10w linear amp from QRP-Labs now available to purchase

Hans G0UPL who operates QRP-Labs has released his 10W HF Linear Power Amplifier.  This kit comfortably produces 10W from a 12V supply.  It is a compact design with huge heatsink included, which will not overheat even on continuous 100% duty-cycle operation.   The amp provides 26dB gain with +/- 1dB gain flatness from 2 to 30MHz.

This 10W HF Linear Power Amplifier kit has no Surface Mount Components (SMD) to solder. There are a number of small transformers to be wound, and assembly requires care and patience.

The push-pull driver stage uses two BS170 transistors in the amplifier design used in the SoftRock transmitter stage. The final uses two IRF510 transistors in push-pull. Yes, this humble low-cost MOSFET really is capable of excellent performance all the way up to 10m band and beyond! Short lead-lengths and PCB layout are extremely important, they are the key to success.


  • 10W output from 2 to 30MHz, using 12V Supply
  • Generously-sized heatsink, will not overheat even on continuous 100% duty-cycle modes
  • 2-stage amplifier provides 26dB of gain
  • Push-pull driver and push-pull finals, for high linearity and low harmonic content
  • +/- 1dB gain flatness from 2 to 30MHz
  • 4dB down at 6m (50MHz) and 8dB down on 4m (70MHz)
  • Standard 50-ohm input and output
  • Through-hole plated PCB, all through-hole components (no Surface Mount Devices)
  • Standard inexpensive components throughout
  • Tested for 1 hour at full-power 10W, 100% continuous duty-cycle with no forced air cooling
  • Tested for 15 minutes at 20W, 100% continuous duty-cycle with no forced air cooling
  • Tested at 20V supply
  • Tested into open load, shorted load and various mismatches without instability (oscillation)

As Hans advises on the BITX20 I/O Groups list:

I am not suggesting that this amplifier will solve all problems and without wishing to be offensive, the “garbage in, garbage out” rule always applies – in particular, if you feed it a signal containing spurs, harmonics, or other problems… then you will get the same thing at the other end, just amplified 26dB. It also still applies that all stages leading up to the final amplifier must have flat gain. If the pre-drivers cannot provide a big enough signal then you won’t get full power output.

But, it could be useful or interesting, in the BITX context, to look at the details of this amplifier kit and see the short lead lengths, symmetric PCB and extensively “stitched” groundplanes.

For further information see the product on Hans website.


Teensy audio board schematic and simplified audio “board”

Dr Flywheel (aka Ron N7FTZ) has provided some details of the Teensy audio add-on board.  This is used in the upcoming JackAl add-on board expected to be available shortly, but can almost certainly be home-brewed by savvy constructors using the Teensy 3.6 and audio add-on board.

Here is the circuit diagram for the PJRC audio module:

The CODEC part is very much the reference design provided by the part manufacturer. The support for the EEPROM and the SD card is optional.

The libraries will be compatible with any SGTL5000 or for that matter a long list of other I2S compatible CODECs (look at the source code). If you are homebrewing an audio processor on the Teensy you should consider integration of the CODEC. The libraries will support the internal A/D-D/A of the Freescale ASIC with very few components (resistors, capacitors) added.

Sample resolution of 12-bits is sufficient for the type of audio DSP that we need, though the higher bit-rate CODECs are much better. It is a matter of cost effectiveness and board space.

An alternative simpler approach


Below shows mono circuits for a lower grade Teensy, however the circuit is the same for the Teensy V3.6 with the corresponding pins.

The input circuit (mono):

Here is the output circuit (mono):


New 5″ display for the Biteensio processor board

Jim W0EB latest µBITX has been  built with a 5″ RA8875 type Colour TFT display running on the BITeensio Card.

The system shown can be controlled by touching the screen, with an external USB Keyboard or an external USB mouse.  This almost completely eliminates the need for front panel controls and helps make a very clean looking µBITX build while keeping full functionality.

In using a 5″ or larger display, Jim and Ron have run into several problems . The 5″ display draws almost half an amp from the 3.3V rail and the even larger 7″ displays draw even more current. Using the 5″ display requires the BITeensio card to be modified to provide a high current, on-board 3.3 volt regulator as the Teensy 3.6’s on-board regulator would be severely strained trying to power this display.

In doing this we build the new 3.3V 1.5amp  regulator by isolating and using the pads for the 5 volt I2C bus on the card.  We do lose 5 volt I2C capability by doing this but the 3.3V I2C bus is still available so 3.3V I2C peripherals can still be used.

Another disadvantage of the larger display, other than its higher cost and higher current requirement, is that a fan is needed to cool the regulator’s heat sink. This makes retrofitting the bigger displays very difficult to say the least.

The Triumvirate Skonk Works has no plans at this stage to support any colour displays larger than the 5″ one.  Even this display is only for experienced experimenters, because of the mods that need to be done on the BITeensio card.

We’re also recommending that using the 5″ display be considered ONLY with a new BITeensio build rather than modifying a previously constructed one as there is much less chance of messing it up this way.

The modification instruction manual, with pictures is located in the “Documentation” directory of their website under the “Files” section. Like the previous 2.8″ Colour version and our older 4 Line I2C LCD display version, this version is capable of utilising an optional external USB keyboard for rig and CW control. Mouse control is only available with the Color TFT versions.

