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Why do oscilloscopes have bandwidth limits?
bdunham7:
--- Quote from: Anding on November 02, 2022, 04:48:36 am --- yes but why should it be the case that an oscilloscope is a low pass filter?
--- End quote ---
Because everything is a ultimately a low-pass filter? The analog front end (input, amplifier, scaling) will have limitations that depend on what you are willing to spend on them.
As BW goes up, it becomes increasingly expensive to make these analog components linear and coherent, especially in the case of DC-to-BW limit designs. Not only do you get loss of amplitude, you get group delay and and other distortions that make it pointless to continue beyond a certain point. Your inexpensive 8GSa/s 350MHz scope (gee, I wonder which one that is? :) ) doesn't even have 50R inputs, so it is going to be very difficult to get a 1GHz signal cleanly to its inputs, let alone scaling and amplifying it within the scope. For the digital signals you want to look at, just one probe that could connect those signals to an appropriate input will cost more than your entire scope.
TimFox:
Even if we have a good noise figure in the amplifier, the fundamental thermal noise from a 50 ohm resistor is 0.9 nV/Hz1/2.
If you had an infinite (or much too high for the application) analog bandwidth, the amplified thermal noise added to your MHz-bandwidth signal would be huge.
This is why many analog oscilloscopes have a front-panel switch to limit the bandwidth to much less than the full bandwidth: you can see easily the reduction in trace width (at sensitive V/div settings) with the lower bandwidth.
Circlotron:
--- Quote from: Fungus on November 02, 2022, 09:28:48 am ---Why can't my car go faster than sound?
--- End quote ---
Tractive effort goes up with the square of the speed.
Power required goes up with the cube of the speed.
So if your boxy, mid sized 1970s car needs 15 horsepower to travel at 100kph then it will need 28,255 horsepower to travel at 1235kph.
Oscilloscope cost vs performance goes up similarly.
SL4P:
Read about dV/dT
AG6QR:
Series inductance, parallel capacitance, and resistance turns everything into a low pass filter. Any wires will have at least a bit of series inductance, parallel capacitance, and resistance.
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