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Noise Figure Measurement - A Practical Approach
6. Noise Figure Measurement Techniques:
There are atleast three ways to measure the noise figure of a device. These are:
Each method has its own advantages and disadvantages.
1. Noise Figure Measurement using Noise Figure Meter:
The equipment connections are as shown in the diagram below. A mixer may be necessary if you want to convert the RF frequencies to desired IF frequencies. In case you dont need a frequency mixer, simply connect the IF OUT cable to the Noise Source, and calibrate.
a. Excess Noise Ratio (ENR): ENR is frequently used to denote the noise that a calibrated noise source delivers to a DUT.
Where Th is the hot temperature (Corresponds to Noise source ON)
Tc is the Cold temperature (Corresponds to Noise source OFF)
At room temperatures, Tc = T0
The ENR needs to be entered into the Noise Figure Analyzer corresponding to the frequency of measurement. Normally, the ENR table is prominently displayed on the Noise Source.
b. Calibration procedure: First the equipment needs to be calibrated. The calibration procedure normally involves inputting the ENR (Excess Noise Ratio) as given on the Noise Source in to the Noise Figure Analyzer at the desired frequency range. This will establish the base level against which the DUT noise figure will be measured.
c. Measurement of Noise Figure and Gain:
The following figure shows the diagram with required connections with DUT. Once the test equipment is calibrated for Noise Figure, (and Gain, as it is normally measured along with the Noise Figure) by simply connecting the DUT in to the calibrated set up as below will display the Gain and Noise Figure of the DUT.
It is possible that you use any connector adapters or attenuators during the measurement process to ensure that the measured values are within the range of the Noise Figure Analyzer. In such case, you may to include the adapters or attenuators during the calibration process itself (not shown in the figure).
d. Factors that you need to consider when choosing equipment for Noise Figure measurement:
e. Advantages and disadvantages of this method:
1. Simple to measure Noise Figure and Gain
2. Accurate for small NF measurements
3. NF could be measured across a wide range of frequencies with external mixers.
2. Gain Method:
a. Formulas Used:
The Noise figure measurement using gain method is based on the following formulas:
=1+ Ndut]/[ GNin]
Or 10logF = 10log Ndut 10 logNin
=10log Ndut 10 log(KT0D fG)
If you consider per Hertz (normalized) bandwidth,
=10log Ndut 10logKT0 10logG
=[10log Ndut 174 10logG ] db/Hz
We know the constant KT0, which is equal to 174 dBm/Hz. Therefore, we can compute the NF in dB by measuring Ndut.
b. Measurement using Gain method:
The diagram for measurement of NF using Gain method is shown below:
c. Advantages of using this method:
d. Disadvantages of using this method:
3. Y-Factor Method of Noise Figure Measurement:
a. What is Y-Factor:
The Y-Factor is the ratio of Hot and Cold noise powers (in watts) and is defined as
If the Noise source is at room temperature, then Nc = N0 and the equation becomes,
Note that the Y factor method is a relative method and does not depend on the rest of the equipment. All you need is to measure the power levels accurately while the noise source is OFF and ON.
The noise figure is related to the Y factor as below:
F = ENR/[Y-1]
Note that the above parameters are in linear units. Normally, the ENR provided on the noise source is in decibels. This needs to be converted to linear units for computing the noise figure.
b. Advantages of this method:
c. Disadvantages of using this method:
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