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

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. 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 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.

IMD using an alternative MMIC amplifier

Warren WA8TOD has now tested the MMIC based wideband amplifier board from SV1AFN in two different IF stages in the TX chain.

First Test

This is a replacement for the 45 MHz transmit amp comprised of Q20 – Q22 on the µBITx main board.    Warren removed C20 and C22 and used two short lengths of miniature coax to take the signal off-board to a 7 dB attenuator and then on to the MMIC amplifier, and then back onto the board.

The resulting MMIC amplifier gain was +16 dB to match that of the BI-DI amp on the stock µBITx.

Test 1

The yellow trace is before the amplifier board was inserted and the purple trace after. 3rd order IMD was reduced by nearly 10 dB over the stock µBITx by using the MMIC amp.

Second test

Warren restored the Q20 – Q22 amplifier and moved the MMIC amp to the Q40 – 42 amp with similar results. Here he found that he required the full 23 dB of gain provided by the MMIC to achieve the same level of main signal.

Test 2

Yellow and purple traces are as before with the new measurement indicated on the blue trace. Results are almost identical indicating replacing these two amps together would provide 19 – 20 dB of IMD improvement which would make the transmitter completely viable and, in fact, better than some commercial radios in terms of IMD.

In both cases the indicated power out from the two tone test was a little over 4 watts.

Self-oscillation test

Warren also tested this MMIC board for its susceptibility to oscillation.  He connected a 60 dB attenuator between the input and output while feeding a signal into the input.  He then gradually reduced the attenuation one dB at a time until oscillation was visible on a 1.5 GHz spectrum.

The amplifier broke into oscillation very reliably when the attenuation was stepped below -14 dB. Higher than that and it was completely stable.

In the course of these two tests the board was hanging unshielded about 3 inches from the PA heat sinks and the output was a little over 4 watts. In both cases he saw no indication of oscillation.


Kees K5BCQ 6x LPF board now available for purchase

If you are wanting one of Kees’ K5BCQ 6x LPF boards (too big to fit in the corner where the existing filters are located) then Kees K5BCQ has posted an early LPF order status as “K5BCQ LPF Kit Orders” in the uBITX files section under his call.

A 4x board will also be available shortly which will:

  1. fit in the space cleared on the uBITX board
  2. select 3 of the relay sets with the TxA, TXB, and TxC uBITX drivers for 80/60m, 40/30m, 20/17/(15)m
  3. select the correct relay:  if any one of those 3 drivers is picked it selects the TXD driver which is driven by a 3 input diode dot-OR from the TXA,TXB, TXC inputs.  The TxD driver has the 4th 10/12m filter on  the NC contacts so you don’t have to have 1 LPF relay energized at all times and may not require K3 if it’s the highest band LPF, like on the present uBITX. …..haven’t decided yet).
  4. have an on-board replacement to K3.  This will require disabling audio M1/M2 and the audio pop fix if you have a v3 board.

Both the 6x filter and 4x filter boards are set up to accept the QRP Labs footprint plug-in LPF and BPF filters (1.5″ x 0.5″). This is to allow users to use those LPFs or design their own. The QRP Labs LPFs will handle 10W, the QRP Labs BPFs won’t according to Hans Summers. A blank” LPF board is also used to accept the existing LPFs transplanted from the µBITx board.

ALL the LPF components need  to be moved over from the µBITX board.  The toroids and capacitors are placed in the same locations on the blank LPF boards. The relays can also be brought over on the 6x filter board because it’s larger. 

The 4x LPF board has been designed to fit in the existing µBITX location and uses smaller relays (Omron G6H-2F type relays).


About the IMD and where it is being generated

Warren WA8TOD previously raised concerns about the level of intermodulation being generated in the µBITx TX signal path.   IMD may not be as pressing an issue as harmonics and spurs, but the µBITx seems to generate more IMD than it should.  This can cause audio distortion and splatter (including out of band transmission if you are operating near a band edge).   IMD only occurs in SSB modes, not on CW.  If you are CW operator you only need to address spurs and harmonics for which there are now solutions.

General expectation about intermodulation

The generally accepted limit for SSB intermodulation products is a minimum of 24 dB between the lowest of the twin tones and the highest of either the third order (products immediately adjacent to the twin tones) or fifth order (products next over above and below the third order) products.

