ND6P Volume limited and distortion reducer

ND6P came up with this mod after testing his uBITX on the bench to determine how much signal it could handle before distorting at the speaker due to clipping of the audio signal.  What he found is that RF input signals greater than S9+5dB (-68 dBm) become distorted due to the audio output clipping on the negative side.

What the mod does is insert enough attenuation at the RF input to prevent the audio from clipping for signals stronger than S9+5dBm. Signals S9+5dBm and weaker are not attenuated. He’s tested with RF input as strong as S9+40dBm (-33 dBm) and gets a clean output.

He says he can now listen to a weak signal without having to reach for the volume control when a strong signal comes on. So effectively this is an automatic RF attenuation circuit to provide AGC functionality for the uBITX.

Points connected to the volume control are in parallel with existing wiring.

The pin diodes (MA27B) are available at RF Parts Co.


Having fun with the uBITx

Woody KZ4AK tell us that he is very happy with his µBITx.
He has completed many of the board mods, added RX amp/attn/agc, changed and added to the RX audio chain, added forward and reverse power meter & S meter, VOX, sound card interface, and fan.
Woody says the TX power is still about 6db down from what he expected and he plans to address that later.  He is achieving  about 3 watts out on 80m SSB which is a bit of a challenge, but it works!  On the other hand he is getting near 10 watts on 30M FT8.

Most of the details are in two ZIP files  that can be downloaded.  The uBITX is at the top of the list.


RF preamp

Woody KZ4AK says “My µBITX has always been a little deaf (couldn’t figure out why) so I built a RX front-end addition”.

Woody also wanted AGC so it became an amplifier followed by an attenuator. As with prior AGC / RF Gain mods, it inserted in the RX antenna trace (Ripley, et.al.). As built, it provides a maximum gain of ~ 8 db (+12db from mmic – 4 db from atten) with a 3.5 db NF, which can then be attenuated to more than (about) 50 db down.  Woody has not measured this exactly… but it really wakes up the RX!

Attenuator Control: >5 volts for minimum attenuation, approaching zero volts for maximum attenuation (at 22 to 5 ma, depending on attenuation setting). For manual control would use a potentiometer and voltage follower for current buffer.

Woody made no attempt to minimize the current draw. It is a bit of a pig, consuming 70 ma + control current (22ma).  Also, no attempt was made to minimize the board physical size.

The circuit uses a MAR3 SM MMIC followed by a HSMP-3816 pin diode array attenuator.

If anyone is interested,  he has a to-scale PDF for the board layout, a parts placement image, full resolution schematic, etc.  Woody can ZIP up a package for you …

He is also currently working on an audio derived AGC amp/controller for this thing. Have it sort of working, but he is not happy with the performance yet.


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:  Kit-Projects.com


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 uBITx.net 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.”