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This is an extension of transient analysis rather than a separate analysis mode. When activated, real time noise sources are added to all noisy devices with a magnitude and frequency distribution calculated using the same equations used for small signal analysis. This allows noise analysis to be performed on sampled data systems and oscillators.
To use real time noise analysis, the following parameters may be added to the .TRAN analysis line.
| RTNstep | Source step size in seconds. This will need to be small enough to cover the frequency range of interest. The noise magnitude starts rolling off at about 1/3*stepsize. Default=0 i.e. real time noise analysis disabled. |
| RTNstart | Optional. Time after analysis start at which the noise sources will be enabled. Default = zero |
| RTNstop | Optional. Time after analysis start at which the noise sources will be disabled. Default = stop time. |
| .TRAN 0 1m RTNstep=1u RTNstart=500u |
Analysis time 1m, RTN step size 1u, real time sources start at 500u. The step size parameter - i.e. the first parameter on the .TRAN line - must be supplied if real time noise parameters are to be included. This is only to comply with the syntax rules not because the step size is needed for any other purpose. In most cases, just set it to zero as in the above example.
To test real time noise and verify it's accuracy we ran a test on a number of circuits which compare AC noise with real time noise. The procedure was to run real time noise analysis 50 times then plot the averaged Fourier spectrum. This test was repeated for different transient run times and step sizes to build a noise spectrum over several decades. The graph below is the result of one such test. This was carried out on the BSIM3 buffer circuit provided in one of the examples except that a value for AF - the flicker noise parameter - was added to the models. As can be seen in the graph below the real time noise results strongly follow the AC noise results.
Similar tests were performed on circuits containing each of the major noise generating devices including diodes, BJTs, JFETs, resistors (including its flicker noise parameter) and also the NXP MOS9 and MEXTRAM devices. All showed results similar to below with a close similarity between AC noise and real time noise.
These tests were performed using a simple script. This script is called rtntest.sxscr and can be found on the installation CDROM at SCRIPTS/EXAMPLES. This can also be found at our web site. Please refer to Further Documentation for details.
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