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III. Gm-C Filtering - Epublications - Université de Limoges

III. Gm-C Filtering - Epublications - Université de Limoges

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A.6 Noise Measurements<br />

To measure the NF of the filter, the popular Y-factor method is used [A.5 and A.6]. It<br />

consists in using an ENR (Excess Noise Ratio) source. In the present case, a noise dio<strong>de</strong> has<br />

been used. It actually is a Zener dio<strong>de</strong> un<strong>de</strong>r a high DC voltage (28V). According to the<br />

working frequency, the ENR of such a dio<strong>de</strong> is standard and can be found in a table.<br />

The dio<strong>de</strong> is directly connected to the input of the filter as shown in Figure 1. The<br />

output of the filter is then connected to the spectrum analyzer.<br />

Figure 196. NF Measurement<br />

The noise source is successively turned on and off, by turning on and off the DC<br />

voltage. The change in the output noise power <strong>de</strong>nsity is measured with help of the spectrum<br />

analyzer.<br />

The formula to calculate the noise figure is given by:<br />

ENR<br />

⎛ ⎞<br />

⎜ 10 10 ⎟<br />

NF = 10log⎜<br />

Y ⎟<br />

(A.15)<br />

⎜ 10 ⎟<br />

⎝10<br />

− 1⎠<br />

in which ENR is given in a table and Y is the difference between the output noise power<br />

<strong>de</strong>nsity when the noise source is on and off.<br />

The major advantage of this technique is that is can measure a wi<strong>de</strong> range a NF in a<br />

simple way. It is also able to measure it at any frequency regardless of the <strong>de</strong>vice un<strong>de</strong>r test<br />

gain. However, when measuring very high NF, error could be large.<br />

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