Electrical "noise" covers a range of natural and artificial electrical changes or disturbances from analogue equipment, such as amplifiers. The goal for engineers is to reduce this excessive, unwanted noise to its lowest possible point for any given circuit. Knowing how to do this means knowing how much additional, unnecessary noise there is at the present time. When the current noise level is depleted as close to zero as possible, you will have a circuit that can process electronic signals at optimal efficiency for your given set-up. Every circuit has such a "noise figure": it defines the lowest limit of a reliable signal.
- Skill level:
Things you need
- UUT e.g. linear amplifier
- Noise source
- Noise figure meter
Specify the bandwidth over which you will measure unwanted, excess noise. Set it to a measurable range you are comfortable with. Ignore all other frequencies during the calculation to make the process clearer and more straightforward. A reasonable frequency range to measure is 1 Hertz.
Set up or prepare your existing circuit using a linear amplifier. Measure the amplifier's gain over the bandwidth of interest using the figure meter. Measure the signal at the starting point and then the finishing point of the amplifier and calculate the gain, which is the output signal minus the input signal. Record the value.
Introduce a noise source such as thermal noise, a temperature-dependent "white" noise --- also called "Johnson Noise" --- that is produced from unpredictable fluctuations in voltage or current, into your unit under test (UUT), such as the amplifier. Ensure the noise source produces a signal in the required range of frequencies, or you will not be able to detect it.
Calculate the amplifier's noise in volts from its gain value using a calculator and the equation: voltage is equal to the square root of four times the Boltzmann's constant --- 1.374 multiplied by 10 to the minus 23 Joules per Kelvin --- multiplied by the temperature of the set-up, the resistance of the amplifier in ohms and the bandwidth you selected.
Use a noise figure meter to measure random noise within the circuit at the frequency you defined for yourself. This noise figure is the total power output minus the known output from the noise source. The calculated figure represents the amount of unwanted noise being generated internally through the circuit as a direct result of the amplifier. Noise power is measured in decibel minutes.
Tips and warnings
- Take care with units and powers during the calculations. Room temperature --- temperature of the circuit set-up --- measures about 290 degrees Kelvin, so use this as a basis for the measurement.
- Make sure voltages are safely insulated and earthed before switching circuits on. Use a transformer to step down any high voltages, such as 120-volt household power supplies. Electricity is dangerous when used improperly.
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