Though it took a little longer time, finally my uBitX seems to be working very well now, for which thank you for advices here so far.
Then, another question to ask you who experienced the same issue.
I hear high frequency noise behind the band noise that my iPhone freq meter says it’s about 2,700Hz. (I’m not sure if this app is reliable or not)
My uBitX is not in a metal case, yet. And I use a small powered speaker.
Maybe it comes from Raduino or main board of uBitX, or possibly the 12v power supply.
Since I’m planning to bring my uBitX to a local lab in order to measure its spurious emission this coming Wed, I’d like not to mod much things on hardware coz it may possibly bring another trouble to me.
I just hope that some capacitor in the line of speaker or some small mod can solve this issue at this moment.
Looking forward to your advice.
// hiro, JJ1FXF