Hello everyone How can I simulate Noise figure and OIp3 with open-source-silicon.dev #analog-design

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Hello everyone. How can I simulate Noise figure an...

# analog-design

wisla morais

07/01/2022, 6:41 PM

Hello everyone. How can I simulate Noise figure and OIp3 with NgSpice?

Tim Edwards

07/02/2022, 2:20 AM

At least for noise figure, I use something like this:

Copy code

noise V(VBGP) VVDDA oct 10 0.1 10 1
let vn = sqrt(onoise_total)

The arguments for

noise

you can look up in the ngspice manual. The important thing is to understand that running the

noise

analysis creates this (non-obvious) value

onoise_total

, that you need to take the square root of it, and the final value is then in volts RMS.

👀 1

Harald Pretl

07/02/2022, 12:24 PM

I am not sure what the more obvious value is in terms of noise (as it is a stochastic signal) 🙂 V/sqrt(Hz) seems strange, too Anyway, the behavior can be changed with

set sqrnoise

unset sqrnoise

according the ngspice manual.

Harald Pretl

07/02/2022, 12:25 PM

For calculating NF or other noise simulations the spectral view is usually more helpful than the integrated value.

Harald Pretl

07/02/2022, 12:27 PM

For OIP3 take two sinusoidal input signals, run a transient long enough to contain one beat frequency-time plus settling (so ca. 1.5/f_beat), then take an FFT on the output signal of interest and calculate the OPI3 from there. Make sure that the OIM3 products stick out of the FFT noise floor sufficiently.

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Rana Muhammad Shahid Jamil

07/02/2022, 7:27 PM

I once used something like this to simulate noise figure for a LNA. It uses the inoise (input referred noise) variable. The details can be found in the noise analysis section of ngspice manual.

wisla morais

07/04/2022, 2:06 PM

Thank you for the answers.

wisla morais

07/04/2022, 2:08 PM

@Rana Muhammad Shahid Jamil what would that 8.3e-19 value be?

Harald Pretl

07/04/2022, 2:12 PM

That would be the noise of the source at the input: u_in^2/df = 4 k T R with T=300K and R=50Ohm, k=Bomtzmann constant

👍 1

wisla morais

07/04/2022, 2:18 PM

So, this value is considering the 50 ohm source resistance?

wisla morais

07/04/2022, 2:33 PM

I think I understood @Harald Pretl. Thank you so much!

Harald Pretl

07/04/2022, 2:38 PM

The noise figure is NF = 10 log (total_noise / noise_from_input). You can calculate this either input- or output-referred. Most fundamental definition is NF = SNR_in / SNR_out; you can derive the rest from this.

🙌🏼 1

wisla morais

07/04/2022, 2:47 PM

I understood. Thanks for clarify with your detailed explanation.

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