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Friday, 24 December 2010

Balanced Modulator Musings

I've been fiddling further with the balanced modulator from the SSB exciter in my previous post to try and get some better carrier suppression. The first thing I have done is to match some IN5711's - these were the diodes used in the RadCom article I am copying. These diodes didn't make much difference, so I then decided to remove the balancing components from the circuit and re-wind the toriods. I originally used some FT50-43's as I had loads of these, but I have re-wound the toroids with FT37-43's and much tighter wound trifilar wire.

After all this I still didn't like the output much, so I then tried matching some good old IN4148's and still with no balancing components, and probably a more symmetrical layout:

This is now the output of the crystal filter on LSB:


And here on USB:


You can see that the carrier suppression is now very significant, so I think I am quite happy with this aspect of the exciter now.

The next part of this project will be to make the triple balanced modulator (TBM) as described in January 2011 RadCom. This will need five bifilar wound transformers on FT37-43's which I will need to have a good rummage to find! There is also some discussion on VHF transistors and the selection of the MPSA10 - which it just so happens I have about 100 of!

So onwards with this project over this coming holiday given suitable breaks from family commitments.

More soon.

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