1)? Again,.....and,.......again,? 28 kHz is WELL beyond the specified performance frequency of the TinySA.?
2)? Have a SERIOUS read of the specifications at: ??
??????
3? Any and all digital circuits may and will produce non-operational associated spurs.? Again,....., have a good visit to Mr. Fourier and the Fourier series.?
So what if there is a 28 kHs spur?? Consider 1 through 3, above.? No problem!!!