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Test & Measurement System

Noise Figure Measurement Setup (Y-Factor)

NOISE FIGURE MEASUREMENT — Y-FACTOR METHOD NOISESOURCE Calibrated ENR 28V bias ON/OFF DUTAmplifier Device Under Test POWERMETER or NF Analyzer CALCULATION Y = P_hot / P_cold F = (ENR) / (Y - 1) NF = 10·log(F) PROCEDURE 1. Measure P_cold (noise source OFF = 290K) → 2. Measure P_hot (noise source ON = T_hot) 3. Calculate Y-factor → 4. Apply ENR correction → 5. Compute NF
Component Descriptions

Signal Chain Walkthrough

The Y-factor method is the standard technique for measuring the noise figure of amplifiers, receivers, and other active RF components. It uses a calibrated noise source switched between two known noise temperatures.

Calibrated Noise Source

A solid-state noise source (typically a reverse-biased avalanche diode) produces excess noise with a known ENR (Excess Noise Ratio). When biased ON, it produces T_hot (typically 10,000-20,000 K). When OFF, it presents T_cold (290K, room temperature). Common ENR values are 5, 6, and 15 dB.

Y-Factor Calculation

The Y-factor is the ratio of output power with the noise source ON (P_hot) to output power with it OFF (P_cold). From Y and the known ENR, the DUT noise factor F = ENR/(Y-1), and NF(dB) = 10·log(F).

Second-Stage Correction

If the DUT gain is low, the measuring instrument's own noise figure affects the result. The Friis equation correction: F_total = F_DUT + (F_meter - 1)/G_DUT must be applied.

Typical Specifications

Component Specifications

ComponentParameterTypical Value
Noise SourceENR5 - 15 dB
Noise SourceFrequency10 MHz - 110 GHz
NF UncertaintyBest Case±0.1 - 0.3 dB
Measurement RangeNF0 - 30 dB
Design Note: Y-factor accuracy depends on: ENR calibration accuracy (±0.1 dB is excellent), source VSWR (mismatch adds error), DUT gain (low gain increases second-stage correction uncertainty), and connector repeatability. For sub-1 dB NF measurements, use a low-ENR source (5 dB) and ensure the DUT has at least 20 dB gain to minimize second-stage effects.
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