Noise Source
Understanding Noise Sources
Noise sources are essential calibration tools for noise figure measurement. They provide a precisely known noise power level that serves as the measurement reference. The accuracy of any noise figure measurement depends on the accuracy of the noise source ENR calibration.
How They Work
A noise diode (typically an avalanche or Zener diode) generates broadband noise with a flat power spectrum when reverse-biased. The noise power is calibrated at the factory across frequency, providing a table of ENR values. During measurement, the diode is switched ON (hot) and OFF (cold), and the Y-factor is computed.
Noise Source Selection
- Low ENR (5-6 dB): Best for low-NF devices (LNAs with NF < 3 dB). Minimizes measurement uncertainty.
- Standard ENR (15 dB): General purpose. Suitable for NF > 3 dB.
- High ENR (25+ dB): For measuring high-NF devices (mixers, passive components).
ENR = 15 dB: Th = 9,454K
ENR = 6 dB: Th = 1,445K
Y-factor measurement:
Y = P_hot/P_cold
NF = ENR - 10 log10(Y - 1)
Frequently Asked Questions
What is a noise source?
A noise source is a calibrated broadband noise generator used for noise figure measurement. It switches between a high-noise state (DC-biased diode) and a low-noise state (unbiased, at room temperature). The excess noise ratio (ENR) specifies its calibrated noise output.
How accurate is noise figure measurement?
Accuracy depends on ENR calibration accuracy (typically +/-0.15 dB), measurement technique, and DUT noise figure. For NF < 1 dB, measurement uncertainty can exceed 0.2 dB. Using low-ENR sources and proper mismatch correction minimizes uncertainty.
Do noise sources wear out?
Noise diodes degrade slowly over time and use cycles. ENR should be re-verified periodically (annually is typical) against a reference standard. Changes in ENR directly translate to noise figure measurement errors.