Wednesday, July 10, 2024

Homebrew sBitx on 64 bit and Subthreshold Conduction Resolved

KK4DAS sBitx on 64 bit build by W9JES

The homebrew sBitx is now on the 64 bit version of the OS and software. Thanks to W9JES, JJ and team for their work on this upgrade to the factory sBitx Raspberry Pi image and sofware distribution. JJ and team have made some significant upgrades to the core - most important bringing it to the lates stable 64 bit release of the OS. In addition JJ has developed a suite fo add on applications that provide additional functionality. When I first booted it up I was not able to decode FT8 sigals in teh native sBitx app. JJ worked with me and we found that since I did not have a RTC clock installed the new version of the app was not using the correct time - and for the WSJT protocol to work the time must be in sync wiht UTC. He made the fix so the software will fall back to NTP if no RTC is installed and I am back in business! Thanks JJ.

While I was testing the 64 bit build the rig begain to exhibit another RFI gremlin - the Tx/Rx relay would begin to click randomly - sometimes one click at a time and other times it became a real chatter - annoying enough that I temporarily disconnected the connection from the Pi to the relay so I could get on with testing. I thought it was RFI from the Pi -- maybe related to the Pi's wifi - it seemed to get better when I turned off wifi. My build of the sBitx puts the rig in to Tx by putting a signal on a GPIO line that goes through a dropping resisitor to the base of an NPN transistor that turns on the relay. I suspected RFI was getting into the GPIO line. I put the scope at the base on it and I could see the voltage spikes that were triggering the relay. I put a .1 cap to ground where the line connects to resistor. That didn't help. I put ferrite beads on the line. That didn't help. I switche from a single piece of stranded wire to shielded coax. That didn't help. So I did what I often do when stuck on a thorny circuit problem, I asked my friend Pete, N6QW. Once again the Wizard of Newbury Park diagnosed my problem fromm across the country.

Here is Pete's response in its entirety:

"It may be a case of subthreshold conduction (a leaky transistor), 

I would do the following. 

Use a 2.2K versus a 220 Ohm. Get some small ferrite beads and slip those over the base lead and change out the 2N3904 transistor to a TIP31C.

Your narrative indicates it is a recent problem and you didn’t see spiking on the GPIO. The signs suggest a leaky transistor (or not). But the above steps are positive and should be done as good practice.

Leaky transistors are not limited to digital electronics and in fact this problem is in the analog hardware."

I didn't have a TIP31C in the drawer and since it was working previously I replaced the no-brand 2N3904 with a new known-good one from my MOUSER hoard, I changed the resistor value per Pete's advice and left the ferrite beads on the signal line.

Bob is my uncle!  No more chattery relay.  Key point above - this is an analog circuit problem that likely had little or nothing to do with the digital circuit - subthreshold conduction just means the transistror turns on when it is not supposed to.

Thanks Pete!

Here is Pete's blog post explaining in detail:

July 10, 2024. Subthreshold Conduction (

Next steps - install the RTC module and the directional coupler for the SWR and power meter 73 from Great Falls

Sunday, July 7, 2024

Homebrew to Homebrew with KA4KXX - sBitx to Bitx


Walter, KA4KXX and I had an extended QSO yesterday from my QTH in Virginia to his in Florida.  And the most remarkable thing is we were both working homebrew version of  BITX rigs.  I was using my homebrew sBitx and Walter was using his most recent 20 meter SSB and CW rig. 

Here are pictures of the two rigs as used on the air!


KK4DAS sBitx

Thanks Walter for the fun QSO.  A rare pleasure talking with another homebrewer - particularly one who isn't afraid to let his geat go al-fresco!

In other big hombrew sBitx news I completed a clean sweep of the 13 Colonies special event last night - all on SSB working from my garage workshop location wiht my 80M off center fed dipole.  I managed to bag the elusive K2K, New Hampshire last night after many tries over many days.  So once again the colonies are united and ready to win the war!  Fun fact - New Hampshire was the 9th state to ratify the US Constitution making it the official law of the land.  So why then were they so reluctant to close the deal with me?  

Still waiting for 10 meters to open again so I can check out the peformance on the high bands.

