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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