It's been a busy several months - all of my recent internet activity (and much of my homebrew time has been spent on the SolderSmoke Challenge direct conversion receiver project that I am working on with Bill, N2CQR. Since January more than 40 homebrewers from all around the world have completed the challenge and more are finishing up every day. If you haven't followed it check out the SolderSmoke blog and YouTube channel and search for SolderSmoke Challenge. Join us on Discord if you too would like to complete the SolderSmoke Challenge an prove that "When you know stuffm you can do stuff!"
Schematics, instructional videos and group support on the SolderSmoke Discord Server:
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 groups.io 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.
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.
I've spent the last few weeks putting the finishing touches on what has become the Furlough 2040 Supercharged SSB transceiver. Originally designed and designated the SimpleSSB by my friend and elmer N6QW Pete Juliano I have extended and supercharged his excellent modular design in a number of ways. The final (although Pete advises you should probably never say 'final' when talking about a hombrew rig) configuration is as follows:
20 Watt SSB and digital dual band transceiver for 20 and 40 meter bands
Arduino Nano Every microcontroller and SI5351 clock generator
9 MHz IF - filter
Nextion Color Touch Screen Display
AGC and S-Meter
3 stage Tx amplifier chain, 2N2219A 1st driver, IRF-510 driver stage generating 1-2 watts driver for the 20-watt push pull RD16HHF1 push-pull final amplifier designed by EI9GQ Ed Skelton
Firmware includes full CAT control for digital modes, frequency scanning, persistent frequency and mode memory stored in EEPROM
Custom enclosure with a CNC cut front panel
KK4DAS Furlough 2040 20-Watt SSB and digital mode transceiver
Walkthrough of the transceiver and the final bench test:
Here is a look at the voice modulation envelope:
And a few more pictures to wrap it up:
Final Bench Test
Final assembly in the enclosure - two levels - Tx board on top
On the air peaking 16 watts SSB phone.
This has been an incredible 2-year learning journey. Many thanks to Pete and also to Bill Meara N2CQR of the SolderSmoke podcast who started out humoring me as I built my Michigan Mighty Mite and have become great friends. Also, many thanks to the members of the Vienna Wireless Society Makers group who joined me on the journey at the beginning of the year and we now have about 15 more SimpleSSB transceivers on the air. Watch this space for more on the VWS maker group progress including KA4CDN, Mike's addition of CW just completed yesterday!
Michigan Mighty Mite - December 2019
73 from Great Falls,
Dean
KK4DAS
UPDATE - For those interested in building a similar rig you should know that this is not a kit nor is there a single master schematic or bill of materials. The vast majority of the components are common resistors, transistors, capacitors, voltage regulators, etc. This is a modular build with each module built and tested and integrated as you go along. Each module is well documented and by virtue of the Vienna Wireless Society Makers group build there is a great set of documentation and examples for each module. Where particular components are required (ADE-1 Mixers, SI-5351 clock module, etc) they are called out in the documentation below.
For builders, I highly recommend you build the original SimpleSSB as specified below. This is a 10 transistor, Arduino controlled 5-watt SSB phone and digital mode rig for 40 meters. Once you have the basic rig built then decide which enhancements are right for you.
It has been too long since the last update but I have some exciting news and also a classic homebrew “tale of
woe” to share.
First, my friend Mike KD4MM, here in Vienna, VA was the
first member of the Vienna Wireless Society Makers group to complete the
receiver side of N6QW, Pete Juliano's SimpleSSB transceiver – check it out:
Mike shared his progress with us and also this demonstration
of decoding FT8 with simple audio coupling:
And the tale of woe shared by Don, KM4UDX, reminds us that
not everything goes smoothly when building a homebrew transceiver. He
reminds to “Don’t do what Don Did!”
It has been just over a year since I completed my initial build of the Furlough 40 with much coaching and assistance from Pete. Mike and Don are two out of 20 members of the Vienna
Wireless Society Makers group that is working on a group build the SimpleSSB project as enhanced by me to include features like CAT control for
digital modes. Beginning about six weeks
ago the group is progressing module by module per SolderSmoke best practice
advice. We started with the audio amplifier, followed that with the
Arduino/SI-5351 based controller module and the builders are just completing
the IF module. This week at our weekly meeting, Mike proudly showed off
his success.
Pete was gracious enough to provide tribal knowledge and
encouragement to the group a view weeks ago:
The group is generating a ton of great material on the
project including photos and videos of the in-progress builds, technical
documentation, test equipment, procedures and more.
Here just a few pictures of the in-progress builds
Just think of this – in another couple of weeks we may have
as many as a dozen SimpleSSB transceivers on the air…..we are giving serious
competition to the big rigs everywhere.
And a very warm welcoming of to the new members of the much sought after, rarely granted membership in to the International Brotherhood of Electronic Wizards.
It has been nearly a month since the completion of the receiver beginning to build the transmit modules. I am super excited that this morning I had my first two confirmed QSOs. I was putting out just over 2 watts of RF magic and confirmed two contacts in Ontario - about 400 miles from here.
