[Elecraft] 250 Hz and 400 Hz Filter Measurements

[David Gilbert]

Why not just tighten the DSP on the 450 Hz filter instead of kicking in 
the 250 Hz filter?  It's just as easy to do for when that occasional 
S9+30 signal pops up nearby, and at both 30db down and 60 db down you'd 
have exactly the same performance.

I happen to have the 250 Hz filter, but after seeing K9YC's data I'm 
convinced I would have been just as well off with the 400 Hz fllter (I 
don't see the need for both) and I would have saved myself 3db insertion 
loss to boot.

73,
Dave   AB7E



On 7/16/2010 11:38 AM, Guy Olinger K2AV wrote:
> In my radio I have the 400 set as 450 Hz, and I have the 250 set at
> 350 Hz, well aware of how close they are.  I use the 350 setting for
> running when the stuff above or below starts to infringe and the
> difference between the two. Pulling in the roofing filter against a
> 9+30 signal just above or below is very noticeable and extremely
> useful.
>
> Each to his own, just don't start a ban the 250 campaign.  I know a
> lot of contest operators using the 400/250 8 pole combo in the manner
> I described.
>
> 73, Guy.
>
> On Fri, Jul 16, 2010 at 2:47 AM, Jim Brown<jim at audiosystemsgroup.com>  wrote:
>    
>> Crew,
>>
>> I finally got around to dragging out my precision audio spectrum
>> analzyer to check out the 250 Hz filters. I have two K3s, one
>> with a single RX and one with two RS. I have 250 Hz and 400 Hz
>> filters in all three RX.
>>
>> First, I tweaked the centering of each filter (that is, the
>> offset), then I measured bandwidth at -6dB, -30dB, and -60dB with
>> IF bandwidths of 1kHz and 250 Hz.
>>
>> Results: There was some variation from one filter to another, but
>> trends are quite consistent. The only significant difference
>> between the 250 and 400 Hz filters is at their -6dB points with a
>> 1kHz IF, where the average bandwidths were 311 and 412 Hz
>> respectively. This is essentially the bandwidth of the roofing
>> filters themselves.
>>
>> Once you crank the IF down to 250Hz, there is no significant
>> difference between the two filters. The average -6dB bandwidths
>> were 193 and 200 Hz; at -30 dB, both filters averaged 300 Hz; at
>> -60dB, they averaged 381 and 397 Hz respectively.
>>
>> There IS one repeatable difference between the 250Hz and 400Hz
>> filters -- their insertion loss, which is 3dB. That is, the 250Hz
>> filters have 3dB more insertion loss.
>>
>> After doing these measurements, I firmly agree with W4ZV, W0YK,
>> and others, who have noted that there is no good reason for
>> having both of these filters in a radio. Indeed, there is no good
>> reason for the EXISTENCE of this particular 250 Hz filter,
>> primarily because it is NOT a 250Hz filter by any reasonable
>> measure.
>>
>> It is long past time for Inrad to lean on their filter supplier
>> and get them to ship 250 Hz filters. Failing that, I want my
>> money back for three filters. BTW -- all of these were purchased
>> from Inrad, not Elecraft.
>>
>> 73, Jim Brown K9YC
>>
>>
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