Johnson and Shot noise the Frequency Term

In summary, the frequency term in Johnson and Shot noise formulae is the bandwidth of the detecting/analyzing system/circuit. This could also be the bandwidth of a desired signal since you must use the same bandwidth to detect the signal correctly.
  • #1
Master J
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I am a bit confused over the frequency term that appears in Johnson and Shot noise formulae. What is this term?

Is it bandwidth? Of what? In relation to say a photodiode, is it the range of frequencies being detected?

And how would this relate to say a DC circuit which no frequency terms at all?
 
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  • #2
Strictly it's the bandwidth of the detecting/analyzing system/circuit. This could also be the bandwidth of a desired signal since you must use the same bandwidth to detect the signal correctly.

There are cases when it's something a bit different when you have a internal device feedback loop that can see the full noise bandwidth even when the external system has a lower bandwidth - but for most purposes it's the above.

In terms of DC, DC does have frequency. In the words of a microwave engineer I knew who was presented with presentation about "DC testing": "Yes, but what frequency DC?" The guy presenting wasn't a microwave/RF guy so he didn't understand the question. You see for her, 30 MHz was DC. That's the cut-off of an HP 8510 network analyzer's "DC input" that she used. It's all relative.

And at some point in even a "DC" circuit you had to turn on the DC and then later turn off the DC so you minimally have a transfer AC component equal to 1/(time on). But even a DC source isn't perfectly constant - there is always some d/dt.

When you deal with low frequency noise, DC is also relative. Consider that 0.1 Hz is 10 seconds. So when you measure low frequency noise you have to sample (in this case) for say 100 seconds to get 5 point (Nyquist sampling). This is part of the "fun" of measuring noise. This "low frequency/long time" integration is especially common with 1/f noise measurement.
 

1. What is Johnson and Shot noise?

Johnson and Shot noise are two types of noise that can affect electronic systems. Johnson noise, also known as thermal noise, is caused by the random motion of electrons in a conductor. Shot noise, on the other hand, is caused by the discrete nature of electrical current, where electrons move in discrete packets or "shots".

2. What is the frequency term in Johnson and Shot noise?

The frequency term in Johnson and Shot noise refers to the fact that these noises are dependent on the frequency of the electrical signal. As the frequency increases, both Johnson and Shot noise also increase, making them more noticeable in high-frequency electronic systems.

3. How does Johnson and Shot noise affect electronic systems?

Johnson and Shot noise can affect electronic systems by adding unwanted fluctuations or variability in the electrical signal. This can cause errors or distortions in the signal, which can impact the performance of the system. In some cases, this noise can even limit the sensitivity or accuracy of the system.

4. Can Johnson and Shot noise be reduced or eliminated?

While it is not possible to completely eliminate Johnson and Shot noise, it can be reduced through various techniques such as using low-noise components, shielding, and signal filtering. Additionally, selecting the appropriate operating frequency and minimizing the temperature of the system can also help reduce the impact of these noises.

5. Are Johnson and Shot noise the only types of noise that can affect electronic systems?

No, Johnson and Shot noise are not the only types of noise that can affect electronic systems. Other types of noise, such as flicker noise and excess noise, can also impact the performance of electronic systems. It is important for scientists and engineers to understand and mitigate the effects of all types of noise in order to optimize the performance of electronic systems.

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