Rig control now allows use of a mouse for almost everything.   Purchasing an inexpensive, wireless keyboard/mouse combination will allow simultaneous keyboard and mouse operation. Using the keyboard or mouse for control, you won’t have to even touch the screen unless you want to. This will reduce wear and tear on the touch panel.

All documentation for modifying the BITeensio card and the just released firmware for the 5″ display version (ubitx_5_0_T_V1_00R.zip) is available on the TSW website in the files directory.

The 5″ display we use is the “ER-TFTM050-3 from Buy Display dot com.  If you decide to order one you need to get it with the 4 wire SPI interface, Resistive Touch Screen, 3.3V power option (5 V won’t work) and the ER3304-1 font chip. The SD card option is not  necessary  as all SD card  operation is contained  on the Teensy 3.6 itself.

Ordering information for the BITeensio kits is also available on the website and kits are in stock.


Q90 failure and prevention measures


Lightning took out Q90 on Mike K5ESS µBitx.  It also fried the switching power supply he was using.   Others have reported similar frying of Q90 due to lightning.   

Doug WA8UWV suspects any strong signal or static pulse on the antenna during receive will damage Q90.  He has another ham 2 houses away and DX Engineering is also located near by.  He contemplated a reverse connected diode across the base and emitter of Q90  to see if it eliminated failures.

Gordon KX4Z has speculated before that the failure mode is a REVERSE voltage on the base-emitter, as the series capacitor charges up due to rectification by the base-emitter junction.  This document on pp18-19:


seems to suggest that indeed, overloaded reverse voltage bipolar junctions fail SHORTED, while forward current would be expected to OPEN them.  The reverse diode would eliminate the charging of the series capacitor and protect the junction against reverse voltages, and someone tested this for ill effects and found none.

Gordon has put the reverse diode on his µBITx and similar radios and has had no failures as yet.


Plot of the fall-off in gain from Q90 through to the finals

The following diagram from Henning DK5LV provides an insight into the final four stages of the µBITx PA chain.  This stretches from Q90 through to the finals.

Warren WA8TOD has measured the gain and drive requirements of the transmit chain at operating frequency from Q90 through the finals.  A sweep generator was inserted at C80 and  the level adjusted first for a nominal 1 watt indicated on the wattmeter (Purple trace) and then five watts indicated (Blue).

– Gain of the chain varies from 56 dB at 3.7 MHz to 52 dB at 28.2
– Input levels at C80 were -24 dBm (purple) and -15 dBm (blue); note 9 dB input increase generates 7 dB output increase….. 2 dB com

pression and resulting unacceptable IMD (not shown)


Video on the JackAl system

Jack W8TEE and Al AC8GY have been working on a display replacement for the µBITX for quite some time. They now have a preview video that gives an overview of the JackAl system.

There are some software hiccups in the preview that should be fixed soon. We just sent Rev 4.0 of the JackAl board off to the PCB fabricator.  They hope that this will be the final version of the board. All SMD’s will be included on the board.

JackAl makes use of either 5″ or 7″ 800×480 displays (US$34-$44) using the Teensy 3.6 microcontroller (US$30) and its companion audio board (US$14) for the DSP.

The Teensy has 1Mb of flash memory and 256K of SRAM, or which they are using less than 20% and 15%, respectively.  Jack and Al have also brought out a dozen “empty” pins for experimentation, so along with the I/O pins, there plenty of resources left to play with.

Pay particular attention to the ALS Tuning it uses. It really makes it so much easier to zero in on a station. Details are all in the video.


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.

IMD using an alternative MMIC amplifier

Warren WA8TOD has now tested the MMIC based wideband amplifier board from SV1AFN https://www.sv1afn.com/wideband-mini-amplifier.html 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).


Details of the anti-spurs mod (added 45MHz filter)

The spurs can be conquered by a relatively straight forward modification.  The circuit diagram for the mod is shown above and the photo below:

The filter is a 45MHz 15Khz type filter identical to the existing roofing filter in the µBITx and replacing R27 in the original circuit.   Warren WA8TOD encapsulated the filter, transformer, and capacitor in shrink wrap and connected the ground of the filter to the nearby junction of R13 that is grounded.

Warren’s implementation of the filter uses a single transformer on the output of the crystal filter.     A capacitor is also needed between the low impedance transformer output and the center tap of the mixer transformer.  He soldered a 0.1 uF SMD, one side to the output pad of R27 and the other side served as a connection for the transformer low impedance output.  No doubt a through-hole component could also be deployed.

The filter input is attached directly to the other R27 pad.  Warren elected to not use a second transformer because the source is much closer to the filter impedance than it is to 50 ohms.

In Warren’s case the transformer does not dramatically reduce the spurs versus the bare filter. The primary effect, and one that is desirable, is that a much more reasonable level of audio drive is available. The radio is still susceptible to intermodulation distortion because the levels are increased closer to the stock configuration, giving only slightly reduced power output.

Other implementations of the anti-spur filter

A number of others have now implemented the anti-spurs 45 MHz filter and demonstrated that it reduces spurs.   uBITx.net believes this modification should be included in the production of the µBITx immediately, in order to clean up the output on SSB.

Glenn  VK3PE has made his own board to mount the components for the filter mod on, and Nick VK4PP has designed a board (which will be for sale or included as a freebie with his LPF board).  Photos of their board designs follow: