噪音系数测量

1RF & Microwave e-Academy Program

Powerful tools that keep you on top of your game

Technical data is subject to change. Copyright@2004 AgilentTechnologies

Printed on Jan, 2004 5988-8495ENARFMW 202: Noise Figure Basics

2RFMW 202: Noise Figure Basics

Welcome to RFMW 202, the module on the basics of noise figure. This module will take you about 60 minutes for you to

complete.

If you have not already done so, we recommend that you study themodules RFMW 101 and MEAS 102 before this one.

3Fundamental Noise Concepts

Fundamental

noise

conceptsHow do we

make

measurements?What DUTs

can we

measure?What influences

the measurement

uncertainty?

In this module we will first look at the concepts of noise (why is it important), then on to how to make measurements and we will

conclude with some detailed information on measurement uncertainty and tools.

Let’s now go straight into concepts of noise.

4What is Noise Figure?

Imperfect

AmplifierSignal larger

But Noisier

Agitation of Electrons adds

noise to the signalSmall

Signal

Noise is undesired information that gets added to any signal –this has the effect of distorting or obscuring our signal. All

devices add noise. Let’s consider the example of an amplifier.

In this example, a perfect amplifier would add no noise, and the signal would be an amplified replica. However, in practice,

noise is present, and can mask the wanted signal. The noise floor, as seen in a given bandwidth, limits the detection of weak

signals.

All electronic systems are subject to noise. Receiver systems have to process very weak signals and any noise added by the

system will obscure these weak signals.

5DUTEMC Noise

Power supply Noise

Phase Noise

DUT NoiseV+

We will

derive a

figure of

merit for thisCauses of Noise

Noise comes from a variety of sources. It can be picked up from the emissions of nearby electrical equipment, or from the

phase noise of downconverting synthesizers. Noise can even come from the power supplies of active components in the

receiver. In this presentation we will NOT be considering these types of noise although they are very important to understand

and control.

Instead, we will concentrate the type of noise caused by ordinary phenomena in active electrical circuitry caused by random

fluctuations in charge carriers caused by thermal, shot and flicker noise.

We will define a figure of merit called Noise figure which a unique way of characterizing systems and also the components

within systems. When you know the noise figure of the system, you can easily calculate the system sensitivity from the system

bandwidth.

6Noise Contributors

Thermal Noise:(otherwise known as Johnson noise) is the kinetic

energy of a body of particles as a result of its finite temperature

P

therm=kTB

Shot Noise:caused by the quantized and random nature of

current flow

Flicker Noise:(or 1/f noise) is a low frequency phenomenon where

the noise power follows a 1/fαcharacteristic

Thermal noise is a function of the kinetic energy of a body of particles. The noise power available is equal to kTB and is the

maximum rate at which energy can be removed from the body. Boltzmann’s constant is defined as the average energy per

particle that can be coupled out by electrical means per degree of temperature.

The power is related to temperature and that makes intuitive sense. Thermal noise is evenly distributed across the frequency

spectrum (1% variation up to 100GHz) and therefore B specifies how much of the spectrum power is available.

Shot noise occurs in active devices and is caused by the randomness of current flow. Shot noise is flat with frequency and a

function of the current level. Flicker noise is a function of frequency and is a low frequency phenomenon. The value of alpha is

close to unity.

7Noise Power at Standard Temperature

R -j X

R+jXLL

Available Noise Power,

P

av= kTB

At 290K P

av= 4 x 10 -21W/Hz = -174dBm / Hzk = 1.38 x 10-23joule / k

T = Temperature (K)

B = Bandwidth (Hz)

In deep space kT = -198dBm/Hz

Here is a schematic representation of a noise source. Noise follows the normal power transfer laws. kTB is termed the

available noise power. A conjugate match is needed for an optimum noise power transfer from the source to the load. This

gives us the figure of -174dBm / Hz as the universal noise floor at standard temperature.

Note that defining noise threshold at -174dB/Hz in space applications is not applicable because 290K isnot the ambient

temperature in deep space! In deep space the ambient temperatureis around 4K and satellite earth station receivers the

temperature 30K

Noise power is not a function of the size (or resistance) of thebody. Imagine if you connected a large body to a smaller one. If