[K3] RE: [Elecraft] How do you measure filter offset

[Bill W4ZV]

On Tue, 19 Aug 2008 13:26:25 -0700 (PDT), Bill W4ZV wrote:

>Well why not the -60 to -80 dB points?  :-)  

Because -35 dB or so is the best I can do. If you lived close to W0YK, you'd 
WANT to know what's happening at -60dB!  :) 

        It actually doesn't matter beyond about 30 dB down ina roofing
filter...the DSP filters handle things beyond that.

>Two points are entirely
>sufficient to **center** the filter's 6 dB bandwidth, which was the
question
>asked. 

I don't think -6dB bandwidth was the question, but rather how to measure the 
filter. Whatever -- a single number description is quite limited. 

        The question was how to center filters...not make detailed plots of
them.

>Regarding Ed's comment about ripple, we'll never see 6 dB ripple
>excursions unless there is a defective filter. 

I've not seen enough going on in the passband to even begin to call it 
ripple. BUT -- when I'm measuring something, the more detail I can see the 
more I know about it. The fact that the filters ARE quite flat in passband
is 
something that I want to know!  Having read LOTS of product data sheets that 
looked like they were drawn by an artist rather than measured, its quite 
encouraging to see a product that actually measures like it's data sheet! 

        If you want full plots of 8-pole filters, go below and click on "See
Details" under each filter (I think the 250 is missing but the others are
there):

http://www.inrad.net/home.php?cat=140

...and plots of the 5-pole 200 and 500 are shown here:

http://f1.grp.yahoofs.com/v1/YEKrSIMok74B_mjAO8WAb3OqdauIV7jhVEWFK5zAcLSr3JJoFH79RRz_dbxeVUNQicFDDsQu2hGN42lvYFAIIJUmHw/Elecraft%205%20Xtal%20CW-filters%20for%20K3.jpg

73,  Bill
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