Re: Digital Interface - transformer seems to drastically weaken signal


Rob French (KC4UPR)
 

Hey Evan,

Really I think the only difference between EasyDigi and my setup, is that the Teensy + Audio Adapter takes the place of the sound card.  The added capability of the Teensy is that I plan to (and have previously) implemented standalone DSP filters for transmit and receive in the Teensy.  I'll have a mic input option to the Teensy, with S/W speech compressor, as well as variable width receive filters.  Right now, the part that I'm having issues with is in fact the "EasyDigi" part (i.e. the transformers).

As you noted, yes, the Teensy Audio Adapter is capacitively coupled with 2.2uF capacitors on all of the audio input/output lines.  If I can't figure out this transformer thing, I'll probably just directly connect to the Teensy.

I noted recently that, when transmitting into a dummy load, everything seemed fine (without the transformers installed).  However, when I transmitted through my antenna, I had a terribly distorted audio waveform.  I traced this to the unused wire in the stereo audio cable I was using... when I grounded that unused wire, the distortion went away.  I have a suspicion that issues with RFI are contributing to my woes.

I'm going to try prototyping a quick setup where I (a) add a couple 0.01 uF bypass capacitors at each side of the transformer, and at each end of the audio cable, and (b) use a good, shielded mono audio cable for each of the RX and TX audio connections.  If that doesn't work, then I'll try swapping out one of the El Cheapo 1:1 isolation transformers I'm using, for an ostensibly higher quality Xicon transformer I have lying around from Mouser.  That one isn't a 1:1 transformer (I think it's 4k:600 ohm), but maybe I can use the transformation ratio to my advantage as well (e.g. step down the transmit audio).

Rob KC4UPR

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