[Elecraft] K2 filter attenuation

[Larry East]

At 07:25 PM 10/16/02 -0500, Rich Lentz wrote:


>Back to the original subject -
>
>I also hear and see (with spectrogram) a noticeable drop in signal (in
>excess of 10 db) on the narrowest filter.  I am positive the filter is
>properly aligned, and this happens on normal, reverse, USB, and LSB. My
>filter numbers are almost the same as that in the manual, with a slight
>difference for a 750 Hz CW tone. The same decrease is also shown with
>the noise generator on the narrowest filter.  I cant believe it is
>"tuning" or the filters are "miss tuned," I might believe there is an
>improper component or miss-wiring that I can't find. But I have checked
>each component at least 8 times (counting the four times that I checked
>every part when the K2 was built. The level matches the other filters
>when it is as wired as the other filters which makes me believe there is
>no incorrect components or miss-wiring.
>
>As the first question in this thread "Any suggestions?"

I also see significant attenuation with the 150 kHz filter -- but not as 
much as you are apparently seeing. Here's a copy of measurements that I 
made on my K2 in May of 2001 (using a 700 CW Hz tone):

  Filter    Rel. Attn.
  1000Hz -  0.0dB (reference)
   600Hz - -2.5dB
   400Hz - -4.5dB
   150Hz - -9.0dB
(values rounded to the nearest 0.5dB)

Each filter was centered on 700 Hz within 10Hz using Spectrogram. According 
to Gary S., there shouldn't be as much attenuation as I measured and he 
sent me a new set of crystals to try. However, I've never installed them 
for one reason or another.

So... your problem could well be a poorly matched set of filter crystals.

Even with the 9dB of attenuation, I haven't had any problems "losing" weak 
signals when switching to the 150 kHz filter. And with the narrow audio 
filter also in use, signals just pop out of the noise!

Maybe one of these days I'll try the new set of crystals...

72/73,
Larry W1HUE/7