73 from Great Falls,



Saturday, June 29, 2024

Homebrew sBitx Walkthrough


Here is a video walkthrough and quick homebrew sBitx update.  I've intergrated the PA into the rig and calibrated the Tx levels.  I am very pleased to get 20 clean watts CW out on all bands from 80-10.  All modes are working well.  Final calibration meant deriving all my own band settings and updating the sBitx hardware configuration file hw_settings.ini.  Since my analog build is unique I had to start from scratch with the configuration.  I tested transmit on each band through the LPF and check the signal quality and strengthh on the scope.

When I first installed the PA it worked great on 80 and 40 but on the higher bands the relays were chattering and buzzing.   I added .1uF caps to the DC lines and that fixed the problem on all bands except 10 meters.  Farhan suggested adding bypass caps to the GPIO lines that drive the transistors that control the relays.  As N2CQR says - "Bob was my uncle!"  That fixed it.  

The radio sounds great and I have been getting good audio reports.   Had a QSO yesterday with PV8AL,Helio in Brazil on 17 meters SSB.  I got a 5:7 signal report plus good audio.  If you've ever worked Helio you know you can sometimes here a rooster crowing in the background.  Not so this time.

Its scary - I'm running out of bugs and problems.  I better be careful what I wish for.




Monday, June 24, 2024

Homebrew sBitx 20 Watt Power Amplifier


20 RD15HVF1 HF Power Amplifier

Between Field Day and other volunteer activities this past weekend I managed to complete a build of a 20 watt RD15 amplifier for the hombrew sBitx transceiver.  I spent the last week or two studying these amps and looking at various implementations. I had previously built EI9GQ, Eamon Skeleton's amp from his book  "Building A Transceiver which I documented earlier.   That build was copied directly from his schematic and I didn't give much thought to the design.  

Push-pull MOSFET RF amplifiers pretty much all share a common architecture.  The transitors are configured as common source amplifiers and biased for class AB.  The differences are in how the input an output transformers are configured and how the bias circiut is designed. Compared the the EI9GQ amplifier I simplified the input and output transformers and tripled the value of the feedback resistor to get more gain.  I  also learned that adding a cap across the drain side of the output transformer will flattend the gain curve and increase gain at higher frequencies. Using cut and try I ended up at 330 pF which results in flat gain from 3 MHz to 21MHz and about 3db more gain at 28 MHz.  This was a very satisfying build experience. Essentially everything worked as expected almost right off. 

73 from Great Falls



Thursday, March 14, 2024

Homebrew SBITX - Tx Modules PA, LPF and Mic

Much progress since the last post. After resolving my ground-bounce and hallucinations with wisdom, I moved on to the transmitter which consisted of a 5 watt power amplifier, straight from the UBITX 40 module.  Thanks to Bill, N2CQR for the recommendation and his excellent Manhattan layout.

The three stage PA starts with a 2n3904 pre-driver followed by a 2N2219A driver with a heatsink cap and the fnal is an RD0HHF1 biased at around 6.5V - which puts it squarely in class A.  Bill's build and other bias more for class AB which I may experiment with later.

Next I needed a microphone and microphone amplifier.  I built the microphone amplifier straight from the SBITX schematic and built a homebrew electret mic.

The homebrew electret capsule in a 3D printed case  The PTT is a SPDT microswitch.  When I first hooked it up the mic did not seem very sensitive - almost had to shout to see output on the scope. A little googling and noodling and I figured it out.   In brief - when I finalized the mic amp build I mistakenly powered it from the 5V rail instead of the 12V rail, that accounted for most of the low output. By then I had read through the long thread from last year on electret bias and the recent thread on SSB audio quality and had ordered a ag of the -25dB capsules that Gordon recommended.  They accept VCC of 2-10 V through a 2.2K resistor.  So replaced the R21 10K biase resistor with 2.2K to the 5V rail.  With that setup the mic is working great.

With the mic amp and PA done I began work on the LPF module. The module consists of 4 filters covering the 80 meter to 10 meter bands.  In the original SBITX it is a diode switched module. I spent some time understanding the diode switching but decided instead to use relays - it takes away a lot of the complexity of the module.

Next lesson learned: when I first built the LPF board I had the relays only on the input and had tied all of the outputs together.  This created all kinds of problems with the filters interferring with each other even with their input switched out of the circuit.  Solution and best practice is to switch both the inputs and outputs and ground the unselected filters.  Problem solved.  But not out of the woods - still too much loss in the filter module.  I put that asisde for now and moved on to fixing the frequency alignment. 

