Akira JJ1EPE had spuries on his BITX40 at 75kHz.
The modification for suppress the spurious signals provided by a friend involved the following steps:
1) Add 3.3uF electrolytic capacitor to VCC line of Arduino board
2) Add 0.1uF x 2 bypass capacitors to the VCC line (fuse point)
3) Insert high frequency choke of about 100 uH in the power line of Arduino
4) Connect the minus line of power supply line to the ground
5) Connect the minus line of BITX40 main board to the chassis ground
6) Add 0.1 uF × 2 bypass capacitor to VDD of BITX 40 board
7) To prevent common mode noise, add 0.01uF x 2 bypass capacitor to the power connector
This series of steps could equally well be applied to the µBITx if you have noise from your power supply and want to get rid of it with a very thorough strategy.