How is noise figure measured?

From RF Design Wiki · By the RF Design Wiki editorial team · Published October 21, 2026 · Last reviewed October 21, 2026 · How we research

The Y-factor method uses a calibrated noise source switched between hot (on) and cold (off) states. With excess noise ratio ENR (linear) and measured ratio Y = N_on/N_off, the noise factor is F = ENR/(Y − 1) when T_cold = 290 K. Instruments then subtract the measurement receiver’s contribution using the Friis formula (second-stage correction).^[1]

What is ENR?

ENR = (T_hot − T₀)/T₀, given in dB at calibrated frequencies on the noise source’s label. Typical sources are 5–6 dB ENR for low-NF devices and 15 dB ENR for higher-NF devices.^[1]

What is the cold-source method?

Measures output noise power with only a cold (room temperature) termination and uses the DUT gain measured by a VNA to compute NF. It handles mismatched devices better and is fast for multiport fixtures, with the VNA providing vector correction of source mismatch.^[2]

What causes uncertainty?

Mismatch between noise source, DUT and analyzer; on/off source-match changes.
ENR uncertainty (typically ~0.1–0.2 dB).
Insufficient DUT gain relative to the analyzer’s NF.
Interference and temperature of the cold source differing from 290 K.

Keysight provides uncertainty calculators for these terms.^[1]

Frequently asked questions

Can a spectrum analyzer measure NF?

Yes, with a noise source and NF personality, or by the gain method for high-NF devices.

What is the gain method?

NF from measured output noise density and gain: NF = Pout_noise − (−174) − G.

Why does low-NF measurement need care?

Small uncertainties are large relative to a 0.5 dB NF.

References

Online sources accessed October 21, 2026; printed works cited by edition. See all sources.

“Fundamentals of RF and Microwave Noise Figure Measurements (Application Note 57-1)”. Keysight Technologies. application note.
“Understanding the Fundamental Principles of Vector Network Analysis (Application Note 1287-1)”. Keysight Technologies. application note.

Categories: Test and measurement

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