Since a couple of months I have been building the uBitx. Not the kit version, but from plain PCB and a mill. This gives me flexibility to add things, such as a narrow crystal filter later. The first version I built had lots of birdies but only had the bidirectional amps in SMD. I restarted the project and now used far more SMD layout. This really helped as the rig works much better now, at least the receiver, TX is not finished yet.
What I did find out is that there was a lot of noise in reception, but clearly in the audio chain, not from the HF section. You could hear this at best when the antenna was disconnected and no RF noise was coming in. However, when receiving on battery the noise was gone. First I thought it was the charging section, the internal batteries are charged when external supply is applied. Later I found another strange thing.
I use a voltage/current regulated power supply. If I set this power supply in the mode that the voltage was regulating, I heard the noise. When I reduced the current regulation until the level that the supply voltage started to drop (current regulation mode), the noise was gone. I had clearly to do with noise coming from the power supply.
I tried a standard 13.8 robust (2N3055....) supply and even had more noise. At An external 12 volt battery, noise was gone.
The problem concentrated around Q70, the first audio amp after detection. I increased C52 to 2200uF and the audio noise became muffled. It had some effect but not as much as running on batteries. I think the supply noise from the regulator in the power supply is adding the noise with very low impedance to the DC. It is difficult to get rid of it using the RC filter made by R52 and C52.?
As an experiment, I changed the circuit so that Q70 has its power from the 5V regulator which also drives the Arduino. The noise was gone.?
End conclusion: if there is a big difference in noise sounds when powered from battery or normal power supply, consider adding extra 'clean' stabilized power for Q70. I don't know yet if the same applies for the MIC preamp as that could be sensitive as well. Working on that now.
73, Alex
PA1FOX
