eldo_user

Setting Up An Analysis
Pole Zero Analysis
will focus our attention, while all the parasitic and less meaningful roots will also be detected.
For these reasons, the following reduction mechanisms are available:
• “Frequency Limit” on page 234
• “Pole-Zero Cancellation by Threshold” on page 234
• “Hand Selection” on page 235
• “Select Index” on page 235
Frequency Limit
Eldo offers the option to limit the analysis up to a certain frequency, as shown in the following
example:
Do you want to set an upper frequency limit for the selected poles and
zeros? [yes/no]:
y
So give the highest frequency to be considered:
5e7
Roots up to 0.5000E+08 Hz will be examined.
If the roots of the circuit are spanned up to a frequency significantly higher than the upper
frequency of the AC analysis, this limit can be very useful in simplifying textual output and
FNS models without meaningful loss of accuracy.
Pole-Zero Cancellation by Threshold
Very close poles and zeros can be deleted, as their influence on the circuit behavior
compensates. Big circuits with a lot of roots often have closely spaced poles and zeros, which
can be deleted to show the most meaningful remaining roots. A reminder about the threshold
mechanism shows the condition to delete a pole, P, and a zero, Z. The condition is that:
abs(RE[P]-RE[Z]) < abs(RE[P]) * TH + TH
and the same for the imaginary part.
Therefore, if a threshold of, for example, 0.1 is given, there are two possibilities:
• For very low frequency roots, for example, fractions of 1 Hz, TH has the meaning of
maximum difference between the real parts of the pole and the zero, and the same for the
imaginary part.
• In most cases, there are roots at quite high frequencies so the first term on the right side
of the relation dominates and TH takes the meaning of maximum ratio between the
distance amongst the roots and their value, so in this case a 10% tolerance.
At this point, a table of remaining poles and zeros (after the two elimination steps) is displayed.
234
Eldo® User's Manual, 15.3