I thought that I would do a little more side-by-side comparison between the transformer and the capacitor methods of signal isolation. My test bench consisted of:
- DG1022 function generator
- DS2202 oscilloscope
- Sound Card Interface v0.11
Testing the sound card interface
Fldigi reference waveform showing 1.9Vpp amplitude
As I found that there was a direct voltage saturation type effect of the DA102MC transformer in previous testing, I decided to measure a baseline peak-to-peak signal level to compare against. I ran Fldigi and measured the output of the audio output using the oscilloscope and found that at 100% volume, a SoundBlaster SB1140 will output 1.9Vpp which is what the DG1022 will be set to for the comparison tests.
Comparison of inductor (transformer) and capacitor isolation methods
Using a 1kHz tone at 1.9Vpp on both channels (phase aligned), the resulting waveforms were visualized.
The blue trace was the resulting output from the transformer. The signal was found to be 896mVpp which is down from the injected 1.9Vpp.
The red trace was the output from the capacitor isolators and had the output voltage decreased (using the on-board 10kΩ potentiometer) from a maximum of 1.9Vpp to 832mVpp for a closer comparison.
I used the external trigger output of the DG1022 to ensure consistent triggering and to show the phase difference. This is one of the times I wish that I had forked out the extra money for a 4-ch oscilloscope – oh well :).
In any event, I will be looking at a replacement SMT transformer as it is apparent that at a typical 100% output from a USB sound card the transformers are not able to handle the full-swing of audio at 1.9Vpp. More to follow on this …