There are several sources of noises in electronic systems. Noises are unwanted signal polluting random or not signals that reduces overall signal quality.

Thermal Johnson/Nyquist noise

This noise is actually discovered by two scientists Johnson who did the experiments and Nyquist who developed the formula. Thermal noise present at all frequencies (has a constant power density at all spectra) and is so called white noise. This noise can only be reduces by reducing temperature, resistance and bandwidth. Lets see at the Noise voltage RMSformula:

V_RMS=(4·k·T·R·Δf)^1/2

where k – Boltzmanns constant (1.38·10^-23J/K);

T – Absolute temperature (K);

R – Resistance (Ω);

Δf – Bandwidth (Hz).

For example if we have 10kΩ resistor at 300K. Bandwidth 10kHz then we get V_RMS=1.3μV.

Shot noise

This type of noise occur because of random charges moving across potential barrier.

I²_n= 2·e·I·Δf

where e- charge of electron (1.6·10^-19C);

I – DC signal current (A)’

I_n – Noise current (A);

Δf – Bandwidth (Hz).

Usually Shot noise is more due to high frequency component. So reducing bandwidth may reduce high frequency noise.

Flicker Noise

This noise increase due to decreasing frequency. So measuring sensitive measurements ir is better use DC. Flicker noise origins isn’t known and not well understood. Usually it is not important compared to noises over 1 kHz.

Environmental noises

This type of noise comes form outside sources. This type of noise is often called interference. Interference may be mechanical or electrical. Electromagnetic interference (EMI) is most common that arrise from:

• Radiation from switching devices;

• Radiation from AC power lines, rectifiers;

• Lightnings;

• Transmitter RF noises(radios, cell phones);

• Electronic devices working at high frequencies;

• Electrostatic discharges.

Reduction of environmental noises should start form noise sources. If it’s possible the best practice is to remove it. It is not always possible to do so, then second step is to divert noise signal to ground using filters. If we talk about measurements then it is important to remember that the major noises contribute at first stage of amplification. So preamplifiers should be differential and with good CMRR and located closer to transducer. Cables between amplifier and further stages should be shielded. Shields should be grounded to common pint to eliminate ground loops.

Signal to noise ratio

When we talking about noises there is a need to measure it. It is common to measure a relative magnitude of the noise given by signal to noise ratio SNR (dB). Signal to noise ratio (SNR) is the ration of signal power over the noise power:

SNR=Ps/Pn

So the larger SNR is the better.

If expressed in decibels:

SNR_dB=10·log₁₀(Ps/Pn)=20·log₁₀(Vs/Vn),

Vs and Vn are RMS values used. Noises are always present in signals. In the circuits with amplifiers new noises can be added by amplifier itself. This is called SNR degradation. This degradation from amplifier input to output is called Noise Figure (NF). NF less than 3dB is considered good:

NF=SNR_I/SNR_O

SNR_I – signal in the amplifier input;

SNR_O – signal in the amplifier output.

Signal degradation that increases NF of device raises because of thermal noise and flicker noise (< 1kHz) because of semiconductor resistance and randomness of the diffusion processes in PN junction.

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This entry was posted on Wednesday, March 21st, 2007 at 6:21 pm and is filed under Electronics Tutorial. You can follow any responses to this entry through the RSS 2.0 feed. You can leave a response, or trackback from your own site.