Category: uBitx News

The number of posts on the BITX20 IO Groups list hasn’t gone down very much over the last month,  but the ideas that swirl are all about potential fixes to the µBITx spurs and harmonics.  And a new problem has been identified … very poor intermodulation results.

The µBITx is simply not putting out the nice pure signals we would like.   We are now trapped in a pattern of waiting for the  “magic bullet” solution to our woes, but sadly this seems unlikely to result.   There are just too many issues that need to be resolved.

Somewhat understandably after months of exciting developments, new contributions from constructors with substantive mods and hacks making the whole package sing are now no longer emerging.  We had become somewhat used to regular exciting new updates on processor or screen additions, or simple fixes to niggly issues (such as low audio, or distorted audio, or the annoying pop on toggling between RX and TX), and now we are becoming a bit more despondent about those more serious problems of compromised output.

Some progress is being made in solving spurs and harmonics.  We know that the built-in LPF filters and relays in the output stage can simple be disconnected and bypassed by adding a new external filter set (e.g. using Hans G0UPL’s filter board and filter kits for LF through VHF) on the antenna output.  After doing this the rig should work just fine on CW operations on all bands.   Of course, it may need either a hack to the firmware to get automatic band switching to work, or we can just use a rotary switch.

Alternatively, there has been work on modifying the filters in situ.  By changing the relay switching logic (e.g. mounting the relays under the main board and rewiring the switch points) and at the same time adding a second set of relays on an add-in daughter board we can restore effective functioning of the existing LPFs.   They work fine, but the switching system creates unwanted blow-by.  However, this involves quite a bit of hacking on the main board that many constructors may baulk at.

And none of this solves the issues with the spurs on SSB on 15m and above, that requires yet another fix of a second filter board at the output of the 45MHz mixer, and then there is the probably unrepairable issue with intermodulation products are probably way above what would normally be expected.

So where does this all leave us?   We have had a lot of fun on the journey to date.  We all acknowledge that most of the fun has been in the journey, but what’s the point in a journey that has no end?

Should we wait a bit longer hoping for a fairy godmother and a magic bullet solution to appear?   We  could spent more money to acquire the new QRPLabs transceiver  kit when it comes out (hey, all bands all modes and around US$150!)?    However, we already know that Hans G0UPL won’t release the firmware source code – therefore, there will be no add-ons and feature enhancements unless they come from Hans.

We could now just bite the bullet and hack our µBITx to turn it into a CW rig?  But it is supposed to be a phone rig as well.

Hold in there a bit longer, the “magic bullet” may eventuate.  Who knows? And I am sure there are plenty of other projects under the workbench for you to get on with in the meantime.   At this QTH there’s a couple of mag loops under construction (Teensy controlled) and a bench power supply project …

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.