Noise Source Symbol
Definition: The Noise Source symbol represents an idealized or physical circuit element that generates a specified broadband random noise signal — such as white noise, thermal (Johnson–Nyquist) noise, or shot noise — used in circuit simulation, RF system analysis, and test equipment schematics, drawn as a circle or source block with a single output pin labelled Out.
Also known as: white noise generator, thermal noise source, Johnson noise source, shot noise source, RF noise source, broadband noise generator.
What the Noise Source symbol means
The Noise Source symbol denotes a circuit element that produces a random, spectrally distributed electrical signal. In theoretical analysis and SPICE simulation, a noise source models the unavoidable random voltage or current fluctuations present in resistors (thermal noise), semiconductors (shot noise), and amplifiers (flicker noise). The symbol conveys that the element injects noise power into the circuit under analysis.
In practical RF and microwave test schematics, the Noise Source symbol represents a calibrated hardware module (such as an Agilent 346B) that produces a precisely characterised broadband noise signal for measuring the noise figure of amplifiers, receivers, and other active components. The single output pin (Out) connects to the device under test or the circuit node being analysed.
How to identify the Noise Source symbol
The Noise Source symbol is drawn as a circle (similar to an AC voltage source) or a rectangular block, with a wavy or noise-squiggle graphic inside or the label 'Noise' or 'N'. A single output pin (Out) extends from the symbol. In SPICE netlists it appears as a current or voltage noise source appended to a resistor or independent source. The circle-with-noise-label distinguishes it from a sinusoidal AC source (sine wave inside the circle) or a DC voltage source (+ and − symbols).
Function in a circuit
A Noise Source generates a statistically random electrical signal with a defined power spectral density. Thermal (Johnson–Nyquist) noise has a flat power spectral density across frequency equal to 4kTB (where k is Boltzmann's constant, T is temperature in Kelvin, and B is bandwidth in Hz). A noise source is used in circuit simulation to model inherent device noise, to specify system noise performance via noise figure (NF) measurement, and as a stimulus in noise immunity testing.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617 does not define a specific dedicated symbol for a noise source; in SPICE and simulation tools it is represented as a voltage or current source with a noise spectral density parameter. IEC 60268-1 defines noise measurement methods for audio systems. |
|---|---|
| ANSI/IEEE 315 | ANSI Y32.2 / IEEE 315 has no specific noise source glyph; the component is represented as a circle source symbol with an appropriate label. IEEE 60068-2-64 covers noise vibration testing; IEEE Std 521 defines radar noise figure. |
| Key difference | Both IEC and ANSI/IEEE represent the noise source as a generic source symbol with a descriptive label; there is no standardised unique glyph, and representation varies by simulation tool and application. |
Terminals / pins
| Pin | Name |
|---|---|
| out | Out |
Typical values
Noise power density: kT = −174 dBm/Hz at 290 K (17 °C). Excess Noise Ratio (ENR): 5–35 dB for calibrated RF noise sources. Output impedance: 50 Ω (RF instruments) or matched to circuit node impedance. Frequency range: DC to GHz for broadband sources. Thermal noise voltage across a resistor R at temperature T in bandwidth B: Vn = √(4kTRB).
Where the Noise Source symbol is used
- SPICE simulation of amplifier noise performance to predict output SNR and noise figure before fabrication
- Noise figure measurement of LNAs, mixers, and receivers using the Y-factor method with a calibrated noise source (hot/cold state)
- RF receiver design schematics showing cascaded noise figure budget analysis across LNA, mixer, and IF amplifier stages
- Audio pre-amplifier analysis where thermal noise of input resistors and 1/f noise of op-amps is modelled
- EMC and immunity test setups where a broadband noise source injects disturbance into power or signal lines
- Random signal generator circuits used in audio testing (pink noise for speaker response measurement)
Example
In an LNA noise figure test setup, a Keysight 346C calibrated noise source connects its Out pin to the RF input of a 2.4 GHz LNA under test. The noise source is switched between hot (ENR = 15 dB active) and cold (50-ohm terminated) states while a spectrum analyser measures the output power change (Y-factor). The noise figure is calculated as NF = ENR − 10·log10(Y − 1) dB.
Key facts
- Thermal (Johnson–Nyquist) noise is the fundamental noise in a resistor at temperature T and has a power spectral density of S = 4kTR V²/Hz, where k = 1.381 × 10⁻²³ J/K and R is resistance in ohms.
- At room temperature (290 K, 17 °C) the available thermal noise power density is kT = −174 dBm/Hz; this sets the noise floor of any 50-ohm RF receiver.
- Calibrated RF noise sources specify Excess Noise Ratio (ENR) in dB, the ratio of the hot noise power to the standard kT₀ = 290 K noise power, allowing accurate noise figure measurement via the Y-factor method.
- Shot noise arises from the discrete nature of charge carriers in semiconductor junctions and has spectral density In² = 2qI_DC, where q = 1.6 × 10⁻¹⁹ C and I_DC is the DC bias current.
- Flicker (1/f) noise dominates at low frequencies in MOSFETs, JFETs, and BJTs; it is characterised by a power spectral density proportional to 1/f and has a corner frequency below which it exceeds thermal noise.
- In SPICE simulation, noise sources are modelled as current or voltage sources with a spectral density parameter; the simulator performs AC noise analysis to calculate noise figure, equivalent input noise voltage, and signal-to-noise ratio.
- The Noise Source symbol has a single output pin (Out) in schematic representations; the symbol conveys that the element outputs a stochastic signal rather than a deterministic waveform.
Frequently asked questions
What does the noise source symbol mean in a circuit diagram?
The Noise Source symbol represents an element that generates a random electrical noise signal with a defined power spectral density. It models unavoidable thermal noise, shot noise, or flicker noise in a circuit, or represents a physical calibrated noise generator used to measure amplifier noise figure. The symbol appears in simulation schematics and RF test-setup diagrams.
What does the noise source symbol look like?
The Noise Source symbol is drawn as a circle or rectangular block with a wavy or noise-like graphic inside and the label 'Noise' or 'N'. A single output pin (Out) connects to the circuit node. It resembles an AC source symbol but is distinguished by a noise-waveform icon rather than a sine wave inside the circle.
What is the thermal noise power of a resistor?
The available thermal noise power of a resistor in bandwidth B is P = kTB watts, where k = 1.381 × 10⁻²³ J/K, T is temperature in Kelvin, and B is bandwidth in Hz. At room temperature (290 K) and 1 Hz bandwidth, P = −174 dBm. The corresponding open-circuit noise voltage is Vn = √(4kTRB).
What is Excess Noise Ratio (ENR) in a noise source?
Excess Noise Ratio (ENR) is the ratio of the hot-state noise power of a calibrated noise source to the reference thermal noise power at T₀ = 290 K, expressed in dB. ENR = 10·log10((T_hot/T₀) − 1). RF noise sources are supplied with a calibration table of ENR vs. frequency, used in Y-factor noise figure measurements.
What is the difference between white noise and pink noise?
White noise has a flat power spectral density across all frequencies (equal power per Hz bandwidth), analogous to thermal noise in resistors. Pink noise (1/f noise) has power spectral density inversely proportional to frequency, giving equal power per decade of bandwidth. Pink noise is used in audio speaker and room acoustics testing because it mimics the spectral distribution of music.
How is a noise source used to measure noise figure?
A calibrated noise source is connected to the input of the device under test (DUT). The source is switched between an 'on' (hot, high noise power) state and an 'off' (cold, 50-ohm) state. The Y-factor is the ratio of output power in the on state to output power in the off state: Y = P_hot/P_cold. Noise figure is then calculated as NF = ENR − 10·log10(Y − 1) dB.
What standard defines noise figure measurement?
IEEE Std 60268-1 covers noise in audio systems. For RF and microwave noise figure, IEEE Std 519 and IRE (now IEEE) standards define noise figure methodology. The Y-factor measurement method for noise figure is described in Keysight application note AN 57-1 (Fundamentals of RF and Microwave Noise Figure Measurements), which follows the IEEE noise figure standard framework.
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