I've got another grab-bag of miscellaneous news for this post, but I'm going to lead off with the big one: I'm going to be a presenter at the world's preeminent QRP convention: Four Days In May 2012. The tentative topic for my presentation will be about the free and open source tools that I use in the development of my products and how you can put them to use in your own homebrewing endeavors. This will be my first time speaking to an audience larger than about 25 people, so I hope that I can provide an entertaining and informative talk at such a prestigious event. I'll be speaking in front of a lot of people who I consider to be much more capable than I and some who I consider my virtual Elmers. It is my sincere desire to not disappoint.
I am very excited for the opportunity to go back to Dayton so soon after my last trip. I really didn't expect to have the chance to go again for quite a few more years, so the ability to get back to the convention after only two years is a great blessing. I owe a great debt of gratitude to Jennifer, who didn't hesitate to encourage me to go, even though she will be dealing with a 2-month-old baby and a near 2-year-old by herself for a few days while I'm away.
In other news, I feel like I've gotten over the steep part of the learning curve with Kicad, having successfully made PCBs for my little Twin-T code practice oscillator. You can see a short video of it in action above. The output level is suitable for modern, sensitive headphones, but if you want room-filling audio such as in my video, you'll need to connect it to an amplified speaker. The PCB is designed to fit in the ubiquitous Altoids tin, with room to spare for a 9 V battery. I expect that this will eventually make it to my stable of products, but it's low priority considering the long delay on the CC-Series and the need to get it ready to sell by May. If you are really interested in the project, write a comment or shoot me an email (milldrum at gmail) and I'll see if I can't work something out to get you hooked up with a kit early.
The OpenBeacon project is cruising right along. Now that I know that I can successfully make a PCB with Kicad, I've taken the plunge and decided to migrate all of my workflow there (I think this will include the next board spin of CC-Series, since there are so many changes to be made there will be no real advantage to staying with TinyCAD/FreePCB). The OpenBeacon PCB design is nearing completion. Once I get a shipment from Mouser in the next few days to verify that my newly-created PCB footprints match the actual physical components, I'll be ready to submit my CAM files to Seeed Studio for prototype boards. With any luck, I'll have them back within about two weeks. (Protip: it's worth taking the time to place your component against a 1:1 printout of your Gerber to make sure it will fit. Don't ask me how I know this.)
Once those CAM files are off to China, it will be full-bore on the CC-Series. With the deadline of mid-May staring me down hard, I figure I will have to get those CAM files out within no more than three weeks. That will put me into mid-March for getting the PCBs back, which will give a pretty slim margin of time to beta test and prepare the kit for final sale. Going to be pulling some long, late-night shifts...that I can already see.
I've also got a few more projects in the pipeline for after FDIM and the deployment of CC-Series and OpenBeacon. The first is a fairly simple and inexpensive VXO DC transceiver that I hope to initially kit for the high bands of 10, 12, and 15 meters. It uses a topology which is somewhat unique. The other is an extrapolation of the receiver circuitry of this rig to use as a dedicated QRSS grabber receiver. But I may be getting a bit ahead of myself. Let's get this CC-Series launched, then see where the winds take us.
Tags: cc-series, dayton, fdim, OpenBeacon
I'm in the process of selling off some of my ham radio stuff so that I can fund further development of the CC-Series. Right now, I've got a Yaesu FT-817ND along with a few other items for sale. I've also got a very lightly used Kindle with the leather case with integrated reading light. Occasionally I will be posting more items there, so please check back periodically. Thanks!
Progress on CC-Series development proceeds at a reasonably-good clip right now. One of my last big hardware bugs to stamp out is some nasty microphonics that seem to be generated by the combination product detector/BFO. Today, I believe that I made some significant progress towards solving it and wanted to share what I learned.
I've done a lot of reading in Experimental Methods in RF Design (EMRFD) about microphonics in DC receivers (read chapter 8!), and the number one cause of it is poor LO-RF port isolation in the mixer. The CC-Series uses a venerable old circuit which hasn't seen much use in a while. A dual-gate MOSFET is pressed into double-duty as a product detector and BFO (see above). Since the dual-gate MOSFET product detector is in a single-ended configuration, it inherently has bad LO-RF isolation. This allows VFO (or BFO in this case) signal to leak out the product detector input, and have a good portion of that signal reflect back into the product detector. So naturally, the CC-20 could be experiencing the microphonics because of this phenomena. One of the solutions mentioned in EMRFD is to put an amp in front of the mixer which has excellent reverse isolation (signals coming into the amp output don't tend to get out of the input, and therefore can't reflect back in again).