I had previously tried to align the frequency (so that the displayed frequency matches the actual Tx or Rx freqency without success.  The alignment is done by creating a compensation offset for the 25MHz clock of the SI5351.   Its usually straight forward. Inject a known 10MHz signal into the receiver, zero beat and note the frequency display.  The delta between 10MHz and the frequency display can be used to derive the actual crystal value.  Pop that into the code and the the transceiver should be aligned.  The problem was it was aligned at 10MHz but off at 5 and 20.   After much testing and head scratching and with an assist from Farhan himself we discovered the problem was that the MIKROE WM8731 prototype board uses a different clock rate than the Linux driver in the Raspberry Pi.  In brief, the driver expects the codec to use the standard USB clock rate of 12MHz but the MIKROE board uses the more standard audio processing frequency of 12.288MHz.   The fix is to replace the 12.288MHz crystal on the MIKROE board wiht a 12MHz crystal.  Like so:

The surface mount crystal did not come off the board cleanly - in fact it lifed one of the tracks clean off the board.  So I put in a field expedient patch - the red enamled wired to replace the damaged trace.  To my delight it worked straight away.  I used blue painters tape to mask off the adjacent pins and that made the sodering to the chip much easier.

The results is very satisfying -  here is the first loggable QSO I had after completing the rig:

I'm very pleased with the way the rig is working.  I have a few kinks to iron out but we are appoaching the finish line.

73 from Geat Falls
Dean, KK4DAS

Sunday, February 11, 2024

Homebrew SBITX Receiver - Ground Bounce, Hallucinations and Wisdom


After a long hiatus I am back at the blog.  I have a number of projects that I have been working on that I will share going forward, but today I want to talk about my latest project - a homebrew version of the SBITX transceiver designed by Ashhar Farhan.  The SBITX is a hybrid analogue superhet transceiver / software defined radio.   The analogue portion of my build is based on the Furlough 40 / SimpleSSB that I built in 2020.  The SDR software runs on a Raspberry Pi 3 or 4 with software written by Farhan.    I'll explain the title of the post before we are done today but first lets take a look at the KK4DAS SBITX.

Here is a demonstration made shortly after I completed the receiver:

For a quick overview of how it works, lets look at the block diagram.

Beginning with the antenna on the upper right let's follow the received signal path.  First we pass through a single 30 MHz low pass filter which passes the entire HF band.  We amplify the incoming signal with a broad band RF amplifier and then pass it through and ADE-1 mixer to mix the signal up the the 40MHz IF.  The LO and BFO clocks are provided by an SI-5351 PLL controlled by the Raspberry Pi.  The homebrew 40MHz crystal ladder filter is 25KHz wide which controls how much of the spectrum you can see on the waterfall display at any one time. The bidirectional IF board I am using is the first board I built for the SimpleSSB at the beginning of 2020.  I have replaced the 9MHz commercial filter with my homebrew 40MHz filter.  The second mixer then drops the signal to a 24Khz IF which is well within the range of the ADC in the codec board. From the second mixer we go through a low noise amplifier to boost the signal and pass it in to  the left line-input channel of the codec. The 24KHz signal is digitized in the codec and passed on to the Raspberry Pi where further signal conditioning and filtering occurs, the waterfall display is generated and the digital audio is extracted.  The digital audio is sent back to the codec where the digital to analog conversion occurs and the analog signal is sent out the left line output to headphones or to an amplified speaker.   The SBITX software also supports FT8, RTTY and CW decoding natively - no additional software or computer is needed.  For a detailed description of the SBITX you should read Farhan's SBITX description linked above.  Transmit will work much the same but in reverse.  I'll cover that when I get the transmitter implemented.

I'm very happy with how the receiver is performing. Its fun to listen to and sounds great.  But getting to this point has not been without a few stumbles and sidetracks.  I was honored to be included as a guest on the SolderSmoke Podcast Episode #250 with N2CQR, Bill Meara and N6QW, Pete Juliano where I shared my tales of woe - a few of which I will describe here in more detail and a others which I will save for another day. 

