[Elecraft] Question - for Educational Purposes

[Stephen W. Kercel]

Lee:

Since we are trying to stay at the 3rd grade level, I will spare you 
the mathematical definition of the difference.

The observable difference is that if you apply a single pulse to the 
input of a Finite Impulse Response (FIR) filter, the output of the 
filter will be a finitely long train of non-zero pulses, but if you 
apply a single pulse to the input of an Infinite Impulse Response 
(IIR) filter, the output of the filter will be a  train of non-zero 
pulses that never ends. After a settling time, the value of the 
remaining pulses will become very small, but they will never quite go 
to zero. The IIR response is akin to the "time constant" response of 
a linear analog circuit.

(I could get into details like recursive versus non-recursive, but 
that is past 3rd grade level.)

Advantage of the IIR filter: It is computationally cheaper than a FIR 
filter. In other words, if I want to obtain a specified filter 
response, an IIR filter will process the signal in fewer clock ticks 
than a FIR filter.

Disadvantage of the IIR filter: It is susceptible to computational 
instability. In other words, unless you are extremely careful about 
both the design of the filter and the type of signal you apply to it, 
the filter output will gyrate wildly between the largest and smallest 
number that the machine precision allows; this is the machine's way 
of approximating a response that "approaches infinity."

Advantage of FIR filter: It is inherently stable. A bounded input 
must produce a bounded output. In most engineering applications the 
inherent stability is worth the cost of the extra clock ticks 
required for the FIR filter.

73,

Steve Kercel
AA4AK



At 10:26 AM 7/24/2008, Lee Buller wrote:


>I've been doing some reading on the new firmware.  I would like 
>someone to explain (like I am a 3rd Grader) the different between 
>IIR Filtering and FIR Filtering.  Thanks.
>
>Lee - K0WA
>"Liberal Arts Major - Mathematically Challenged - Science Wannabe - 
>Fascinated by Technology"
>
>
>
>
>In our day and age it seems that Common Sense is in short 
>supply.  If you don't have any Common Sense - get some Common Sense 
>and use it.  If you can't find any Common Sense, ask for help from 
>somebody who has some Common Sense.  Is Common Sense divine?
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