Because of the constant 1ohm, we can remove it from the equation all together. The -3dB point is very commonly used with filters of all types low pass, band pass, high pass It is just saying the filter cuts off half of the power at that frequency. The rate at which it drops off depends on the order of the system you are using. Higher order can get closer and closer to a "brick wall" filter. Well, many reasons.
All devices analog or digital have to do something with the signal. You can go as simple as a voltage follower up to something more complex like showing the signal on a screen or turning the signal into audio. All of the devices required to convert your signal into something that is usable have attributes about them that are frequency dependent. One simple example of this is an opamp and its GBWP.
So, on an O-scope they will add a low pass filter so that none of the internal devices are having to deal with frequencies above what they can handle. The point which the two lines meet, when compared to the real line gives us the number of -3db.
This point is called the cutoff frequency. So, lots of systems are designed to operate in normal conditions until they met the cutoff frequency when they lose at maximum 3db. If you operate with signal above that frequency the signal can be more attenuated. More info in Wikipedia about continuous low pass filters. The -3dB, come from 20 Log 0.
Kellenjb's answer is excellent, I just wanted to add a webpage that gave me a "Ohhh" moment when I was reading about this -3db thing. Maybe it helps to visualize.
I read a tutorial on Band Pass filters which includes a great image of a Bode Plot. You can see the key image below. It nicely illustrates how signal attenuation varies depending on the frequencies.
We see there is no phase shift at the center frequency, so we have complete signal transmission. However as we go out of the Pass Band, we get to a point where the Band Pass Filter shifts the signal to lag or lead the central frequency by 45 degrees, and we see our point of -3dB.
The internal of the oscilloscope has an amplifier limitation. They called it dynamic range. If you use your scope pass the limitation, your reading will no longer be accurate. The linear amplifier will start to become nonlinear.
If you look at any block design of the oscilloscope, you will notice the input amplifier or preamplifier. You will not see filter block in front of it. The input signal is too small before it can be process by a filter.
After you amplified the signal then you can use a filter. So the limitation is the preamplifier not a filter. When the o-scope give you a spec of Mhz, 3dB. You can be sure it's referring to the preamplifier.
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Create a free Team What is Teams? Learn more. What is the significance of -3dB? Ask Question. Asked 10 years, 11 months ago. Active 4 years, 9 months ago. Viewed k times. Thomas O Thomas O Add a comment. Active Oldest Votes. So originally we had 36 W, now we have 18 W which of course is half of 36 W. Application of -3dB in Filters The -3dB point is very commonly used with filters of all types low pass, band pass, high pass Why filter the input to an Oscope?
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