Hi Helmut,
Thanks for taking a look at this, and for pointing out that LTspice
changes the rise and fall times if you do not specify the values. I
am still learning how LTspice works, and these unexpected variations
from other SPICE programs can be major pitfalls.
In this case, the amount of overshoot is very small, and I'm not
sure if the 1.6% is not caused by the way the component values are
rounded.
I use this filter program for general filter design. The component
values were calculated for the theoretical values and the inductors
were rounded to 2 significant digits. This reflects the difficulty
in obtaining precision inductors, particularly at VHF and UHF.
The capacitors were rounded to tenths of a pf. This is because the
program is used mostly for work at VHF and UHF, where the caps are
much smaller. It may well be that using the correct theoretical
values for the components may reduce or eliminate the small amount
of overshoot you measured.
However, there is still the practical matter of obtaining inductors
and capacitors with the needed precision, and any practical filter
will have imperfect components. I have found the Bessel and
Equiripple to be much more tolerant of errors in component values
than other filter types, and still give good performance.
If we are concerned about such small values of overshoot, we need to
repeat this using the correct theoretical values for the components.
Even so, an overshoot of 1.6% is not in the same category as the
overshoot from Butterworth or other non-linear phase filters, which
may have ten percent or more, depending on the sharpness of the
cutoff. In practise, the Bessel and Equiripple are considered linear
phase filters, and have negligible overshoot.
For example, a Butterworth or other nonlinear group delay filter may
cause excessive timing errors when used to filter digital data. This
can be minimized by changing to a Bessel or Equiripple filter.
So practically speaking, the Bessel and Equiripple distinguish
themselves from other filters by the constant group delay through
the filter bandpass, and the low or non-existant overshoot.
Regards,
Mike Monett
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