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Design >> RF Design >> Noise measurement/extraction of simple RC circuit
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Message started by VINAY RAO on Dec 9^th, 2012, 11:29pm

Title: Noise measurement/extraction of simple RC circuit
Post by VINAY RAO on Dec 9^th, 2012, 11:29pm

I have simulated simple RC circuit to calculate NOISE CURRENT in HSPICE. In one of 2 cases, RESULTS are not matching with my hand calculated values. Circuit is simulated with 2 cases (figure 1 and 3) which are shown in the pdf which is attached to this post. Hand calculations along with spice code are shown in the same PDF.

Case 1 (Figure 1 in the PDF): When VOLTAGE source is connected at the INPUT:
In this case answers are very well matching. First with the statement “.NOISE V(3) V1 5” I could calculate NOISE VOLTAGE DESNISTY which resulted in 4.57pV/rt(Hz) which is well matched with my calculated values.
For calculating Noise CURRENT DESNSITY, I replaced earlier statement by the following one i.e. is “.NOISE I(C1) V1 5” where only V(3) is replaced by I(C1). With this even I am getting the expected/calculated CURRENT DESNSITY (through CL) that is 574f A/rt(Hz) .

Case 2 (Figure 3 in the PDF): When CURRENT source is connected at the INPUT:
In this case, I am using the statement “.NOISE V(3) I1 5” and “.NOISE I(C1) I1 5” for calculating NOISE VOLTAGE DENSITY and Noise CURRENT DESNSITY respectively. Here both statements yielding a different results i.e. are 3.22pV/rt(Hz) and 405.571fA/rt(Hz) compared to the expected/calculated results of 4.57pV/rt(Hz) and 574fA/rt(Hz) respectively for VOLTAGE and CURRENT density.
Question is “WHY NOISE HAS TO DEPEND ON THE TYPE OF SOURCES CONNECTED AT THE INPUT” ? Actually it must not depends on the source because both cases consists of same resistors. Or Am I doing anything wrong in this noise analysis?

Note : This circuit is taken from the website: http://www.ecircuitcenter.com/Circuits/Noise/Noise_Analysis/res_noise.htm

Title: Re: Noise measurement/extraction of simple RC circuit
Post by VINAY RAO on Dec 9^th, 2012, 11:36pm

Here i am posting the PDF which includes figure and calculation.

Title: Re: Noise measurement/extraction of simple RC circuit
Post by raja.cedt on Dec 9^th, 2012, 11:58pm

hello i didn't understand what you are doing. I suggest it is better to find Rth, and simply use 4KTR otherwise find current of R1 and current noise R2, instead of that you have added voltage noise.

You are not suppose to use current source as an input here, if so noise of R1 will not appear at the input.

Never connect a current source in-series with a resister and voltage source in parallel with an res.

Thanks,
Raj.

Title: Re: Noise measurement/extraction of simple RC circuit
Post by VINAY RAO on Dec 10^th, 2012, 4:21am

Thanks Raj. I was trying to measure output noise due to those resistors. I was thinking spice is taking input source (voltage or current) as dummy in noise calculation and gives same output noise for any type of input sources. You are right that R1 noise isn't appearing in output noise (answers matches if i omit R1 noise) due to current source at the input.

Regards,
Vinay Rao.

Title: Re: Noise measurement/extraction of simple RC circuit
Post by nrk1 on Dec 10^th, 2012, 6:04pm

The two circuits are different. With a current source, the noise from the upper 100k resistor is irrelevant.