Noise Figure And Measurement Techniques

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 don't 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, T_c = 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:

• Expected Noise Figure: NF Analyzer is suitable to measure Noise Figure when the value is small (say less than 10dB typical). If you intend to measure very high value or very low value (say less than 0.05dB), you may need to consult the manufacturer for suitability of the equipment.
• Due to the limitations of the DUT/NF Analyzer, you may need to use external mixer for measuring NF. In such an event, ensure that the NF Analyzer supports external/internal mixer capabilities.
• DUT Connectors: Some times, the DUT may have different connectors, such as waveguide. If so, you may need to procure appropriate waveguide to co-axial adapter. Most of the NF Analyzers use only co-axial connector for interconnectivity.
• Gain Measurement: Normally, you measure gain along with NF. Ensure that the NF Analyzer is capable of measuring expected range of DUT Gain.
• For frequency conversion, you may require external frequency generator, and a mixer. Most measurement (such as Up/Down Converters) require external frequency generator.

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+ N_dut]/[ GN_in]

Or 10logF = 10log N_dut -10 logN_in

₌10log N_dut - 10 log(KT₀D fG)

If you consider per Hertz (normalized) bandwidth,

=10log N_dut -10logKT₀ -10logG

=[10log N_dut - 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:

• This method is very useful for measuring very high Noise Figure (or the order of 10dB or more).
• The method is more intuitive, and useful for experimentation.
• A spectrum analyzer can also be used to make other measurements such as amplifier harmonics, gain, etc. where as a NF analyzer is specifically made for NF measurements.
• Gain method is recommended when making measurements at low frequencies, typically less than 10MHz.

d. Disadvantages of using this method:

• The spectrum analyzer should be able to provide very good resolution bandwidth, and noise floor, typically of the order of -130dBm. A spectrum analyzer becomes very expensive when you need to measure very low signal levels such as -130dBm at high resolution bandwidths (typically few Hertz).
• This method requires that the Gain of the DUT is known already. Also, the accuracy of Noise Figure measured depends directly on the accuracy of the measured Gain.

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

Y=Nh/Nc

If the Noise source is at room temperature, then Nc = N0 and the equation becomes,

Y= Nf/N0

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:

• The equipment required is less. You need a noise source and power meter to measure the power levels with noise source ON and OFF. Ofcourse, you need a mechanism to turn the noise source ON and OFF.
• The method can be used to measure noise figure over a wide frequency range.

c. Disadvantages of using this method:

• Due to the limitation of noise source, if the DUT noise figure is very high, the results may not be very accurate.
• The other equipment needs to be stable so that you can get repeatable measurements.