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10-09-2018, 02:44 PM
White noise is like white light, where all the frequencies are present are once. The noise you hear between radio stations when the tuner has not found a station is white noise and the amplitude of the noise is constant with frequency. This makes it a convenient signal to use to perform a frequency response test on a circuit if IF you have a spectrum analyser.
Spectrum analysers are a specialised oscilloscope with multiple frequency filters at the input. Typically, a single frequency is fed into an amplifier, say, but the amplifier adds distortion harmonics that were not present in the original signal. The analyser shows amplitude on the vertical axis (Y) and frequency on the horizontal axis (X). t each frequency, a vertical line is displayed whose amplitude corresponds to the amplitude of that frequency. In a typical display, the original signal is the "fundamental", or "first harmonic", and is used for the reference level; ideally it has the highest amplitude. The harmonics have much lower amplitudes, so their lines are not as tall as for the fundamental.
The vertical scale is often in deciBels, but can be in volts. The dB scale is more convenient if the amplifier is very good, exhibiting very low distortion, as the "linear appearance" of the logarithmic dB scale allows the low levels of the harmonics to be visible.Were the scale truly linear, the harmonics might be buried in the noise of the zero line across the bottom of the display. The displayed output is also called a "fast Fourier transform" (FFT), which is a mathematical derivation that would result in the same graphical output.
So, the spectrum analyser is great at showing harmonics of a single frequency input. if we feed white noise into the amplifier we see a displayed line that represents the frequency response of the amplifier. This is also called a "Bode" plot. Spectrum analysers are a bit expensive for most labs to have and unless they use it constantly it is difficult to justify the expense. Fortunately, modern DSOs (digital storage oscilloscopes) have a computer core and computers are good at doing any math the designer wishes to have it do. As a result, most DSOs will do FFTs - maybe not as well as a dedicated spectrum analyser, but well enough for most techs and hobbyists.
The only DSOs I found that specifically lists being able to display Bode plots are from Keysight (formerly Hewlett-Packard, HP - not the brown sauce from England... mmmm), EDU1002G - 50MHz, ~$900cdn and DSO1102G - 70/100MHz; ~$1200cdn. Both have abuilt-in 20MHz arbitrary wave generator (AWG) allowing the scope and generator to be perfectly synchronised to do a frequency sweep. Keysight isavailble through Mouser; HP sauce is availble at your local grocery store.
Spectrum analysers are a specialised oscilloscope with multiple frequency filters at the input. Typically, a single frequency is fed into an amplifier, say, but the amplifier adds distortion harmonics that were not present in the original signal. The analyser shows amplitude on the vertical axis (Y) and frequency on the horizontal axis (X). t each frequency, a vertical line is displayed whose amplitude corresponds to the amplitude of that frequency. In a typical display, the original signal is the "fundamental", or "first harmonic", and is used for the reference level; ideally it has the highest amplitude. The harmonics have much lower amplitudes, so their lines are not as tall as for the fundamental.
The vertical scale is often in deciBels, but can be in volts. The dB scale is more convenient if the amplifier is very good, exhibiting very low distortion, as the "linear appearance" of the logarithmic dB scale allows the low levels of the harmonics to be visible.Were the scale truly linear, the harmonics might be buried in the noise of the zero line across the bottom of the display. The displayed output is also called a "fast Fourier transform" (FFT), which is a mathematical derivation that would result in the same graphical output.
So, the spectrum analyser is great at showing harmonics of a single frequency input. if we feed white noise into the amplifier we see a displayed line that represents the frequency response of the amplifier. This is also called a "Bode" plot. Spectrum analysers are a bit expensive for most labs to have and unless they use it constantly it is difficult to justify the expense. Fortunately, modern DSOs (digital storage oscilloscopes) have a computer core and computers are good at doing any math the designer wishes to have it do. As a result, most DSOs will do FFTs - maybe not as well as a dedicated spectrum analyser, but well enough for most techs and hobbyists.
The only DSOs I found that specifically lists being able to display Bode plots are from Keysight (formerly Hewlett-Packard, HP - not the brown sauce from England... mmmm), EDU1002G - 50MHz, ~$900cdn and DSO1102G - 70/100MHz; ~$1200cdn. Both have abuilt-in 20MHz arbitrary wave generator (AWG) allowing the scope and generator to be perfectly synchronised to do a frequency sweep. Keysight isavailble through Mouser; HP sauce is availble at your local grocery store.
