Products > Test Equipment
Why do oscilloscopes have bandwidth limits?
ejeffrey:
--- Quote from: glarsson on November 03, 2022, 08:51:51 am ---
--- Quote from: TimFox on November 02, 2022, 03:55:37 pm ---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.
--- End quote ---
I don't have anything infinite at hand to test with, but wouldn't infinite bandwidth result in infinite thermal noise?
--- End quote ---
No the plank law cuts in above a few THz and the thermal noise drops off exponentially and has a finite integral out to infinite frequency.
CatalinaWOW:
--- Quote from: Anding on November 02, 2022, 10:05:13 am ---
--- Quote from: Fungus on November 02, 2022, 09:28:48 am ---Why can't my car go faster than sound?
--- End quote ---
No reason why it shouldn't but that would require an engine sufficiently powerful to accelerate the car against the restive forces of friction and air resistance which increase with velocity (reference Newton II: F=ma), and sufficient fuel onboard (mass which itself needs to be accelerated) to complete the acceleration. In the case of a car, it would require rolling gear that maintains it's mechanical integrity at the high RPM for travel above 330 m/s.
Can you put the equivalent physics for a 'scope in two sentences?
--- End quote ---
Sort of. You might add some car details such as suspension capable of maintaining stability with the bump frequency that will be encountered. So now your horsepower limit is representing the sampling rate, the running gear speed capacity is bandwidth and suspension limitations are rise time. These are not direct analogs, but perhaps an illustration that the performance of a device requires several attributes which may be loosely or tightly related. The car example is probably more relatable to a lot of people. Having a great excess of horsepower or sampling rate does not achieve all of the desired performance. (The reverse, however is true. An insufficiency of any one of the elements will prevent the desired performance.)
james_s:
--- Quote from: CatalinaWOW on November 04, 2022, 05:07:50 am ---Sort of. You might add some car details such as suspension capable of maintaining stability with the bump frequency that will be encountered. So now your horsepower limit is representing the sampling rate, the running gear speed capacity is bandwidth and suspension limitations are rise time. These are not direct analogs, but perhaps an illustration that the performance of a device requires several attributes which may be loosely or tightly related. The car example is probably more relatable to a lot of people. Having a great excess of horsepower or sampling rate does not achieve all of the desired performance. (The reverse, however is true. An insufficiency of any one of the elements will prevent the desired performance.)
--- End quote ---
One can look at the various land speed record cars to get an idea of the real world challenges. One of the issues you run into is making wheels that can spin as fast as they need to without the tires and even the rims themselves breaking up from the centrifugal force. Then you have to keep the car on the ground, not easy when it is more like an airplane than a car in the first place. Then as you actually approach the speed of sound you have the shockwave to deal with.
Fungus:
--- Quote from: james_s on November 04, 2022, 06:24:04 am ---One of the issues you run into is making wheels that can spin as fast as they need to without the tires and even the rims themselves breaking up from the centrifugal force.
--- End quote ---
That's the reason CD drives (remember those?) stopped getting faster at 56x.
David Hess:
--- Quote from: kcbrown on November 04, 2022, 01:20:55 am ---
--- Quote from: David Hess on November 04, 2022, 12:33:04 am ---Linear circuits have gain-bandwidth product limits. Bandwidth can be increases by sacrificing gain, but then more stages are required and this quickly reaches diminishing returns.
I say linear circuits because *sampling* oscilloscopes avoid this limitation by sampling before amplification, and the result is massive bandwidth determined only by sampling strobe width.
--- End quote ---
How does the A to D converter play into the bandwidth characteristics? Surely it has some effect, no?
--- End quote ---
With an exception which does not matter here, ADCs have the same limitation and contribute to the bandwidth like any other linear stage.
Starting on page 24, Circuit Concepts - Vertical Amplifier Circuits from Tektronix discusses how several cascaded single-pole responses result in close to a Gaussian frequency response.
A point made a couple pages later is that this response is deliberately chosen because it results in little to no overshoot.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version