What is Noise Figure?
Small Signal
Imperfect Amplifier Agitation of Electrons adds noise to the signal Signal larger But Noisier
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 ise concepts
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
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.
How do we make measurements?
What DUTs can we measure?
What influences the measurement uncertainty?
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Fundamental noise concepts
Fundamental Noise Concepts
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Fundamental noise concepts
Causes of Noise
EMC Noise Power supply Noise Phase Noise DUT Noise
V+
DUT
We will derive a figure of merit for this
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.
Here is an example of an amplifier connected to an antenna. Let us assume that the antenna and amplifier are perfectly matched. Let's also assume that the measurement bandwidth is 25MHz - so add 74 to -174dBm. The noise at the input of the amplifier will be kTB which in log terms is -100dBm. The signal being picked up the input is -60dBm. The carrier to noise ratio at the input is 40dB. If the amplifier was perfect it would amplify the gain and noise by equal amounts and maintain the same C/N at the output. In reality the amplifier will add some gain of its own. It this example the gain of the amplifier is 20dB so the signal has risen from -60dBm to -40dBm. The noise however has risen by 30dB rather than 20dB. The C/N has dropped to 30dB because the amplifier has added 10dB noise of its own. Friis in 1944 defined noise figure as the ratio of signal to noise at the input to signal to noise at the output. I.e. 40dB minus 30dB. We can say the noise figure is 10dB.
5
Fundamental noise concepts
Noise Contributors
Thermal Noise: (otherwise known as Johnson noise) is the kinetic energy of a body of particles as a result of its finite temperature Ptherm=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/fα characteristic
Fundamental noise concepts
How do we make measurements?
What DUTs can we measure?
What influences the measurement uncertainty?
3
Fundamental noise concepts
RFMW 202: Noise Figure Basics
Technical data is subject to change Copyright@2003 Agilent Technologies Printed on Dec. 4, 2002 5988-8495ENA