[Elecraft] K3 filters

[Alan Bloom]

For the same passband ripple and bandwidth I think more poles pretty
much invariably means more ringing.

By the way, many people think that DSP-based filters don't ring. 
Actually, a digital filter's impulse response, measured at say the
half-power point, is pretty comparable to an analog (e.g. cyrstal)
filter with the same ripple and bandwidth.  However, the ringing from a
digital FIR (finite impulse response) filter eventually drops all the
way to zero, while an IIR (e.g. analog) filter theoretically rings
forever.  Since human sound perception tends to be logarithmic, the
ringing _sounds_ longer with the analog filter.

Al N1AL


On Fri, 2007-11-30 at 12:06, Darwin, Keith wrote:
> Yes, that is what the theory says, but I think there is variation in
> filter implementations such that some 8 pole filters ring worse than
> others.
> 
> With the DSP, one approach is to use a 5 pole filter to get a less ringy
> first filter followed by the DSP which adds the extra selectivity to
> achieve the ultimate rejection and elimination of signals outside the
> roofing filter.  This might give great selectivity with less ringing
> than trying to put all that selectivity in the crystal filter.
> 
> - Keith N1AS -
> - K2 5411.ssb.100 -
> - K3 Wave 3 -
> 
> -----Original Message-----
> From: Alan Bloom [mailto:n1al at cds1.net] 
> 
> An 8-pole filter should have more ringing than a 5-pole filter of the
> same bandwidth and ripple.  That's because the loaded Q of each pole
> (crystal) is higher (bandwidth of each pole is narrower).  You can think
> of it as each crystal covering 1/8 of the bandwidth rather than 1/5,
> although that's an oversimplification.
> _______________________________________________
> Elecraft mailing list
> Post to: Elecraft at mailman.qth.net
> You must be a subscriber to post to the list.
> Subscriber Info (Addr. Change, sub, unsub etc.):
>  http://mailman.qth.net/mailman/listinfo/elecraft    
> 
> Help: http://mailman.qth.net/subscribers.htm
> Elecraft web page: http://www.elecraft.com