Having recently explored the signal integrity issues in the Interface v0.11, I’ve re-tooled the board to use the TY-311P transformers in v0.1 which meant a total redesign of the layout. In the most recent version, I’ve grouped the components in functional blocks which in principle should reduce any digital noise in the analog side of things.
It is a bit of a shame to go with such large (by comparison) transformers, however, they afford 1.5kV isolation and have a very flat frequency response from 300Hz to 3.5kHz.
When examined on the DS2202, the difference in amplitude was found to be ~10mV on the peak, so this can be extrapolated to be ~20mVpp or roughly 1% which is quite acceptable.
The transformers are 600Ω:600Ω impedance which is a great match for a radio such as the FT-857D which has a 600Ω data output, and is well within the microphone input range of 200-10kΩ (with 600Ω being the nominal). Matching impedances so closely should serve to avoid the production of any harmonic content due to any non-linearity in the circuit.
In addition to changing the transformer itself, I’ve added the provision to decouple any DC bias from the microphone circuits of both the sound card and the radio.
Most modern microphones are of the electret (or condenser) variety. An electret microphone works by biasing a FET (field effect transistor) in a common source configuration. As the schematic suggests, a voltage on the gate will allow a proportionate flow of electrons thru the device, acting as an amplifier.
Although there may not be a DC bias on the output, it is best not to make this assumption and add the provision of decoupling the signal using a capacitor to prevent possible damage to the transformer due to circulating current. In the event the radio and/or sound card are DC blocked at their outputs (as suggested by the above schematic), the DC block can be shorted with a piece of jumper wire at virtually no loss to the signal in question.
Once I have this new version of the sound card interface rolled out and field-tested, I will begin the fabrication process for the all-in-one board where the USB sound card is built-in to the interface. This is just a warm-up project!