[Elecraft] Wrong Filter Question - 8 pole vs. 5 Pole Filters

[Bill W4ZV]

Lee Buller wrote:
> 
> 
> Which is better a 5 pole filter or an 8 pole filter.  Supposedly the 8
> pole would have sharper skirts....and for only 20 more bucks...well....I
> would like to know the difference between the two filters.  Is it worth
> the 20 bucks?  I am sure it is, but as you all know there is some economic
> rumbling and need to watch the pennies.
> 
> 

This question seems to be asked at least once per week...maybe we should
learn how to use search engines!

http://groups.yahoo.com/group/Elecraft_K3/message/901 (earlier today)

73,  Bill

Re: K3 CW Filters Performance

--- In Elecraft_K3 at yahoogroups.com, "John Rogers" <johnr1g3pqa at ...> wrote:

> Has anyone compared the similar bandwidth 5 pole and 8 pole Ist IF
filters for weak CW readability under normal and noisy conditions?
> ie can anyone comment on practical difference between the 500 and
400, and 200 and 250Hz filters for this usage?

Hi John,

Remember that in a rig like the K3 or Orion, roofing filters do NOT
determine ultimate selectivity. Their purpose is mainly to prevent
strong signals from entering the IF chain and causing IMD or BDR
problems. W2VJN of Inrad says the main area of interest for roofing
filters is only about -30 dB down on the skirts from the center
frequency, so there is really not a significant difference between
5-pole and 8-pole (remember that all of Orion's filters are 4-pole).
In other words shape factor and ultimate selectivity are not critical
for roofing filters.

What you are really asking is what the DSP filters look like, which
actually determine ultimate selectivity. I have not seen any
measurements of these yet but I'm sure someone will eventually post
measurements of them.

Here's a simple block diagram to remember:

Antenna > 1st IF roofing filters > 2nd IF DSP filters
Roofing filters prevent IMD & BDR from entering IF chain
DSP filters provide ultimate selectivity

More info on roofing filters here:
http://www.zerobeat.net/mediawiki/index.php/K3_Roofing_Filters

5-pole plots by OH9NB here:
http://groups.yahoo.com/group/Elecraft_K3/files/

8-pole plots by Inrad here:
http://www.elecraft.com/K3/K3_8_pole_plots.htm

73, Bill W4ZV



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