CW Zero Beat Kit

David  N8DAH has been working on a few projects.    The first of these is a CW Zero Beat

If anybody is interested in a CW Zero Beat addition to their µBITx, then he  has nine in kit form and one built for testing available.   The kits are US$13 shipped inside the US.  Add actual shipping costs outside Continental USA. He can build them for you for $2 extra or only solder the surface mount and allow you to build your own.

J1- Power in 9-12V

RV1- Audio in adjustment

J2- Audio in

RV2- Center Freq Adjustment

Questions please e-mail David direct

David is also working on a new digital controlled audio level project (AGC) and hope to have more info soon but this is still only in the prototype stage.

David’s website is:


Further information about fixes for Harmonics, Spurs and IMD

Removing spurs

Raj VU2ZAP has confirmed that surface mount components can be used in the LPF after the 45MHz filter.

He used SMD Yellow shielded inductors 331nH  x 2 + 51pf to ground.

Summary of recommended fixes to address unintended spectrum products

  1. Replace K1, K2 and K3 with Axicom 12v relays (low current models are fine) to remove harmonics to acceptable levels in most rigs on most bands.
  2.  Install a 45MHz filter across TP13 (v3 board).   Ashhar Farhan VU2ESE inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board.   This is on the output of the 45 MHz IF amp going to the front end of the mixer.   The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).   Or use Raj’s alternative as above.
  3. Consider also changing out L5 and L7 with shielded surface mount inductors (yellow type).  See this article.


G4USI Go Box

Several constructors have put their µBITx in a Go Box for portable/emergency use, but this is one of the best looking results yet.

Daimon G4USI has used a 3D printer to produce a very professional looking front panel.

This is a re-mix of DU2RK’s uBitx Case, and the re-worked front panel of this case by AngelDMercedes.

Daimon wanted a different case, one he could build into an existing flight case to create a Go-Box for HF.   He remixed the ideas above to create a case which fitted his flight case perfectly, but with every control and function sitting on the top panel.

In the strictest sense this is not a full case. It is a top and two vented sides. There is not a bottom, front or back panel –  the flight case provides the structure.

On the top photo you can see a 12v 5a power supply and mic in the space to the left of the rig.  Daimon now has a home-brew EFHW multi-band antenna, SOTA Beams ATU and miscellaneous portable QRP operating items in the right hand compartment.   Everything, in fact, that he needs for portable QRP HF.


A uBITx alternative: the BITx40

It is a while since featured a story on the BITx40.

Jerry KE7ER posted the following on the BITx20 IOGroups list:

“Most traffic in the forum is now all about the uBitx, a very capable rig for all of HF.  Those put off by the need for mods to reduce harmonics and spurs on the uBitx should consider the simpler BITx40.”

Usable out of the box, only issue is that the stock firmware shifts around occasionally by 50 Hz due to noise when reading the tuning pot
with the Nano’s Analogue to Digital Convertor.

Jerry recommends Allard’s basic BITx40 firmware, requires no mods, fixes the operating frequency drift issue, adds a number of other new features. Some minor hardware mods are optional, adding the Function switch is a good idea.

If so inclined, Allard’s  bitx40_raduino_2  firmware adds even more features, though does require some minor mods:

Jerry notes that the Bitx40 is a very good deal at US$59.  Being a single band 40m rig it does not have the complications that come from the wide band approach of the µBitx.

Bruce KC1FSZ is also a fan of the BITX40/20 architecture and has had good luck on other bands. One thing he did was to create a “mainframe” of the core of the design (see the manhattan breadboard style construction above) that starts right before the first mixer and ends right after the product detector/balanced modulator.

In his experimentation it seems like 90% of the modification are happening in the “peripherals” around the core (different LC filters, different PAs, different bands, different audio/mic configurations, different software, different enclosures, etc.) so I am able to cover more ground without needing to re-build the inside of the inside every time.

Bruce’s core uses two SI5351 ports so the BFO is tunable just like the VFO making it easy to fool around with different IFs, filter widths, etc.  He also based the chain on the W7ZOI TIA amp which seems to improve performance a lot. There is a bit of the uBITX design incorporated  in his bottom up construction.

The crystal filter is on a daughter card for easy swapping in and out .  He is using ADE-1 mixers on both sides, which are easily and cheaply obtained on eBay, and adjustable gain VFO/BFO buffers (a la N6QW LBS design). The core is 100% symmetric so it’s quick to build and test.

MVS Sarma notes that “try using the  3 transistor bi-directional amp version and see the performance difference. A friend has indicated that the noise level comes down.  You could even try with single dual gate MOSFET in each direction.”


Important  Information provides help for constructors:  Preventing catastrophes and providing guides, fixes and modifications for your µBITx.  We are an information site only and not associated with HF Signals.

WARNING:  As shipped, your µBITx  probably will not be compliant with emission requirements.  It has been found to generate spurs and harmonics.  There are now simple fixes available to address these issues.  Make sure you apply them prior to operating the transceiver on an antenna.

CAUTION : If you power up the µBITX without the pull-up resistor the µBITX is likely to go into CW transmit. Make sure you install the 4.7k pull up resistor on the CW key pin!

v3 Board?  – There are known issues with WX brand TDA2822 chips (U1): Read this.  Earlier UCI and later socketed TDA2822 are not affected.   You will probably want to do the audio pop mod at least.

v4 Board? – If your audio is distorted see this article.  Note that audio output is reduced over the v3 board.  If you use a low impedance speaker you may need an additional amplifier module (LM386, TDA2822, etc.) to get adequate volume. The audio pop mod is not required on v4 boards.

