standards rather than the symbol library. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 defines the AC/signal source as a circle enclosing the waveform, with test-equipment usage annotated by legend (e.g. 'SIG GEN', frequency, level). The 50 Ω source-impedance convention is an industry practice inherited from RF instrumentation rather than a symbol-standard requirement. |
| Key difference | IEC and ANSI drawings of a signal source are near-identical circles with an internal waveform; IEC additionally sanctions the circle-G generator form. The meaningful distinction on real schematics is not IEC-versus-ANSI but idealized AC source (circuit theory, phasor value) versus instrument source (test setup, with frequency/level/impedance settings called out). |
Terminals / pins
| Pin | Name |
|---|---|
| out | Output |
| gnd | GND |
Typical values
Typical bench function/waveform generators: frequency range 1 µHz–25 MHz (DDS models; RF generators reach 3–6 GHz and beyond), amplitude 1 mVpp–10 Vpp into 50 Ω (20 Vpp into high-Z), DC offset ±5 V, output impedance 50 Ω, sine flatness ±0.3 dB, square-wave rise times under 10 ns, and AWG sample rates from 100 MSa/s to several GSa/s with 14–16-bit vertical resolution. Common test levels: 1 kHz at 1 Vrms for audio work, 0 dBm (224 mV into 50 Ω) for RF, and 3.3/5 V logic-level squares for digital clocking.
Where the Signal Generator symbol is used
- Amplifier gain, distortion, and frequency-response measurements on the audio and RF bench
- Filter characterization, sweeping frequency while plotting output amplitude to trace the response curve
- Injecting clocks and test patterns into logic prototypes and microcontroller peripherals
- Signal-tracing in repair work, injecting a known tone stage-by-stage to localize a dead stage
- Sensor and transducer simulation, replaying arbitrary waveforms that mimic real-world signals
- Modulation testing of receivers and demodulators using AM/FM/burst outputs from the generator
Example
In an amplifier test schematic, the Signal Generator symbol's Output pin drives the amplifier input through a DC-blocking capacitor while its GND pin bonds to the amplifier's ground plane; the procedure calls for 1 kHz sine at 100 mVpp from the 50 Ω output, and because the amplifier input is high-impedance, the operator sets the generator's load menu to High-Z so the displayed amplitude matches the real 100 mVpp delivered.
Key facts
- The symbol is a circle containing a sine wave (or the waveform provided), with Output and GND terminals; IEC also uses a circle enclosing G for a generator.
- Bench generators almost universally present a 50 Ω source impedance; displayed amplitude usually assumes a 50 Ω load, so a high-impedance load sees twice the displayed voltage.
- A function generator supplies sine, square, and triangle waves; an arbitrary waveform generator (AWG) replays any sampled waveform from memory through a fast DAC.
- DDS (direct digital synthesis) generators derive all frequencies from one crystal clock via a phase accumulator, giving microhertz resolution and instant, phase-continuous frequency changes.
- Sweep mode ramps frequency between set limits, enabling frequency-response plots of filters and amplifiers with an oscilloscope in X-Y or a scope's FFT.
- It differs from the circuit-theory AC source symbol by context: it represents adjustable test equipment with real output impedance, not an ideal phasor source.
- Square-wave testing exposes amplifier ringing, overshoot, and slew-rate limiting that sine tests hide.
- Typical instrument spans: 1 µHz–25 MHz function generators, multi-GHz RF signal generators, AWGs sampling at 100 MSa/s–several GSa/s.
Frequently asked questions
What does the signal generator symbol look like on a schematic?
A circle with a sine wave drawn inside it, with an Output terminal and a GND terminal. Variants show a square or triangle wave for function generators, add an arrow for a swept/adjustable source, or use IEC's circle containing the letter G. Test-setup schematics usually annotate it with the required settings, such as '1 kHz, 100 mVpp, 50 Ω'.
Why does my generator output twice the voltage I set?
Because of the 50 Ω convention. The instrument contains a 50 Ω series source resistance and calibrates its displayed amplitude assuming a matched 50 Ω load, which halves the open-circuit voltage. Driving a high-impedance input (a typical oscilloscope or amplifier input) removes that halving, so the load sees double the displayed value. Set the generator's output/load setting to 'High-Z' to correct the display, or halve your requested amplitude.
What is the difference between a signal generator and a function generator?
'Signal generator' is the umbrella term for any calibrated waveform source. A function generator is the classic bench subset producing standard shapes — sine, square, triangle, ramp, pulse — typically up to a few tens of MHz. RF signal generators produce calibrated-level sine carriers with modulation up to GHz frequencies, and arbitrary waveform generators (AWGs) replay user-defined waveforms from sample memory. Schematically they all use the same circle-with-waveform symbol.
What is the difference between the signal generator symbol and the AC voltage source symbol?
Graphically they are nearly identical — a circle with a sine inside. The distinction is semantic: the AC voltage source of circuit theory is an ideal element with zero source impedance and a stated phasor value, used in analysis; the signal generator represents a physical instrument with 50 Ω output impedance and adjustable frequency/amplitude, used in test and service diagrams. Labels and context (instrument settings versus a phasor like 10∠0° V) tell them apart.
What is a DDS signal generator?
A direct digital synthesis generator creates its waveform numerically: a phase accumulator increments at each tick of a fixed crystal clock, its value indexes a sine lookup table, and a DAC converts the samples to an analog output. Frequency is set by the accumulator increment, giving microhertz resolution, crystal-derived stability, and instant phase-continuous frequency changes — which is why DDS displaced analog oscillator-based generators in almost all modern bench instruments.
Why test an amplifier with a square wave?
A square wave is a harsh, information-rich stimulus: its fast edges contain high-frequency energy that exposes ringing, overshoot, and instability; edge rounding reveals limited bandwidth; tilted flat tops reveal poor low-frequency response or coupling-capacitor droop; and asymmetric edge slopes expose slew-rate limiting. A single 1 kHz square-wave photo of the output tells an experienced engineer more than several sine measurements.
Related symbols
- Astable Multivibrator symbol
- Colpitts Oscillator symbol
- Crystal Oscillator Module symbol
- VCO Block symbol
- Voltage Source (AC) symbol
- Wien Bridge Oscillator symbol
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