V15 – How to Detect Precision Problems

Known Bugs


With release 8 and above, precision problems only occur at extremely low frequencies (in the KHz range) or at medium-low frequencies (in the MHz range) when using the memory save option (Analysis => Setup). With releases 6 and 7, this can also occur when you are not using memory save. As a rule of thumb, you should only get precision problems when the cell size is less than about 1e-4 wavelengths and memory save is turned on. When this occurs, the response data usually looks noisy. For example, it may look something like this:

images/precision_plot.gif

Notice that the data looks smooth at the higher frequencies (where there is no precision problem), but looks noisy at lower frequencies (less than 1 GHz).

If you are using an ABS sweep, then the analysis may require a lot more frequency points to resolve the noise in the data. The response might look something like this:

images/prec_abs_memsave_noprt.gif

You can also look at the current density. At 10 GHz the circuit shows nicely behaved current densities:

images/prec_density_plot_10.gif

Notice the higher current on the edges of the lines, and on the inside edges of the bends. When precision problems occur, the current density is usually spotted and makes no sense. For example, the current density of the same circuit looks like this at 100 MHz:

images/prec_density_plot_01.gif

If you are having trouble with precision, you need to turn off memory save.