BITX40: SOTABEams DSP filter module for CW Operation

 

I just successfully integrated a SOTABEAMS Laserbeam DSP Audio filter module into my BITX40. 

http://www.sotabeams.co.uk/dual-bandwidth-filter-modules-ssb-cw/

I am using Raduino V 1.20.1 with the CW mods suggested by Allard in the accompanying documentation and up to now have been
using the same filter (the General Purpose Module with 2.4 Khz WIDE and 300 Hz NARROW passbands) externally mounted into a small
box. I decided that it worked well enough and is cheap enough ($US32) that it warranted installing one directly into the rig for convenience.

I inserted the filter into the circuit at label M2, by removing the wire on pin 1 of  the VOLUME POT and connecting that to the DSP Filter input and then
connecting the Filter output back to Pin 1 of Volume control.  I did need to put a 0.1uF coupling capacitor in series with the output of the filter as I experienced
some weird interactions between the filter and the Volume control / LM386 operation (audio was cutting out with Volume control rotated past 11 o'clock).
The coupling capacitor in series fixed that.  I also opted to provide  Bypass switching for the filter via a DP3T switch, simply moving M2 between the input and output of the filter while keeping the filter powered (not enough poles on my switch to also control the filter power). The filter only draws 30mA of current.
What I discovered  with the filter bypassed yet still powered, is that it was generating some hash on its power leads that was getting back into the BITX40 audio.
With the filter in-line it conveniently seems to filter out its own hash ;-)  I resolved this issue by using VK3YEs  Digital VFO power lead filter in series with the power leads going to the filter. It  consisting of two 220 uF electrolytic caps to ground with a 12 Ohm resistor connected in series between the + leads of the caps.  

In the WIDE setting the filter does cleanup the SSB audio making it sound better, but where it really shines is for CW.  
The 300 Hz passband is centered at 700Hz so it is necessary to set the Raduino Sidetone frequency to be the same value (this also controls the TX offset).
Installing the optional  signal LED gives you visual indication that the CW signal you are tuned to is in the middle of the filter passband, making CW  tuning very simple.  The end result is a much more pleasant experience for operating CW on the BITX40.  
Also if you have any remnants of clicks from tuning etc, the filter does a great job of stripping those out of the RX audio. 

This is by far he best audio filter I have encountered. It outperforms any analog audio filter I have ever used. 

The only minor issue that I still have is that the filter will sometimes overload on very strong signals. The filter input is internally protected from overload but it will
cause some audio distortion, it seems mostly on SSB. I am using the default gain setting on the filter (RB12 floating not grounded) and this works well.
At this point I am simply using the switch to BYPASS the filter on exceptionally strong signals but I am thinking as a future enhancement perhaps a
properly adjusted audio-derived AGC circuit inserted ahead of the DSP filter would resolve this and eliminate the need for bypassing.

Cheers 

Michael VE3WMB / VA2NB 

P.S. SotaBeams also makes a similar SSB-specific filter with WIDE=2.4 Khz and Narrow = 2.1 Khz and another version with a continuously variable
bandwidth and passband centre frequency, that is controlled via a rotary encoder. This would be a little more versatile. 

P.P.S. The usual disclaimer .. I have no connection to SOTABEAMs, financial or otherwise, I am just a very satisfied customer. 

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