Ground Bounce - shortly after completing the receiver I made the unsettling discovery that signals that were being transmitted on 20 meters were being received on 20 meters but also at exactly half the frequency on 40 meters. This was not good - it seemed that it had to be strange mixing products in the first mixer, but I had tested the entire IF before hooking it up to the digital board - and this very same IF board was pulled from a working receiver. I looked at the output of both mixers and I couldn't see how the the 20 meter signal was leaking in on 40.  After thinking about it for a bit I decided to look at the SI5351 outputs on my TinySA Ultra spectrum analyzer and instead of seeing one clean signal on each of the LO and BFO clocks I saw both signals on both clocks.  This was clearly the source of my problem.  Skipping over a day or two of troubleshooting I sent a note to Farhan and he immediately identified the problem.  It was "ground bounce.  Apparently if the clock outputs are not properly grounded it causes current to rise internal the SI-5351 and signals to bleed between the clocks.   In following a separate piece of advice from Farhan on buildiing the digital board I had very carefully insured that there was one and only one ground connection in the digital board and that was directly back to the main DC input.  I had installed the SI-5351 directly onto the digital board and it shared that common ground.  But that meant that I couldn't also ground both ends of the coax shield between the SI-5351 and the mixers.  That was the cause of the ground bounce.  The solution was to remove the SI-5351 from the digital circuit and put it on the analog circuit - with the only connection between the SI-5351 and the Raspberry Pi were the two I2C control lines.  And also to ground the coax connecting the SI-5351 and the mixers at both ends.  That fixed it - the ground is no longer bouncing!

Hallucination - after curing the ground bounce I spent an evening listening to the rig enjoying the glow you get after fixing a thorny problem.  But my enjoyment was short-lived.  I noticed that from time-to-time that the waterfall display would go a little crazy,.  It appeared as if the the receive signal was being duplicated all up and down the band somewhere internal to the SBITX.   It looked like this:

The signal at the center is the received signal - all of the mirror images are false.  Those are the hallucinations.  Farhan identified that fairly quickly and let me know about a software fix in the SBITX 3.2  which led me to:

Wisdom - I don’t have a complete understanding, but the hallucinations are artifacts created during the Fast Fourier Transform of the received signal under certain circumstances.  The SBITX uses the open-source FFTW (Fastest Fourier Transform in the West) library.  There is an extension to the FFTW library called FFTW-Wisdom that is used tune the FFT algorithm the first time it is used.   The tuning  parameters are saved in what is known as an FFT “Wisdom” file .  The Wisdom file, which only has to be computed one time, contains saved information about how to optimally compute Fourier transforms of various sizes. The FFTW Wisdom File man page has more details.  That was what was implemented in SBITX V3.2 which eliminated the hallucinations.

I'm still chasing a few problems in the receiver.  Top of my list is a tuning problem. When I zero beat WWV on exactly 10 MHz, the displayed frequency on the SBITX is a few hundred Hz off of 10MHz, and when I tune to 15MHz WWV the display is off by a different amount. So, the delta between the displayed frequency and the frequency the radio is receiving changes with frequency – but not in any linear way.  I’ve tried several different ways to align the radio but have not yet been successful  The last thing I did was disconnect the analog receiver entirely from the SDR and used a signal generator to put a fixed 24KHz signal into the audio codec which should result in a signal displayed dead center on the waterfall – but it did not – it is a few hundred Hz off.  the current suspicion is that the crystal on my WM8731 protottype board is out of spec.  Farhan has offered to send me one of the codec boards he produced for the early SBITX prototype.  When that arrives, I will replace my audio codec with the one he sends.  That should resolve this last issue but it still doesn’t explain why the delta moves with HF frequency.  That’s what was puzzling me and what I was referring to on the podcast. 

That's it for now,

73 from Great Falls

Sunday, December 5, 2021

Furlough 2040 Supercharged - the Red Roadrunner is on the air.

I've had several comments over the last 24 hours to the effect of - "it looks good, but how well does that pretty red roadrunner work on the air?"

Here is one so you can hear it for yourselves. 

I've made a couple of dozen contacts putting on the air with very good signal an audio reports. Here is one memorable one with the W8A special event station celebrating the 21st amendment to the US Constitution.  The 21st amendment repealing prohibition in the United States was ratified on December 5, 1933 - thus my opening comment about striking a blow for liberty. 

It is often the case that when I tell someone that "the rig here is homebrew" they can't quite get their head around the idea that hams still build radios.   I've also noticed that if I ask for a signal and audio report after they know it is a homebrew rig they are much more critical of the audio - "sounds pretty good for a homebrew" is something every homebrewer has heard.  One contact was so astonished he accused me of surreptitiously running an IC-7300 - I'll take that to the bank.

73 from Great Falls