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
--
View this message in context:
http://n2.nabble.com/How-do-you-measure-filter-offset-tp733034p734358.html
Sent from the [K3] mailing list archive at Nabble.com.
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