[Elecraft] Question - for Educational Purposes

[Alan Bloom]

Good explanation.

Another advantage of the FIR filter is that it has "flat group delay". 
That means the delay through the filter is independent of frequency,
unlike in an IIR filter.  Flat group delay results in less distortion of
the modulation, which is an advantage for digital modes.  It also makes
analog SSB signals sound better.  (Although you might not notice the
difference since the signal also passes through the crystal filter
which, like all analog filters, has an infinite inpulse response.)

The infinite response of the IIR filter is usually not, in and of
itself, a problem.  Even an FIR filter has to have a long "tail"
(ringing) in order to get a good shape factor.  An FIR and IIR filter of
the same bandwidth and shape factor will tend to have similar ringing
characteristics out to the point where the FIR filter's ringing ends
(and the IIR filter's ringing continues).  But the ringing is often
inaudible by that time anyway.

Al N1AL


On Thu, 2008-07-24 at 09:10, Stephen W. Kercel wrote:
> 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"
> >