Furlough 40 First Contact - Thanks to Garry VE3XN!
In the last month I have built the three final modules to enable the transmitter. The second stage amplifier / TX Driver, the IRF 510 based final amplifier and the 40 meter low pass filter. Note the fancy brass home-brew heat sink on the TX Driver, courtesy of the plumbing department at Home Depot. As a reminder, this is my build of N6QW, Pete Juliano's Simple SSB Transceiver and inspired by N2CQR, Bill Meara and Pete's Soldersmoke Podcast.
Furlough 40 TX Driver, Final Amp and Low Pass Filter
There is a tremendous amount of tribal knowledge in Pete's design and I'm really please I went with it for my first transceiver project, because I really, really needed that tribal knowledge to get the rig built. There are things that a first-time homebrewer like me would never think about, like cutting the drain pin off the IRF 510 and using the tab to connect. And using the entire PCB as the heat sink for the transistor. And Pete has been an amazing Elmer, coach, nag and grief counselor when things didn't go quite right - like when I sacrificed the IRF 510 while over exuberantly trying to coax an extra watt out of the not-quite-ready-for-prime-time-rig. The good news was, due to Pete's excellent circuit layout, it only took about 10 minutes to replace the dead transistor.
The sacrificial IRF 510 - IRF, I hardly knew ye!
I didn't get to video of the first few contacts but I will make one in the next few days so you can see the performance. In the meantime, here is the final pre-air transmit test.
The trace shows flat-topping which means I needed to back off on the microphone gain. After I made that adjustment the output dropped to about one watt. I tried to compensate by increasing the bias on the final and I pushed just a little too hard - at about 4.75V DC on the bias, the IRF 510 gave up the ghost. With a little more advice from Pete I managed to get everything more-or-less aligned and am now getting 2-3 watts of output pretty consistently.
Next step will be optimizing each module to see if I can get the output up to the 5 watt design spec.
Introducing the "Furlough 40" scratch-built SSB rig:
Greetings!
I know this is beginning in the middle and the best time to plant a tree was 10 years ago, but here goes...
I'm Dean, KK4DAS. I 've been a licensed ham for about a year and a half, although amateur radio has been part of my life forever. My Dad, Vic W6MYE/SK was a ham for most of his adult life - at least 60 years (my age now) and it is one of the ways I will always remember him. My brother Chris, W6CS has been a ham fore many years and I was very happy that he and I could share the hobby with Dad. I'm sure I'll write more about that over time, but today I have some exciting news to share.
Inspired by Bill Meara, N2CQR and Pete Juliano, N6QW, the Wizard of Newbury Park, and the Soldersmoke Podcast, for the last eight or nine months I have been on a homebrew journey that has led to the completion of the receiver side of a 100% scratch-built 40 Meter SSB Transceiver. I've been working on it since January and thinking about it even longer. The design I selected, after advice from Bill and Pete, was Pete's SimpleSSB Transceiver. It is a wonderfully elegant design that uses 10 common transistors total - which one of the reasons Pete calls it "simple". It was far from simple for me - I should tell you that the most complicated electronic project I had built up to that time was a Michigan Mighty Mite, 7 component, crystal controlled, CW transmitter, which I actually got on the air and Bill was kind enough to blog about. I'll talk more about that project in a later post. Although I have a technical background, it is largely in computer software and networking. I've been building and delivering complex software systems all of my adult life. But electrical engineering was not my thing. W6CS is an electrical engineer, but I am strictly a software guy. Until now...
I am proud to unveil the "Furlough 40" SSB rig, christened in honor of the fact that the current crisis afforded me some stay-at-home time to get the rig finished.
My original goal was to be ready by June, but circumstances have allowed me accelerate that schedule considerably. I've been documenting the project along the way and after today I will back up and tell you a little about the journey.
For the more technically minded the rig is a 40 Metter SSB phone transceiver. The rig is controlled by an Arduino Nano microcontroller driving an SI 5351 clock generator for the LO and the BFO. The IF is at 9 MHz utilizing a crystal filter purchased from the GQRP Club. The IF is bi-directional as is the RF amplifier with relays switching between transmit and receive. It uses two ADE-1 double balanced mixers and the bi-directional amplifiers are Pete's implementation of the Plessy amplifier design from "Experimental Methods in RF Design." I'm building the rig in modules, per the sage advice of Bill and Pete, and testing as I go. Each working module can be used the test the next module. The construction style is nearly 100% Manhattan style - save for the Aarduino and SI-5351.
Thanks to Bill for all of his support and thanks, especially, to Pasta Pete Juliano, master Italian Chef and the true wizard of Newbury Park. Pete continues to patiently answer my questions and guide me through testing each module as it was completed, and gently providing encouraging instruction when I veered too far left or right from plan - even to the point of sending me a hand-drawn sketch of the bottom view of a N2219A transistor so I would know, always, how to wire it in correctly in the future.
More later, but for now - best 73s from Great Falls, VA
