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Manual noise-figure measurements are often necessary when automated measurement systems are not available. One of the more popular techniques for making noise-figure measurements is the Y-factor method, which is accomplished with the aid of a hot/cold noise source to provide two different noise power levels (for this article, cold will be assumed at 290K). The difference between the two noise power levels is the excess noise ratio (ENR), which is given numerically by (TH - Tc)/ Tc, where TH represents the hot temperature and Tc is the cold temperature. Under true impedance-matched conditions, the actual available noise power would be equal to kTB, where k is Boltzmann’s constant (1.3806503 × 10-23m2kg/s2K), T is the temperature (in degrees Kelvin) or S/B (in Kelvin), and B is the bandwidth of the circuit under test.

The Y-factor is the ratio of output noise power from a device under test (DUT) with the noise source set to hot, divided by the output noise power with the noise source set to cold. Using the Y-factor approach, the noise figure of a DUT can be calculated by using Eq. 1:

Noise figure of DUT = 10log10[ENR/(Y - 1)]      (1)

where ENR is the numerical ratio of the ENR in dB, or ENR = 10(ENR/10). The ENR is defined as (TH - 290)/290. Figure 1 shows a typical setup for measuring the Y-factor. The resistor in the noise source is a fictitious component with a body temperature that can be changed to produce different levels of output noise power. The resistor at the input of the DUT, TE, represents a fictitious resistor at a temperature of TEK that, when multiplied by the gain of the DUT, would produce the proper amount of noise at its output corresponding to its internally generated noise. The input noise temperature is related to the noise figure by Eq. 2:

TE = 290(NFac - 1)     (2)

where NFac, the noise factor, is the numerical value of the noise figure—e.g., NFac = 10[(Noise Figure)/10].

1. This basic circuit is used to evaluate the Y-factor of a DUT when using a calibrated noise source.

As shown in Fig. 1, the standard circuit for a noise figure measurement is simple and can provide an accurate measurement of the DUT’s noise figure if one were able to accurately measure the output noise power of the DUT. Since an instrument must be used to make the noise power measurement at the output of the DUT, the noise power measurement is a composite of the noise power coming out of the DUT, along with the internal noise generated by the measuring device (for manual measurements, the measuring instrument is usually a spectrum analyzer).

2. A spectrum analyzer can also be used for measuring the noise figure of a DUT.