Warren’s observed IMD

Warren’s µBITx shows -12 dB at 3.6 MHz. Conditions are: 30 mVrms audio input and RV1 set for 5 watts RF output through a 4 MHz LPF. His board has the onboard PA filters removed and strapped and has the additional 45 MHz filter with 12:1 output impedance transformer in place of R27.

Where is the IMD being introduced?

In order to better understand where the IMD is being introduced in the radio Warren started all the way back at the balanced modulator output and measured IMD at thoughtfully provided test points  (on the v4 main board) up through the driver output. In general, once IMD products are introduced at early stages they tend to only get worse as the signal progresses down the chain. The key to fixing it is finding the root cause as early in the process as possible.

TP17 is the output of the balanced modulator and the 12 MHz SSB filter. IMD products here were below the noise floor of the measurement configuration.

TP16 is the output of the first bi-directional amplifier and IMD at this point measured -35 dB, already much too high and and indication of non-linearity in the amplifier that must be addressed.

TP14 is the output of the onboard 45 MHz filter following the 2nd mixer. This actually shows a slight improvement but probably within measurement error at -37 dB. This measurement pretty much exonerates the 2nd mixer as a significant contributor to the IMD issue.

TP16 is the output of the second bi-directional amp and again there is a serious deterioration in IMD with the amp adding 11 dB to the problem.

TP1 is the output of the 2nd transmit mixer (labelled 1st mixer in the text) and is the first time we see a signal at air frequency of 3.6 MHz. The mixer added 5 dB of IMD to the total… too much and probably indicative of low injection levels as has been stated in the past. On the other hand, it is not the primary culprit by far.

TP3 is the output of the first pre-driver and of RV1 and it adds a little over 1 dB of IMD. The total IMD at this point is 5 dB less than the acceptable amount and it is only beyond this point that we are able to control power levels with RV1 which would normally be the adjustment point for controlling PA IMD. In other words we are starting out with an unacceptable signal from the low level stages and only now getting to where IMD is normally introduced. A contemporary radio would show normally show IMD levels at -45 dB or better at this point.

From this point forward, at the five watt level, the combination of pre driver, driver, and PA added 5 dB of IMD. This amount would be perfectly acceptable in most radios starting out with clean drive and would allow the total power to be increased by RV1 adjustment to significantly higher levels.

Cause of the IMD

The IMD problem is rooted first in non-linearities in both bi-directional (bi-di) amps and then in both mixers.  The focus is likely to be initially on addressing concerns about linearity in the bi-di amps (on TX only of course).

Possible improvements to the bi-di amps

Glenn VK3PE notes that the original article by Wes Hayward and Bob Kopski on bidirectional amplifiers shows a slightly different biasing arrangement and feedback in the first stage compared to that used in uBITX. As shown, gain is 15dB and flat to 100MHz within 1dB.

A more conventional resistive voltage divider is used and the feedback is AC only. Two extra parts are used. The article doesn’t mention IMD though specifically, as a performance target or measure it.

Glenn has plotted the  gain difference between uBITX and Hayward versions of the bi-di amplifier after building a prototype.

There is a difference in input levels before output clipping occurs. The biasing arrangement also gives different Iq.  Gain is reduced about  4dB in the Hayward version over the uBITX. It accords closely with test results from Hayward’s paper of 15.5dB.  Glenn got 16dB of gain at 30MHz.

Haywards paper gives some values for varying the gain to other values also, so there is more experimentation to be done.

Henning DK5LV notes that the designer of the Bi-Di amp states his version is designed for 15 dB gain, which is why he has a series feedback system (680 ohms + cap) from collector to base, and the biasing is done with two extra resistors.

Ashar VU2ESE in designing the µBITx uses only the biasing and feedback with the two resistors.

Henning suggests that the result is that Ashar’s amp has about 20 dB gain, but the frequency response is worse due to the limited fT of the 2N3904 and input reflection (S11).   Due to the higher gain, IMD must be worse for the same input level. If the textbook curve applies (3 dB for every 1 dB of input power change) the IMD will be 15 dB worse.

