Hi
I have been able to model the frequency response of digital filters on my HP48 for years now but thought I would share
this info, make sure you are in radians mode (RAD)
First make a directory, called say, DSP
Go into the directory and make the following
'EXP(X*(0,1))'
'Z'
STO
This means e to the power of -i is substituted if you use 'Z' in equations
Create the following RPL program
<< ABS LOG 10 * >>
ABS'
STO
This program can be use to produce a logarithmic response
for a filter
Now type in a Z transform for a digital filter.
Lets say the old simple LAG filter, the 0.1 of the new value plus
0.9 of the old. In 'C' this would be
lag = lag * 0.9 + newval * 0.1;
But as a Z transform is:
'0.1/(1-0.9*Z)'
Put the Z transform on the stack
Apply ABS
Store the new equation to 'EQ'
Go to plot
Set xrange to 0 3.142
Set y range to -40 40
Plot the magnitude response in DBs of your lag filter
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