I had the suspicion that the common-source JFET amp in front of the product detector might be the culprit. So what's the best type of amp to place in front of a single-ended mixer? The common-gate JFET amp is a good and popular choice. However, VE7BPO notes on a recently published web page that the best commonly found amp configuration for this particular parameter appears to be the cascode (see the bottom of the page).
In order to test this theory, I went to work on a project that I had set aside earier: a direct conversion receiver based on the CC-Series product detector. When there was no preamp in front of it, the microphonics were unbearable. I figured that a good way to test my theory would be to put a cascode amp in front of this mixer and see how much it helped. I decided to put a dual-gate MOSFET preamp in front of it, as this is essentially a cascode amp and it fits with the dual-gate MOSFET product detector. Once the new preamp was added, the change was dramatic. The microphonics were gone.
Next, I decided to be a bit more rigorous in my study and quantify the exact difference between the common-source JFET amp and the dual-gate MOSFET amp. First I breadboarded the common-source JFET amp and ran it through the test procedure in the page linked above (at 18 MHz). The results were atrocious. Only 30 dB of reverse isolation, which is worse than the worst amp listed there (the feedback amp). Next, I dug out an old dual-gate MOSFET amp I had breadboarded for my 2008 investigations and ran it through the same test. As expected, the results were vastly superior: 68 dB of reverse isolation. This lines up nicely with Todd's measured results of >64 dB for the hybrid cascode (I used a spectrum analyzer while he used an oscilloscope, so I was able to get a pretty good measurement down to low signal levels).
So this appears to be strong evidence that the IF amp is the problem. It seems certain that the next version of the CC-Series is going to scrap those awful common-source amps for a much nicer dual-gate MOSFET amp. The lesson to take away from this is that if you are going to use a single-ended mixer for any but the most simplistic applications, it must be fronted with an amplifier with an excellent reverse isolation. While the typical common-gate JFET amp will work OK, for best results it looks like a cascode or dual-gate MOSFET amp is the way to go.
Tags: BF991, BF998, cascode, cc-series, dc, microphonics, mosfet, VE7BPO
I'm happy to report that the second of four CC-20 beta kits is completed and working! Mikey, WB8ICN got his all finished up with no major problems and made a first QSO with N1WPU. It looks like he made a nice custom enclosure out of some very sturdy copper clad. Very, very nice! Please click on over to Mikey's blog and have a look for yourself.
Here it is, the first CC-Series beta unit completed by someone other than me! As is obvious by glancing at the photos, Dave AA7EE has done a magnificent job of assembling the CC-20, as well as creating a custom enclosure for the radio out of red copper clad using the WA4MNT technique. There's really not much more for me to add, except to tell you to get yourself over to Dave's blog to check out his story about the build and to see more shiny photos.
No earth-shattering news to report on the blog, but a few little things to mention (hence the "junkbox" title).
The CC-20 Beta 1 test is proceeding pretty much as planned. As of tonight, AA7EE has his receiver up and running now and a couple of the others are close behind. I'm eagerly awaiting the results of at least a couple of the builds so that I can get moving on the revisions for the Beta 2 circuit (which will hopefully also be the production PCB). I'm anxious to get the business up and running!
I got a very nice mention from Bill Meara on the latest episode of SolderSmoke. He talks up Etherkit and my blog, then mentions that he's going to try to use the single-ended passive MOSFET mixer from the VRX-1 in his homebrew WSPR transceiver. I hope that the experiment works out well for him.
As we approach the halfway point of the gestation of our new little one, I got to thinking about mortality a bit. I hope to be around for a very, very long time to come and have been taking steps to improve my health to make that more probable. But in the awful case that something were to happen to me in an untimely fashion, it seemed that I'd like my family to have a little bit of my own thoughts with which to remember me. At first, I thought that maybe I should do a private journal, but then it occurred to me that wasn't necessary. Barring a complete collapse of civilization, all of my descendants will be able to access an archive of all of my Internet activity. Every blog post, tweet, Google+ post, website comment...and perhaps even my email. If you Google my last name, I'm the first result. I'm active enough online that it's not entirely inconceivable that a reasonable avatar of myself could be created sometime in the distant future (given that Moore's Law holds up in some fashion for the next 50 years or so). Perhaps this is all pie-in-the-sky speculation and will look as foolish as the "flying car future" does to us now, but I'm pretty sure that I'll live on in human information space in some fashion long after I'm gone.
Tags: cc-20, cc-series, soldersmoke, VRX-1
















