K3 - 2.7 kHz Shape Factor

[Bill W4ZV]

K3RIZ:
 >Can anyone give me the shape factor for the
Elecraft supplied, 5 pole, 2.7 kHz filter?

         I was hoping Wayne might respond since we
are now within 1 month of shipments and we'll soon
need to decide whether to go with the 5-pole 2.7k or
the 8-pole 2.8k.  In addition to plots of the 5-pole
filters (like already exist for the 8-poles), I'm
waiting to see some IMD/BDR results for all filters.
Ten-Tec, Icom and Yaesu have had some disappointing
IMD/BDR results published for Orion, the IC-7800 and
the FT-2000 *after* introduction, so I hope Elecraft
does not repeat the same mistake with the K3.

         My guess about the 5-pole is that it will have
a 6/60 dB shape factor a little better than 5:1.  This
is about what the Ten-Tec 4-pole filters are and a
5-pole should be a little better...perhaps ~4:1.

         Let's also remember that the shape factor of a
roofing filter is not so critical as if it were being
used to determine ultimate selectivity.  In rigs like
the K3, ultimate selectivity is determined by the DSP
filters which will have excellent shape factors.  In
DSP rigs, the most critical area of a roofing filter
for it's intended purpose of IMD performance is down
to about 30 dB on the filter skirts (i.e. not 60 dB as
measured by the classical "shape factor").  From the
FAQ of Inrad's paper on roofing filters by W2VJN:

"5. If 6 poles work so well, why not 8 poles?

The most important part of the filter
characteristic is from the pass-band on down
to about –30 dB on either side of center. Eight poles would provide much better
stop-band isolation, but it’s not required in a
roofing filter and would make no
noticeable improvement in IMD performance."

http://www.qth.com/inrad/roofing-filters.pdf

                                 73,  Bill  W4ZV

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[Geoffrey Mackenzie-Kennedy-2]

 From Bill Tippett  <[hidden email]>

>From the
>FAQ of Inrad's paper on roofing filters by W2VJN:

>"5. If 6 poles work so well, why not 8 poles?

>Eight poles would provide much better
>stop-band isolation, but it's not required in a roofing filter and would
>make no
>noticeable improvement in IMD performance."

---------------------------------------------------------------------------

A moot point I would suggest Bill when the symmetry (or lack of) a Ladder
filter's response is also considered. The more poles the better in a Ladder
roofing filter to improve symmetry provided that the IMD generated within
the filter does not become a limiting factor, a point that Jack K8ZOA has
noted. To preach to the choir, filter IMD performance is usually measured
early on in a receiver's design exercise prior to fixing the Gain
Distribution.

73,
Geoff
GM4ESD


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[Bill Coleman-2]

On Jun 27, 2007, at 6:59 AM, Bill Tippett wrote:

> In rigs like
> the K3, ultimate selectivity is determined by the DSP
> filters which will have excellent shape factors.  In
> DSP rigs, the most critical area of a roofing filter
> for it's intended purpose of IMD performance is down
> to about 30 dB on the filter skirts (i.e. not 60 dB as
> measured by the classical "shape factor").

But wouldn't a roofing filter with a small shape factor be "better"  
for IMD performance? Isn't 40 dB of nearby signal rejection better  
than just 30 dB?

Now, this all is keeping other things equal -- that the group delay  
or non-linear characteristics of the filter itself don't become  
unfavorable.

I'm basically unclear on why ultimate rejection in a roofing filter  
is so unimportant. If you are on 40m in Europe, just a few miles away  
from a megawatt shortwave broadcaster, ultimate rejection would seem  
to be of critical importance.

Similarly, if you are at a M/M station on the second rig on a band,  
ultimate rejection is key, as there is another 1.5 kW signal within a  
handful of meters of your receive antenna.

Bill Coleman, AA4LR, PP-ASEL        Mail: [hidden email]
Quote: "Not within a thousand years will man ever fly!"
             -- Wilbur Wright, 1901

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[Bill W4ZV]

At 01:12 PM 7/7/07, Bill Coleman wrote:

>On Jun 27, 2007, at 6:59 AM, Bill Tippett wrote:
>
>>In rigs like
>>the K3, ultimate selectivity is determined by the DSP
>>filters which will have excellent shape factors.  In
>>DSP rigs, the most critical area of a roofing filter
>>for it's intended purpose of IMD performance is down
>>to about 30 dB on the filter skirts (i.e. not 60 dB as
>>measured by the classical "shape factor").
>
>But wouldn't a roofing filter with a small shape factor be "better"
>for IMD performance? Isn't 40 dB of nearby signal rejection better
>than just 30 dB?
>
>Now, this all is keeping other things equal -- that the group delay
>or non-linear characteristics of the filter itself don't become
>unfavorable.
>
>I'm basically unclear on why ultimate rejection in a roofing filter
>is so unimportant. If you are on 40m in Europe, just a few miles away
>from a megawatt shortwave broadcaster, ultimate rejection would seem
>to be of critical importance.
>
>Similarly, if you are at a M/M station on the second rig on a band,
>ultimate rejection is key, as there is another 1.5 kW signal within a
>handful of meters of your receive antenna.

         As I've said many times, let the actual IMD/BDR
measurements (yet to be published) be your guide.  All
else is simply theoretical speculation.

                                 73,  Bill  W4ZV

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