Re: uBitX CW bandwith

 

Derek : 

Regarding the SOTABEAMS DSP filters.

I have a couple of the dual-bandwith ones. The General filter (2.5Khz/300 Hz) is installed in my BITX40
and the CW filter (1khz/300Hz)  in my uBITX. Both are working very well. To others looking for a CW filter solution you, would be very hard
pressed to find an audio filter that works better than this, especially at this price. Performance far exceeds any analog audio filter I have ever used.

https://www.sotabeams.co.uk/audio-filters-and-more/

On the uBITX I removed the yellow wire from the AF Gain pot and connected that to the input of the filter. The output of the filter
is connected  back to the pot where the yellow wire was, via a series 0.1 uF capacitor (this is to isolate DC voltage present on the filter output). 
What I ended up doing in the end is using a 3PDT switch to switch the filter in and out and a separate SPST switch to toggle between WIDE/NARROW bandwidth.
The three poles on 3PDT are used to switch the filter input, filter output and supply voltage to the filter. BTW the filter is powered from 12V
via a small power-line filter (two 220uF electrolytic caps to ground with a series 12 Ohm resistor between them) to prevent any "garbage"
from getting back into the 12V supply (this is a precaution).  

You will want to install the signal LED as that makes a great tuning indicator. It will flash on and off with the CW signal when a station
is tuned in close to the center of the filter bandwidth. 


By grounding RB12 on the filter board you can increase the filter gain. This doesn't actually increase the output of the filter, it decreases the
minimum signal required to get full output from the filter. You might want to play with this to see which you prefer. I went with grounding it, as that allows a lower
setting of the AF Gain control when the filter is engaged and it sounded a lot better to me. This could be wired to yet another SPST switch 
to make it selectable.  It might be desirable to be able to unground RB12 to reduce the internal filter gain on very strong signals. 
I
don't have my uBITX in a permanent enclosure yet so I haven't decided if I will use a switch for RB12. 

Best of luck and let us know how you make out. 

Michael VE3WMB 

P.S. With a "brick-wall" audio filter like the SOTABEAMS DSP,  it is IMHO actually desirable to not have AGC or at least be able to turn AGC off.
Without AGC you don't get "AGC pumping" caused by a strong adjacent signal within the IF filter passband, but outside of the AF filter 
passband. I have actually used my BITX40 in a few casual contests on CW and it works amazing well with the filter in the 300hz position. 

Join BITX20@groups.io to automatically receive all group messages.