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AC Noise : NF from output noise values

AncisMichele

Hello again Stuck out tongue winking eye

There is another thing I would like to understand about how the simulator works in this instance, but I'll keep it separated from the "buggy behaviour" I just wrote about.

From the Manuals, this is the relationship between NF calculation and other AC noise outputs:

(a little unfortunate typesetting here)

So the NF or its linear cousin F, rightfully, do not include the noise due to the load of the DUT, just that of the DUT itself and of the input source.

On the other hand, AC noise calculates the total noise at the output. Moreover, it does not give access to the NI component in the manual's screenshot, i.e. the output noise due to the load.

Since I absolutely want to be able to calculate "NF-like" quantities, but not always having voltage at the output, and I don't want to always depend on the NF calculation, I set out to find an "alternative" way to double-check the NF results.

To this end, I just then thought, in the first instance, to load my DUT with a noiseless resistor, so that the NI part above is automatically zero.

Given that I don't use the "noise separation" option, the noise summary shows (more or less) what I would expect: no trace of Rl:

Now, I have to say that "ext_file_noise" contributor still bothers me, but at least there's no other contributor beyond the input port and the transistor itself.

According to my understanding I should be able now to plot NF and to superimpose a curve to it, by just calculating textbook-like:

output_noise/(input_noise*gain²)

Because - following the manual - the gain is voltage and referred to the internal source of the port:

then the input noise of the source is simply 4*K*T*R (in voltage²) and the gain is just the gain coming out of the noise simulation.

However, when I do all this, I get a curve with a constant difference, and I don't know where this difference is coming from:

This difference is constant.

These are the definitions used for my version of NF:

Can anybody suggest what I might be missing here?

Thanks,

Michele

Parents
  • Andrew Beckett

    Michele,

    First of all, please take into account my response on your other thread - most likely you should just switch to ISR22 or later to ensure the noise summary is giving the right answer.

    If you look (in the results browser) you would be able to see the output referred noise contribution of the input port, and the total output noise, plus the output referred noise from the load (depending on whether you've made it noiseless or not). 

    The main likely reason for the discrepancy is that the noise in the input source is not governed by the simulation temperature, but by the noisetemp parameter on the port, which defaults to 290K (but specified in Celsius). This is a standard way to measure noise figure, given that the input probe is off chip and so not governed by the temperature of the die.

    See my article: Worked example of noise calculations in simple circuit

    Andrew

Reply
  • Andrew Beckett

    Michele,

    First of all, please take into account my response on your other thread - most likely you should just switch to ISR22 or later to ensure the noise summary is giving the right answer.

    If you look (in the results browser) you would be able to see the output referred noise contribution of the input port, and the total output noise, plus the output referred noise from the load (depending on whether you've made it noiseless or not). 

    The main likely reason for the discrepancy is that the noise in the input source is not governed by the simulation temperature, but by the noisetemp parameter on the port, which defaults to 290K (but specified in Celsius). This is a standard way to measure noise figure, given that the input probe is off chip and so not governed by the temperature of the die.

    See my article: Worked example of noise calculations in simple circuit

    Andrew

Children
  • AncisMichele
    in reply to Andrew Beckett

    Thanks Andrew!

    Yeah, first thing is to get basic support in place. I'm working on it, it should be up and running quickly.

    Thanks for the tip, I did not notice that separate noise contributions were available from the browser:

    So it points to some hiccup in the Noise Summary output - However, looking at the browser curves:

    - the "ext_file_noise" is ZERO (unlike in the Noise Summary)

    - the rn component is in V²/Hz

    - the total is in A²/Hz

    Why is that? The noise simulation is defined to output the noise voltage...

  • Andrew Beckett
    in reply to AncisMichele

    It might be related to the handling of aliasTables too - it messed up various things in the reading of noise results - not sure since I can't reproduce your problem. It doesn't happen for me with my hacked version of your netlist. I would expect that the .src components are in whatever units makes sense for that kind of noise source, but the normal noise contributors are in the units of the output noise.

    Andrew

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