Hints, Tips and Solutions - January 2003

Q. How can I get an accurate FFT spectral plot in SmartSpice?

A. The transient simulation uses an adaptive time-stepping algorithm depending on the change in the circuit waveforms (dV/dT). This means the simulated waveform consists of uneven time steps (more time points are required round a rapidly changing voltage to define the detail). If the normal .FFT command is used, these varying time points need to be interpolated to a regular set to allow the FFT plot to be created.

The act of interpolation leads to a distortion of the true waveform which gives an artificially high noise level (~ -60dB). SmartSpice includes an option “fft_accurate” which allows the regular time point to evaluated back on the true waveform and therefore reduce the distortion component. The output waveform now shows a noise level of about –280dB which is below present standard measurement accuracy levels.

Q. How do I simulate across process corners in one simulation deck?

A. The foundry often provides a process model library where parameters are sectioned in terms of their change to variations in say the definition of the N & P type devices in the CMOS process. This is the typical SS, SF, FS, FF & TT that people refer to when showing the process spread in electrical performance. For each of these variations there should be an entry point in the model library and then you can include the following syntax flow in the main deck:

.LIB mod1.lib .ALTER .DEL LIB mod1.lib .LIB mod2.lib

Q. I can create a single voltage waveform source but can I create a single modulated input voltage source?

A. Yes you have the capability in SmartSpice to create a complex waveform in PWL file (piecewise linear) but for a modulated waveform this would be to complex a file for all the data points required. We have the capability to do 2 forms of modulation, Amplitude and Frequency for a single element.

Frequency Modulation:

.param fm =200K
V1 1 0 sffm(1 1 2k 10 fm )
.tran .05U 50m ;sweep fm 5k 20k 5k

Figure 1. Frequency modulated source - single element.

Amplitude Modulation:

.param fr =20K
V1 1 0 am(1 1 1k fr 0)
.tran .05U 50m ;sweep fr 5k 20k 5k

Figure 2. Amplitude modulated source - single element.

 

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