[Elecraft] Re: KX1 IF filter tuning?

[Wayne Burdick]

Hi Jeff,

Jeff Stai WK6I wrote:

> hi Wayne - I understand that a filter can have passband ripple, but in this
> case I seem to be experiencing 10-15dB of "ripple", and the main "trough" is
> right in the middle of recommended 600Hz tone.

I definitely didn't see that in my own tests, but I'll check my prototype out
and let you know what the dip is. You could have an incorrect component value
somewhere, or even a bad crystal (possibly due to overheating during ground wire soldering).


> >If you weren't interested in wider bandwidths at all, you could select a
> >different termination impedance, although I haven't experimented with this. I
> >wouldn't recommend doing this without fully re-testing the passband at all
> >settings of the FILTER control.
> 
> It would be fair to say that I could live with poorer performance at wider
> BWs, and focus on sub-1000Hz for CW - I could even imagine adding a little
> switch for the two BW ranges if I was interested enough.
> 
> Would I be changing R19? Could you suggest some ranges to try?

First we need to make sure you don't have a component problem. Your results are
so much worse than any KX1 I've used that this is still a strong possibilty. 

But assuming you don't, you can experiment with different values of L8, L8, and
C4. You could even try replacing the set of four crytals--perhaps one is bad, or
was damaged during soldering of the grounding wire. Finally, a slighly more
complex topology that I didn't have room for on the PCB is to insert capacitors
between R19/X2 and X4/L8. There may be room on the bottom of the board to add
these. R19 should probably be left at its present value. Anything larger will
reduce the available current (hence gain) through Q8, and a smaller value will
increase current drain without significantly affecting driving impedance, which
is already quite low.

One last comment: I would not suggest that the average KX1 user make any
changes. As I said before, the present circuit is a careful compromise that
allows both narrow and wide-band use. Hundreds of KX1s are in use, with
apparently satisfactory results. Of course even one "unsatisfactory" is too
many, so let's figure out what's up with yours.

73,
Wayne 

-- 

http://www.elecraft.com