Reference #1
Reference #2

Release of CEC firmware v1.1 (non-Beta)

Ian KD8CEC has released version 1.1 of his CEC firmware.  This the first major release since Beta version 1.097.    He has also released new versions of uBITx Manager (for Windows) and Nextion screen firmware.

Version 1.1 includes all additions or improvements from the last non-Beta release version 1.08.   This includes features and bug fixes addressed in Beta versions 1.09, 1.093, 1.095,  and 1.097.   No further changes have been made to the Version 1.097 Beta version.

You do not need to upgrade if you are using Version 1.097. This is the version with only the version number changed.

Major Changes since the last official release

  •  The firmware supports additional features for the Nextion  and TJC LCDs
  •  Read & Backup uBITX, ADC Monitoring, ATT, IF-Shift and more on Nextion LCD (TJC LCD)
  •  Factory Reset (Both Character LCD and Nextion LCD are applicable)
  • Signal Meter using ADC (A7 Port)
  • I2C Signal Meter
  • Spectrum display
  • Band Scan
  • Memory Control on Nextion LCD (TJC LCD)
  • Speed Change CW-Option on Nextion LCD
  • Fixed Band Change Bug (Both Character LCD and Nextion LCD are applicable)
  •  uBITX Manager removed the Encode and Decode buttons. The procedure has become a bit easier.
  • I2C Device Scan on uBITX Manager ( Both Character LCD and Nextion LCD are applicable)
  • Si5351 I2C Address can be changed
  • Recovery using QR-Code Data from Server
  • Nextion LCD and TJC LCD can display Spectrum and CW Decode (using Stand alone S-Meter)
  • Fixes for other Minor Bugs

Please refer to the link below for details.

Please download the related files from the link below.

Nextion LCD or TJC LCD’s GUI have not changes since the 1.097 (Beta) distribution. If you are using a different GUI that is customized by other helpful developers, you should not need to upgrade the firmware.  Some screen sizes are still not supported in ver 1.097 (and therefore in version 1.1).

For further details see Ian’s blog at

External LPF filter board from K5BCQ under test

Kees K5BCQ has received his first set of boards. These boards will fit in the space currently occupied by the on-board LPFs.   The piece jutting out and marked “WOOF” is the space for the TX/RX relay (was K3).  The filter parts need to be transplanted from the main board onto small PCBS that are designed to plug in to the board as illustrated above (they are the same size as QRP-Labs filters).

Kees has already made some modifications for better wireability as follows:

1)   Eliminate R3 on the uBITX board and instead add it to the LPF Relay Board.  This frees up space on the µBITX main board and allows more efficient RF wiring for receive. Since the “old R3″ also switches M1 to M2, this requires modification of the board to short M1 and M2 and insertion of the audio pop mod on a v3 board (a v4 board already has the anti-pop mod included).

2) Replace 3 sets of dual 2N3904 Transistors in the PA driver with 3 sets of dual 2N2222A Transistors. Allison had suggested changing the emitter resistors from 22 ohms to 10 ohms (for dual 2N2222A’s).

3) Add a three 1N4148 diode-OR from TxA,TxB, and TxC driver inputs to a TxD driver to pick that relay, which removes the 10m/12m/(15m) LPF when inserting the proper (A/B/C) LPF during Tx. Yes, that means 4 relays are active during TxA,TxB, or TxC transmit. All four of the LPFs use the existing inductor/capacitor components off the uBITX board. They are just moved over to a small 1.5″ x 0.5″ bare LPF board (that footprint is also the one QRP labs has for their LPF/BPF filters, but they don’t sell blank boards.

4) uBITX Antenna wired to either one of two SMA connector pads.

5) T11 will be connected to the RF input on the LPF Relay Board with about 1″ of coax.

6) The smaller 12V 10 pin DPDT bifurcated contact relays can be found on ebay for about $1 each. They fit nicely.

7) Leave all the traces, just make a cut and remove 1/16” of the trace where needed.

[Photos of installation still to come]


Removing M1/M2 audio switching from Relay

In considering adding a new LPF module to replace teh original on board LPF matrix, it is helpful to remove the audio switching from K3.   This reduces RF signal routing on the uBITX board.  Many have found the signal routing via K3 causes RF to get into the audio so this represents good practice.

