MICRONIX

Relation between noise level and noise figure NF

For example, the input signal is amplified 20dB whe n it passes an amplifier of 20dB gain. This is understandable.
How about a noise ?  This cannot be understood well.

That is, the appearance of signal is same if a noise is also amplified 20dB when a signal is amplified 20dB?

Figure: How the signal looks

It's wrong. Let's consider the definition of noise figure NF.

NF is a ratio of input S/N to output S/N.
NF(dB)=20log[(Si/Ni)/(So/No)]

<Meaning of NF>

  1. A ratio of input S/N to output S/N.
  2. Output S/N is reduced only NF(dB) from input S/N.
  3. Output noise No = Ni + G + NF ...... G,NF and Ni should be dB.

Let's explain concretely.

With the above conditions, the output will be as sh own in the figure below because of <Meaning of NF>3mentioned above.

Figure: The meaning of NF $$ \begin{cases} \text{Input}~S/N=-100-(-150)=50dB\\ \text{Output}~S/N=-80-(-126.5)=46.5dB \end{cases} $$ $$ \begin{align} ∴NF&=(\text{Input}~S/N~50dB)-(\text^{Output}~S/N~46.5dB)\\ &=3.5dB ← \text{Indeed, <Meaning of NF>2 is established.} \end{align} $$

Output S/N is reduced only NF(dB) from input S/N.This's an important point.

Then, the signal and noise at input and output shall be observed with a spectrum analyzer.
The noise level of a spectrum analyzer assumes -110dBm.

When observed with a spectrum analyzer, the appearance of spectrum is related greatly to that noise level.

Case Study:

To verify the effect of the pre-amplifier, following connection of the signal generator, the pre-amplifier MAP301/302 and signal analyzer MSA538 is established. We made the measurement of the signal to noise ration and gain for each signal path. The setting of equipments is as follows.

Relation between noise level and noise figure NF

The measurement results are shown in bellow.

Connection Gain [dB] S/N Ratio [dB]
No Amplifier 0 10
MAP301 52 27
MAP302 21 21

No Amplifier

No Amplifier

MAP301

MAP301

MAP302

MAP302

It is confirmed that we can improve the S/N ratio by using appropriate pre-amplifier.

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