Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Question about thermal noise.

Status
Not open for further replies.

Alles Gute

Full Member level 2
Joined
Dec 4, 2003
Messages
135
Helped
15
Reputation
30
Reaction score
10
Trophy points
1,298
Activity points
1,217
bw*1.57 noise

When calculating the thermal noise of a resistor together with capacitor:
The noise density within the bandwidth of the RC filter is proportional to R, but that the bandwidth is inversely proportional to R, so that the total noise is KT/C independent of R.

But, a capacitor always has parasitic resistance, although it may be pretty small. Does that mean the thermal noise of a capacitor is always KT/C?
 

pixel

Advanced Member level 2
Joined
Sep 16, 2004
Messages
511
Helped
69
Reputation
138
Reaction score
16
Trophy points
1,298
Activity points
3,992
I am not sure that I have understood but:

Noise density er=√(4kTR) [V/√Hz]
Noise Equivalent BW NEB= (1.57*1/2πRC)

Output rms noise:
er*NEB ~1/√R
 

claudiocamera

Full Member level 4
Joined
Aug 19, 2005
Messages
225
Helped
27
Reputation
54
Reaction score
6
Trophy points
1,298
Location
Salvador-BA-Brazil
Activity points
4,282
The question must be improved in order to we have a better understanding, anyway I will try to guess the answer.

Termal noise is due only to resistive components, usually when studying RLC circuits we consider capacitors and inductors as ideal components, the Noise Power spectrum density due to a Resistor is Sn(w) = 2KTR, when the resitor is associated with a capacitor in paralell for instance the NPSD is in the output Sno(w) = Sn(w) * | H(w)|^2 , Where H(w) = 1/( jwRC + 1), It gives Sno(w) = 2KTR/(1+w^2*C^2*R^2) integrating it from - infinite to + infinite we found KT/C just because we have a R cancelation in the integral . It has nothing to do with the capacitor, Therefore there is not thermal noise in capacitor, since in the deduction the capacitor is ideal.

This results has a similar when you consider the available noise power density in a RLC circuit the results is KT/2 no matter resistors, capacitors and inductor values in the circuit.
 

jiaming_lee

Junior Member level 2
Joined
Jan 24, 2006
Messages
20
Helped
2
Reputation
4
Reaction score
0
Trophy points
1,281
Activity points
1,482
Alles Gute said:
When calculating the thermal noise of a resistor together with capacitor:
The noise density within the bandwidth of the RC filter is proportional to R, but that the bandwidth is inversely proportional to R, so that the total noise is KT/C independent of R.

But, a capacitor always has parasitic resistance, although it may be pretty small. Does that mean the thermal noise of a capacitor is always KT/C?

The parasitic resistance can be included, but it still does not influence the total noise KT/C. it only influence the very high frequency noise spectre
 

Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Top