SHIPPING:  µBitx orders have been shipping within one or two working days of being ordered. You should receive a shipping notification through PayPal once your product has been shipped. Estimated delivery times to different countries:

EU:         IndiaPost: 2-5 days  DHL: 2-5 days
US:         IndiaPost: 10 days     DHL: 2-5 days
Asia:      IndiaPost: 2-4 days   DHL: 2-4 days
AU/NZ: IndiaPost:10 days      DHL: 5-7 days

BITX QSO DAY:   Every Sunday – 3PM & 7PM Local Time – 7277 kHz in North America, 7177 kHz elsewhere.

A spur fix and BFO leak fix from Ashhar Farhan

Spurs fix (See update below)

Ashhar Farhan VU2ESE, the designer of the µBITx has been experimenting with solutions to the spurs on SSB mode above 18MHz.   He has posted to the list that one spur fix that is now consistently working for him, and that is easy to apply, is to fix the distortion in the 45 MHz amp.

He inserts a 0.3µH inductor in series with a 10 pf capacitor across TP13 on the v4 board.   This is on the output of the 45 MHz IF amp going to the front end of the mixer.   The inductor comprises 8 turns on a T30-6 toroid (to give 0.3µH or 300nH).

UPDATE But wait … he proposes a better spurs  fix a day or two later

1. Change the resistors R26 and R46 to 220 ohms (from 470 ohms). this increases the darlington pair’s standing current and decreases the Harmonic distortions.

2. The 90 MHz trap was difficult to tune unless you had a spectrum analyzer. Instead, we can use a Low Pass Filter instead. A ‘T’, rather than a Pi is used. This helps improve isolation on both the RX/TX path.

3. The LPF consists of an L-C-L of 300nH, 47pf, 300nH.

Removing the BFO leak into the local oscillator

Ashhar also found that there is cross talk inside the Si5351, whereby the 12 MHz output from the BFO leaks into the local oscillator.

Ashhar used  a 5µH inductor in series with a 47 pF capacitor placed across C202 of the LPF suppresses this. The inductor comprises 35 turns on a T30-6 toroid (to give 5µH).   However,  another constructor found this blanked out the 30m band, and calculations reveal that there may be an error with the calculation of inductance or capacitance that is waiting to be confirmed.  The filter should be tuned to 12MHz suggesting a slightly small capacitance value of around 35pF will be looking for verification from constructors that these mods represent a definitive fix for the spurs and carrier leak problems.


Using a USB hub for a single USB cable to your PC

Nigel G4ZAL has tweaked his ubitx so that he has an ‘all in one’ rig for FT8 and other digimodes, including CAT control (FT-817 emulation).

To achieve this, he added an un-powered 4 port USB hub inside the rig and used a cheap USB sound card (discarded casing shown to right side of the pics below).  He cut the USB cable and hard wired the USB hub from the back panel and into the Raduino via the USB hub.   He didn’t connect the USB 5v power into the Raduino).   He also hacked the USB sound card so he could wire it directly to the corresponding MIC and SPKR wiring (used an old PC CD-ROM audio cable).

He is using the latest firmware KD8CEC in the ubitx and now has CAT control and sound  over a single USB cable to his PC/laptop.

Using the hub, he can also upload new firmware without opening the case.

In response to a question as to whether isolating transformers are required, Nigel noted that he had built a couple of devices like this and never had any issues requiring isolation transformers (He has some, but he never fitted them as the mod worked fine as is).

He used a USB cable “Mini USB 5pin male to female with screw panel mount extension cable” from eBay.   Nigel’s was wired incorrectly as the colours did not match the USB standard.  He had to wire according to how the manufacturer had made it.  He cut the cable to suit and hard wired it to the USB hub removing the original connection.

The USB hub was made by Startech, but any hub that can fit inside your enclosure should work.  Nigel removed 2 of the USB sockets on the hub and hard wired the stripped down USB sound card.   He removed the USB connector and 3.5mm audio/mic sockets and soldered these connections direct to the hub.   He also added connector pins for the audio/mic (CD-ROM) cable.  He went to these extremes so that the hub and sound card would all fit inside his enclosure.  If your enclosure is larger they can all be simply plugged together.

The USB sound card can be found on the famous auction web site. Nigel’s was an “External Virtual USB 3D Sound Audio Card Adaptor Converter Mic/Speaker PC Laptop“.   Nigel hard wired the Raduino USB side of the connector to the hub as well, but he didn’t connect the 5v power line (the Raduino is powered as normal).


The cause of spurs

Ashhar VU2ESE and Raj VU2ZAP have nailed the cause of the spurs.  The 45 MHz  IF amplifier is distorting and producing a second harmonic at 90 MHz,  which mixes with the local oscillator to produce a spur at 90 MHz.
Here is an example of how it happens :
  • the spur moves down as you tune up.
  • when the radio is tuned to 28 MHz, the spur is at 17 MHz.  When the uBITx is tuned to 28.150 MHz, the spur is on 16.850 MHz.
  • At 28 MHz, the local oscillator  is at 73 MHz.
  • There is a signal X such that :

X – local oscillator = 17 mhz

Local oscillator = 73Mhz.

X – 73 = 17,  so X must be 90 MHz.

After checking the spur on a number of frequencies between 15 MHz and 30 MHz, it was confirmed that the above formula works consistently to predict the spur.
Now to confirm a simple solution …  if the problem is in the 45MHz IF, then it may also be possible to address the IMD at the same time.