[Elecraft] Let's Try This Again -- was "[K3] ... A 750 Hz, 8-Pole ... Filter?"

[Joe Subich, W4TV]

 > The issue is that it's wider than than at the -3dB points. So the
 > problem appears to be that it's pretty difficult to build an 8-pole
 > 250 Hz filter at 8.8 MHz, and that Inrad has given up on doing better.

Sorry Jim, amateur practice has been to specify bandwidth at - 6dB not
-3dB.  Inrad use -6dB (within reasonable tolerance) for their other
filters:

        AM (6 KHz):  6.250 @ - 6dB  (+4%)
           2.8 KHz:  2.880 @ - 6dB  (+3%)
           2.1 KHz:  2.175 @ - 6dB  (+4%)
           1.8 KHz:  1.838 @ - 6dB  (+2%)
           1.0 KHz:  1.063 @ - 6dB  (+7%)
           400 Hz:     450 @ - 6dB (+11%)
           250 Hz:     370 @ - 6dB (+48%)!!!

The problem is that "250 Hz" and to a lesser extent "400 Hz" is either 
marketing hype or an outright lie.  I choose to believe it is an
artifact of Yaesu's method of specifying filters bandwidth based on
their cascaded bandwidth - in other words marketing hype.  For example,
the 708/704 pair provide an effective 250 Hz in cascade and 703/701
pair provide an effective 400 Hz at - 6dB in cascade.  However, that
means the 708 and 701 filters fail to perform to their nominal
specifications when used by themselves.

It's high time that INRAD admit to the marketing hype and produce a
filter for the K3 that honestly lives up to its specified bandwidth
values as a 250 Hz (+/- 5%) filter or change their marketing to admit
that the "400 Hz" filter is really a 450 Hz filter and the "250 Hz"
filter is really 370 Hz wide.

The Elecraft produced 5-pole filters are generally specified correctly
at -6 dB ... that it my four 500 Hz filters all measure between 480
and 510 Hz wide at - 6dB and my 200 Hz filters are both just slightly
less than 200 Hz.  If they can be specified correctly, why should
INRAD be given a "pass" on truth in advertising?

73,

    ... Joe, W4TV


On 7/15/2010 1:18 PM, Jim Brown wrote:
> On Thu, 15 Jul 2010 14:53:10 +0100, Ian White GM3SEK wrote:
>
>> Agreed. 250-270Hz would be the "sweet spot" for an 8-pole filter, to
>> guarantee good sales for the manufacturer *and* good performance for a
>> range of users in heavy QRM.
>
>> Now let's see if Inrad or Elecraft take the bait :-)
>
> Hang on a minute, guys. Inrad builds a nominal 250 Hz filter, Elecraft
> tests and re-sells it, and we buy it, because we've agreed that it's a
> sweet spot operationally.
>
> The issue is that it's wider than than at the -3dB points. So the
> problem appears to be that it's pretty difficult to build an 8-pole 250
> Hz filter at 8.8 MHz, and that Inrad has given up on doing better.
>
> 73, Jim K9YC
>
>
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