A full blown mid-life crisis can sometimes only get worse!

Further investigations of the issues surrounding µBITx odd harmonics on CW on the lower bands are showing that the problem is from board layout issues in the relay switching and LPF section of the board.

Allison KB1GMX has cut up her board and done testing on the LPF/relay section. This shows the following:

On 3.5 MHz in TX mode, the 3.5 MHz filter is selected the blow by limits us to barely 25DB of filtering of harmonics at 30 MHz

On 7.0 MHz in TX mode, the 7 MHz filter is selected, blow by limits us to maybe 25db of filtering.’

On 21.0 MHz in TX mode with the 21 MHz filter selected the blow by limits us to about 30db at 30MHz of filtering.

While the “blow by” may vary by individual unit and it is posisble to argue over the magnitude, over all the filters are simply unacceptable.

Allison says “At this point I’d rip out the filters and the relays and even the TR relay (KT1-3 and K3) and route everything to an external low pass board. At that point I think we have a chance with the external board performance being unknown but for certain it cannot be worse.”

Stay calm

For many of us, all of this is simply more bad news. However, we should stay calm at this point and start using our commercial rigs or other kits for a while. With such a large community of constructors, there will be solutions found to some of these problems. You may well need to do some mods though for your µBITx to be fully legal in your country.

Summary

There are two issues known to date:

Spurious emissions (spurs) generated in the double balanced mixer at 45 MHz that exceed US emission standards on SSB above 18 MHz. This issue is exacerbated by higher audio drive levels (e.g. if you have added a mic preamp or voice compressor). This issue is an unnoticed design issue.

It is likely that this issue can only be resolved by adding additional filtering to replace the existing 30MHz LPF following the mixer, or by redesign of the µBITx with a higher IF frequency (maybe 70MHz or higher).

Harmonic output on 3rd, 5th, 7th, 9th harmonic, etc. that exceeds US emission standards on CW on some bands. This issue is caused by “blow by” in the LPFs and the associated relay switching. This is a design issue with the layout of the board and the relay switching design (using only one relay to switch both ends and the sequential routing of relays on lower bands).

It is likely that this issues can only be fixed by disconnecting the existing LPFs and reconstructing the filtering and relay switching system on a new daughter board.