[Elecraft] K3: roofing filter configuration settings

[Guy Olinger K2AV]

What the K3 contesters around here have done with the "400" and "250"
roofing filters is assign them 450 and 350. For someone running in a
contest, those are meaningful settings where is is *desired* to have the
DSP and roofing skirts concurrent for sharp adjacent channel rejection.

As to where those numbers came from they are the exact same filter
(different mounting) as the INRAD 8 MHz 400 and 250 Filters for the Yaesu
MP series. Those are part of a *cascade pair* with a 455 kHz IF filter that
give razor sharp performance at 400 and 250 bandwidth, where the pairs
really are 400 and 250. In the MP you put the "400" filters in the 500
slots. The MP skirts are still going down at -100 dB. I've never been able
to measure the bottom.

In any event, E's use of the 8 MHz as a roofing filter makes perfect sense.
When needing very narrow settings, the DSP skirts inside the 350 roofer's
skirts works well enough for me so far.

73, Guy K2AV



On Wed, Feb 10, 2016 at 8:32 AM, Bob McGraw K4TAX <rmcgraw at blomand.net>
wrote:

> Filter bandwidth is defined by two points either across the top or at the
> knee of the filter.  If the knee is 250 Hz wide at the 3 dB points then
> measuring the 6 dB points may be 370 Hz wide.  So without the position of
> the filter where the measurement takes place, the number relating to
> bandwidth is inadequate to describe the filter.  There is really no
> standard with regard to filter measurements, thus the value is simply
> arbitrary.
>
> Yes a given filter can be 250 Hz at the 3 dB points, can also be 370 Hz at
> the 6 dB points and also 500 Hz at the 18 dB points. So........what is the
> bandwidth of the filter?
>
> 73
> Bob, K4TAX
> K3S, s/n 10163
>
>
>
>
> On 2/10/2016 5:49 AM, Arie Kleingeld PA3A wrote:
>
>> Hi Dick,
>>
>> Just do it. If you need a certain bandwidth for a certain mode, use the
>> narrowest roofer you have in there; just like in any other conventional
>> transceiver.
>> So if you want 350Hz as bandwidth, use that 250 filter (which is actually
>> 370). That's common sense.
>> Don't let the 250Hz tag on the filter fool you. They say they sell you
>> 250Hz, that's just not true. It should have stated 350Hz or so.
>>
>> And in ham spirit: just try out different settings and decide what works
>> best.
>>
>> 73
>> Arie PA3A
>>
>> Op 10-2-2016 om 6:05 schreef Dick via Elecraft:
>>
>>> snip>
>>>   The factory default roofing filter "configuration bandwidth" setting
>>> is to
>>> set the 250hz filter at a setting of "250".  Therefore, the 250hz roofing
>>> filter will engage at the same time as the 250hz DSP filter.
>>> However, since the 250hz roofing filter has an actual BW6 of 370hz,
>>> aren't
>>> there times when it would be more beneficial to have  the "configuration
>>> bandwidth" of the 250hz roofing filter set to 350 or  400?
>>> I'm thinking this may be especially true when operating RTTY since the
>>> average RTTY bandwidth is aprx. 370hz.  Perhaps there are times when
>>> this
>>> would be beneficial when operating other modes as well?
>>>   <snip
>>>   Dick- K9OM
>>>
>>
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