Summary update of key uBITx issues

Despite the strengths of the µBITx (a low-cost all HF band single-board SSB and CW transceiver that is readily modifiable) it has been found to be wanting in a number of critical areas.   These include:

  • poor carrier rejection on SSB
  • IMD that may be viewed as unacceptable at -12dB, that can result in poor audio quality on TX and potential splatter
  • spurs on SSB transmission on all bands above 18MHz
  • harmonics on both CW and SSB on most bands (worse on low bands) generated through board layout issues in the LPF filter stage
  • poor output levels on higher bands due to roll-off in performance of TX stages.
  • lack of any AGC
  • loud audio pops on RX/TX and TX/RX transitions
  • unreliable CW keying with stock firmware

Today is a break-through day. We are much closer to having a handle on solutions for all of these issues.   This is because we now have a good understanding of the causes of each problem, and proven solutions exist to solve each problem. 

Some problems were identified and solved early on.  Others have only just been clarified and solutions put forward.

CW keying issue

CW keying issues were identified as soon as kits were beginning to be assembled around Christmas 2017.   Keying issues were, however, fixed with a simple firmware solution, thanks to Ian KD8CEC back in January 2018.   Other hardware/firmware solutions were also developed, but Ian’s firmware has been very widely adopted by µBITx constructors as the lowest cost solution.

Audio pop issue

A solid audio pop mod took a bit longer to be adopted, but the first audio pop mods started appearing in January 2018.  Wayne VA7AT’s pop mod became popular when  Kees K5BCQ starting selling a surface mount kit in early May 2018.  He sold more than 800 of the audio pop mod boards to constructors.   An even simpler solution was found to work  and the manufacturer of the µBITx (Ashhar Farhan VU2ESE) took on the modification in June 2018 and included it in his v4 board.

Lack of any AGC (or even an RF gain control)

The lack of an effective AGC has plagued all of the BITx kits over the years.  However, a working solution from ND6T in April 2018 offered a much larger dynamic range, and has been widely adopted by constructors.  The kit version provided by Kees K5BCQ was a key reason for widespread adoption.  It is disappointing that the manufacturer has yet to include this in the production board.  It is not a luxury to have AGC.

Poor output levels on higher bands

The promise of 10 watts output on all bands except 10m was never going to be realised in the µBITx.   Getting near even output on all bands probably requires a combination of solutions.   A solution  involving different RV1 (driver gain) settings was implemented back in February 2018 by Bill K9HZ (illustrated above) using a series of relays driven off the transistor drivers for LPF selection.    Allison KB1GMX provided a solution in June 2018 that addressed the root cause of the problems – declining gain with increasing frequency in the various driver and pre-driver stages.  The transistors used in the µBITx are inadequate for the job, but can be replaced with 2N2222A (in a metal TO18 case) to achieve a much flatter response across most bands except 10m (where output can be increased to at least 4 watts).

Harmonic output issues

The µBITx has four Low Pass Filters (LPFs) in its final output stage.  These are switched using a complex relay arrangement designed to use only 3 digital I/O lines.   The filters themselves have proved to be well designed and would have been acceptable except for the layout issues associated with the relay configuration.

Solutions have been under investigation during August and September 2018.  There are two key design ideas:

1) reroute the signal path on the main µBITx board to simplify the RF path using the existing relays on the input of the LPFs, and add an outboard relay switching board for the outputs only.  Testing of this approach was led by Gordon KX4Z who has a board available (in small numbers) to try out.

2) remove the LPFs from the main board and rebuilt them on a daughter board.  Several have developed boards to achieve this, including Nick VK4PP and Kees K5BCQ (who has two boards:  one with six relays the same size as those on the µBITx and one with http://ubitx.net/2018/09/10/external-lpf-filter-board-from-k5bcq/).

Spurs  on SSB modes above 18MHz.

The µBITx produces spurs on SSB on frequencies above 18MHz.  This is due to mixer products from the 45MHz local oscillator combining with the intended transmit frequency and which are reflected back into the mixer.   The LPFs remove spurs at frequencies below 18MHz, but not above this frequency since the spurs are below the intended transmit frequency and cannot be trapped in the LPFs.

The solution is somewhat straightforward and involves the removal of one part (R27) and replacement by a 45MHz 15Khz filter, a transformer, and a capacitor.  This has been shown by several constructors to reduce the level of spurs below the required -43dB.

uBITx.net would like to see this appear on v4 production boards as soon as possible since it does not require changes to PCB production.

Poor carrier rejection on SSB

Poor carrier rejection has been identified as an issue by several constructors (and from immediately after the release).  This is the result of poor board layout, with the LO frequency leaking back in to the signal after the balanced mixer that removes the carrier.  No solutions are likely in the near future (although there is always hope).   This needs to be addressed by the manufacture prior to the release of an updated board as it is primarily a result of board layout issues (paralleled tracks that “leak” carrier into the mixer product.  Poor carrier rejection is unlikely to be a drop-dead issue for most constructors.

Unwanted intermodulation (IMD)

IMD in the µBITx has now been found to be the result of over-driven bi-directional amplifier stages (Q20-22 and Q40-Q42).   There may be more than one solution here, but substitution of these stages with an MMIC removes more than 10dB of intermodulation products in each mixer (20dB overall improvement).

Workable solutions that replace or improve the two bidirectional amplifier stages should be forthcoming in the next few weeks.

HF Signals should now be able to address all of the above issues in a v5 release

uBITx.net believes that now that the causes of these issues have been verified and solutions to all but the carrier rejection issue have been identified, it will be important to address the issues in a new v5 release.  The investment in changes to board production are likely to be hugely beneficial to the manufacturer as many constructors will repurchase the kit even if the price point moves up slightly.

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