[Central-Electronics] 200V transmit bandwidth experiment

[Kihwal Lee]

I've modified the filter module to reduce the bandwidth and I think it still sounds okay.
As explained in the manual, BT-1 and BT2 forms a band pass filter and TN-1 and TN-2 are "peaking" filter. The rest are rejection filters.  The each element designated as TN is called twin-T notch filter. You can easily find a calculator/analyzer online. BT-2 and TN-1 define the upper limit of the passband.
BT-2's corner frequency is 3183Hz and TN-1's notch (peaking, since its used as an inverse feedback) frequency is 3882Hz. I guess overlaying the two makes the response a bit steeper.  By adjusting the parameters, I was able to lower the both to 2273Hz.  The resulting bandwidth is about 2.9kHz.  Now when I tune a receiver 3kHz away from the tx frequency, I no longer hear interference.
You can see before and after picture of the received signal spectrum here:https://k9sul.fayoly.net/projects/ce200v/IMG_5327.JPG

Details of the modification:
TN-1: Change R1, R2, R3, R4 from 82k to 100k. Add 200pF to C1 and C3. Add 400pF to C2.BT-2: Add 200pF to C22.
I am not claiming this is an optimal solution.  I just tried this design and it worked good enough for me. When I tried changing the frequency of BT-2 to 2652Hz (pad 100pF) and TN-1 to 3252Hz (pad 100pF to C1/C3 and 200pF to C2), the response did change, but the bandwidth was still too wide.   This is certainly a compromise to the wonderful audio quality of the 200V, but it makes the TX usable on a busy band without worrying too much about causing inconveniences to others.  My motivation was to use it everyday for all QSOs.
73,Kihwal, K9SUL