It is suggested that Audio M1 and M2 are tied together at R70, and does not  involve Tx/Rx switching them with K3. 

Power is Tx/Rx switched to Q70 on v4 boards, and the audio pop modification does the same thing on the v3 board.

The procedure is:

  1. Remove R70
  2. Bridge the front pad to the adjacent track
  3. Cut the adjacent track and short the relay sides of both tracks to ground.

Testing removal of spurs with additional 45MHz filter

The photo above shows an additional 45MHz filter (15khz passband) inserted in place of R27 (you can’t see the centre wire on the filter, which is attached to the ground end of R13).

Early indications are that this removes all of the offending spurs.   This will make it  a ESSENTIAL MOD.  The mod has, however, yet to be tested by

Warren WA8TOD has completed spectrum analysis plots for each band, and these have been reproduced below.  The plots show removal of all unwanted spurs.

Conditions for the test:

  •  eBay filter in place of R27. No other changes.
  • Audio input: 100 mVrms, 1.5 kHz tone. RV1 adjusted in each case for 2 watts output.

Yet to be verified:

  • 100 mV audio drive, without the filter in place, gave very unacceptable IMD performance.  It may well be in the case of the added filter that the stages preceding the filter have enough dynamic range to work at that level and it is simply compensating for the insertion loss of the filter itself. That can and will be confirmed with two tone IMD testing.
  • Listening to the recovered voice quality and decide if it is adequate.

Adding the filter has introduced low frequency rolloff for LSB and high frequency rolloff for USB. The change is less than 6 dB and may not be objectionable but that will be a subjective judgement.

28MHz results

24.9 MHz results

18 MHz

And a wider scan …

And finally, here is a wider span showing 15 through 10 meters harmonic performance.  Warren’s unit has the onboard filters completely removed so this scan was made with an external 30 MHz LPF plus the new 45MHz R27 filter.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting.

If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.

Comparison of CW and SSB power out using the added 45 MHz filter

The chart was made by adjusting RV1 to maximum key down CW power, and then keying PTT with an input tone at the specified level. There are a couple of caveats here:

1) 120 mVrms is far above the audio level that caused unacceptable IMD before the filter mod. IMD must be checked and the audio levels adjusted to make it acceptable.

2) 120 mVrms is also far above the output level of most microphones, at least without shouting. If IMD is bad at this level then the audio level must be reduced. Before the mod the radio showed terrible IMD at any input level higher that about 25 mV and, at that level, the radio produced less than 2 watts.

If it turns out the filter is a ‘magic bullet’ and the radio can actually sustain this level of input with acceptable IMD, then the input audio stages need more gain.


Why has KD8CEC’s firmware been so successful?

There are several alternative firmware versions available for use with the µBITx transceiver.   So why has the KD8CEC firmware been so successful with uBITx owners?

Some key reasons put forward by

  1. KD8CEC firmware is fully compatible with the standard issue kit.  No hardware changes are required to make it work.  This is a critical point of difference with all of the other variants, and probably the most important factor associated with the success of CEC firmware.
  2. No custom wiring changes are required when using the firmware.  This is a further significant factor in widespread adoption.
  3. The firmware fixes problems that come with the factory firmware – although some argue that CW modes are still not fully addressed in the CEC firmware.
  4. Using CEC firmware means no loss of features from the default factory firmware.   Other alternatives offer fewer or different features from the factory firmware.
  5. Users are familiar with the  user interface, as it reflects the default firmware’s “look and feel” with the standard display supplied with the kit.
  6. It is easy to upload a hex file to the Raduino. Constructors without a working knowledge of the Arduino IDE can upload files easily.
  7. All full releases of CEC Firmware are available as open source firmware.  This follows the same structure as the original code, although most of the code has been replaced.  Open source is not released for beta versions (and for good reason).
  8. No additional processor is required, unlike other firmware variants.  A mechanism for adding additional processors has been added in ver 1.097 (Beta).   This promises a future where multiple processor support will be available.  You won’t be locked into a single processor type.
  9. Addition of a Nextion display or additional processor is relatively straightforward.
  10. The firmware on the Nextion display can be edited by others to provide a different “look and feel” or to add or subtract features.   This is independent of the firmware for the transceiver.