噪声系数测量

Technical data is subject to change. Copyright@2004 Agilent

Fundamental

noise concepts

How do we

make

measurements?

What DUTs

can we

measure?

What influences

the measurement

uncertainty?

What is Noise Figure ?

Noise

Out

Noise in

Measurement bandwidth=25MHz

a) C/N at amplifier input b) C/N at amplifier output

Nin

Nout

Ga Rs

Two examples of Noise Figure
Example 1: In a receiver, the LNA is connected to an antenna which points to earth’s atmosphere (290K) and the LNA has 3dB NF and 10dB gain. Noise power at LNA output is: -174+10+3=-161dBm/Hz Example 2: In a transmitter the modulator noise floor is -140dBm/Hz. The modulator output is amplifier by a linear amp with 3dB NF and 10dB gain. Noise power at amplifier output is: -140+10+3=-127dBm/Hz
-140dBm corresponds to a noise source with a temperature 700 million K, i.e. DUT input is not Standard Temperature and Example 2 is wrong
Just to emphasize this point, noise figure only represents the noise added to the input noise referred to the DUT output when the noise into the device is thermal noise at the standard temperature. So the first example here is correct. In the second example, the noise going into the device is much higher and therefore the noise figure of the amplifier cannot be added to the noise out of the DUT from the modulator. In reality if the noise of the amplifier is only 3dB then it will add practically no noise to that generated by the modulator.
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An Alternative Way to Describe Noise Figure: Effective Input Noise Temperature
Nin
Nout = Na + kTB Ga
Rs
Output Power
Ga , Na
Slope=kBGa
c isti ter c ara
is No
C ree eF
h
Na -Te Te Source Temperature (K)
Let’s now plot the output noise power as a function of the temperature of the noise source. In the equation for Nout I have substituted Nin for kTB where T now varies from absolute zero upwards. It’s a linear curve as we are dealing with very low power levels so all devices are operating in their linear regions. Actually the line is a very standard ‘y=mx+C’. M is the gradient in this case kBGa and c is the point at which the curve intersects the y axis. C is equal to Na. What you can say at T=0 is that no power at the device output comes from the noise source. All the output power at this point is generated within the DUT. This gives us another figure of merit for describing the noise performance of active devices. If you look at the graph I have drawn the characteristic of a noise free device. If you transpose the added noise Na through this line on to the x axis you arrive at Te, the effective input noise temperature. When you multiply Te by the gain bandwidth product of the device you get the amount of noise added. It’s a useful figure of merit because it is independent of the device gain (unlike Na).
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