There are atleast three ways to measure the noise figure
of a device. These are:
Using Noise Figure Meter
Excess Noise Ratio -ENR of Noise Source
Factors that you need to consider when choosing
equipment for Noise Figure measurement:
Advantages and disadvantages of this method
Formulas for noise figure measurement
Measurement technique used
Advantages of using this method
Disadvantages of this method
Y Factor Method
Advantages of using Y Factor method
Disadvantages of this method
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 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
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:
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
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+ Ndut]/[ GNin]
Or 10logF = 10log Ndut -10 logNin
=10log Ndut - 10 log(KT0D
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
b. Measurement using Gain method:
The diagram for measurement of NF using Gain method is shown
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
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:
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.