There have been many postings on the BITX20 IO Groups list following the raising of concerns by a couple of list members about harmonics and spurs emanating from the µBITx.
It is still rather early to be sure that these issues apply to every µBITx produced. They certainly don’t apply to single-band BITx designs like the BITx40. Raj VU2ZAP suggests that his µBITx is pretty clean (except 10m) and has provided spectrum screenshots to demonstrate this. However, several have found issues with their boards. The issues seem to lie with design deficiencies (layout issues, relay switching design and the use of a double balanced mixer without specific BPFs) suggesting the issues can be overcome with future revisions to the µBITx main board, but will require hardware mods on the existing boards in service.
The spurs, as identified by Allison KB1GMX appear on SSB on higher bands (above 18MHz) as a result of unwanted mixer by-products and exacerbatedn by overdriven audio. There may be ways of reducing the spurs sufficiently by modifying the circuitry that injects the VFO output via the si5351a to combine with the 45MHz IF signal. Higher injection levels from the si5151a are being experimented with by Ian KD8CEC, but it is unclear as yet whether this approach could reduce spurs. Others are pursuing alternative hardware solutions, including incorporation of filters (BPFs) to replace the 0-30Mhz LPF following the mixer stage which is in circuit on both receive and transmit.
Several others are working on circuit board options and design suggestions that would address the odd harmonics (3rd, 5th, 7th, etc.) found particularly on CW. This involves reworking the final LPF matrix (four LPFs). This could be a daughter card that plugs into the main board or, alternatively could be a completely separate stand-alone circuit board. There is discussion about whether these should be plugged-in manually, switched using a standard wafer switch, or be relay driven, as in the µBITx design. Several options may well result.
If you are concerned right now about a fix for the harmonic generation on CW, you can use outboard LPF filters. For example, the filters that Hans G0UPL has a 5 LPF board kit available for sale, and also sells individual filter kits for each band. Minor changes to firmware would be required to select the correct filter for each band. You would also need to rescue some additional ports (e.g. by using an i2c display). Alternatively you could insert an LPF in the antenna line and change it out when you change band.
You can use Hans G0UPL’s filter board for switching in and out Band Pass Filters as well. These are best used to replace the existing 0-30MHz LPF. While you may be kept busy for two or three evenings winding toroids, the solutions to these problems already exist. You will end up with a cleaner transmitter and receiver after doing this.