[Elecraft] [K2] Filters attenuation

[Don Wilhelm]

  Dave,

I am wondering if you have some problem with the crystals in the 
variable IF filter.
The variable filter design point is near 500 Hz.  It will normally have 
attenuation below 200 Hz width and the passband will bet more 'ragged' 
above 1 kHz.  What I am saying is that you should have a constant filter 
response at 700 Hz, 400 Hz with perhaps a small bit of attenuation at 
200 Hz.  Below that width, attenuation will increase rapidly.

If your K2 is below SN 2560, and it has not been upgraded, you have the 
older crystals in it and I highly recommend that they be changed with 
the matched set of 7 crystals available from Elecraft.  That will change 
the attenuation.

Of course, if your filters are not aligned to properly center on your 
sidetone pitch, that alone could be the reason for your observations - 
that would be more apparent as the width is narrowed (the signal falls 
on the slope of the passband rather than at the peak).  See the filter 
alignment information on my website www.w3fpr.com - look at part 3  of 
the article dealing with K2 Dial Calibration.   Use Spectrogram or 
Spectrum Lab or similar to give you a visual picture of the filter 
passband placement and the relative amplitude of the filters.

One of the aspects (limitations) of the K2 is that your listening 
sidetone pitch cannot be changed at will without peaking the narrow IF 
filters at that new sidetone frequency.  You may be able to change the 
sidetone pitch by 50 Hz each side of the filter center, but much beyond 
that will require that the filter centers be repositioned.

73,
Don W3FPR

On 11/4/2010 2:12 PM, dave wrote:
> I dunno about the loss you are seeing but I've measured about 8 dB of
> loss in my K2 at 200 Hz vs 1 kHz BW. That does strike me a quite a bit
> of loss. I just don't use the narrow filters often. I know of at least
> one other K2 that has similar loss. Anyone else?
>
>
> 73 de dave
> ab9ca/4
>