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.
| Amplifier Type | Noise Figure | Gain | Output Power |
|---|---|---|---|
| LNA | 0.3-2 dB | 10-25 dB | -10 to +10 dBm |
| Driver | 3-8 dB | 10-20 dB | +15 to +25 dBm |
| Power Amp | N/A | 8-15 dB | +30 to +50 dBm |
| Distributed | 3-6 dB | 5-12 dB | +10 to +20 dBm |
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.