[Central-Electronics] 200V transmit bandwidth

[Kihwal Lee]

 Bob,
Thanks for the reply. I did something similar using my SDR.
https://k9sul.fayoly.net/files/200v_res.jpg

The left one is from a stock 200V and the right one is with a 3kc active audio filter. I am going to build another filter using MAX7400 and try.  There are many ways to do this with a modern circuit, but it will be nice if the existing filter can be tweaked to have a lower roll-off frequency.
I am going to look at BT1, BT-2, T1 and T2 first as they define the passband.
73Kihwal, K9SUL    On Monday, March 1, 2021, 09:58:26 AM CST, Robert Nickels <ranickels at gmail.com> wrote:  
 
 On 2/28/2021 11:15 PM, Kihwal Lee via Central-Electronics wrote:
> I noticed that the transmit bandwidth is at least 4kc.  Is it normal?  The manual says 200-3500kc. Has anyone tweaked the filter module to make it narrower?

I have not, but I recently had the filter module on the bench and swept 
it,  and can confirm that the high end rolloff is around 4 kHz.  I 
didn't take data as I was just looking for distortion but you're seeing 
what I saw from the filter.    I made some IQ recordings on an 
attenuated SDR for offline analysis and as you can see from the image at 
the link below,  transmitted signal with the filter installed does 
extend beyond the 2.7 kHz of the bandpass I used, but with my voice/mic 
at least, not much energy is present there.

https://i.imgur.com/9eimEAA.png

Fortunately it's easy to remove and install something else through a 6 
pin plug that mates with the filter connector as long as  you pay 
attention to signal levels and proper grounding and shielding.   You 
might be able to use a filter design program like Elsie to model each 
section of the CE filter to get an idea of how it works (remember CE 
designed it with maybe a slide rule!)   It should even be feasible to 
route the filter in and out signals to outboard audio  filtering or 
other processing if desired.

The modularity of the 100V certainly is a plus for troubleshooting and 
repair, but is also no doubt one of the factors that affected its 
profitability for CE.

73, Bob W9RAN


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