Re: [ng-spice] Noise analysis
Hello Robert,
I have been using noise analysis before but I never checked it in the way
that You described. Please wait to Monday, I will try to find out what is
going on.
Regards
Michael
Robert Penny wrote:
> I couldn't find any mention of this in the ng-spice archives... my
> apologies if I've overlooked something on this topic.
>
> ******************************************************************
>
> Either I'm missing the point or there is an error in the way noise is
> being calculated in ng-spice rework 13. (I have no idea if this problem
> exists in previous versions).
>
> If I calculate noise using a command of the form
>
> noise v(outamp) vsig dec 10 1khz 2meg 1
>
> my understanding is that the total noise at node "outamp" is calculated as
> caused by the sum of the contribution of all noise generators within the
> circuit. To calculate this sum the transfer function from each noise
> generator to the "outamp" node is calculated and the contribution from
> each noise generator is scaled by this amount and added to form the sum.
>
> This seems to work in a reasonable way for all the output noises. I can
> also pull out the individual contributions to the noise from the noise
> generator output summaries, sum them, and they add up to the total noise.
>
> The noise command, however, also calculates the noise as referred to a
> reference input voltage generator, in this case vsig. Ng-spice calculates
> both the total noise referred to the input "inoise_total", and summaries
> of the various contributions of noise generators within the circuit as
> referred to the input.
>
> Surely though the ratio of each individual noise generator's contribution
> referred to the output to this noise generator's contribution as referred
> to the input should just be the transfer function of vsig to v(outamp).
> (Actually the square of the transfer function because we are working in
> units of V^2 or A^2).
>
> If I take the ratio of individual contributions, some equal the expected
> transfer function and some differ by a large factor. What's more, some of
> the individual inoise_total_(generator) contributions are larger than the
> sum total inoise_total noise referred to the input.
>
> My previous experience has been with TopSPICE, which I recall worked in
> the way I'm describing. Ng-spice does not seem to be working in the
> expected way. Am I seeing a bug or is the noise referred to the input
> different from what I'm describing.
>
> Many thanks for any help,
>
> -Robert Penny (rob@ucsd.edu)
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