It’s nice to see we hams, who I think suffer from a bit of an image as throwbacks in the larger maker community, get some recognition for the good stuff we’ve accomplished. Today on Hackaday, a nice article about Manhattan and Ugly construction was posted, with ample coverage given to the fact that a lot of the best exemplars of these techniques come from the world of amateur radio builders. I’m not certain about how others feel on this topic, but it seems to me that Hackaday is one of the preeminent blogs relating to our hobby, so I get quite excited when we get repped there.
Featured in this article are two names well-known in our circles, and guys that I’m proud to call my friends (although I have never personally met either in real life yet!). Todd VE7BPO, is renowned for his rigorous empirical work in circuit design, as well as his beautiful Ugly circuit creations. They feature one of his designs near the top of the article.
The other is Dave AA7EE, who is probably familiar to almost every reader, unless you just crawled out from living under a rock for the last decade. It’s not difficult to see why they chose Dave’s work for to illustrate Manhattan construction, as his is some of the best out there. Period. Also unsurprisingly, this is not the first time that Dave’s creations have made it to Hackaday.
Well done, gentlemen! Way to show the maker world at large that we’ve got relevant skills for the 21st century hacker community!
It’s late at night here, but I wanted to write a short post, because this is very notable. I have unfortunately found out that the esteemed Todd Gale, VE7BPO took down his amazing site, the QRP/SWL Homebuilder. Todd’s site has consistently been one of the best places on the web to get solid, entertaining, well-documented, and hype-free information on the RF circuits that we all love to build. The QRP Homebuilder has been the premier destination on the web for my favorite hobby for as long as I can remember.
Today’s news is about as sad to me as when W7ZOI removed most of his RF experimentation materials as well. I could only hope to be half as talented as either of these two gentlemen, and we are all going to miss this most essential web site. I pray that some day soon we will find a new generation of experimenters who will be able to bring that same evidence-based RF work to the internet.
The good news is that all of that valuable documentation is not lost. Todd has graciously archived the last version of the site as a full-color PDF, with all of the schematics and colorful illustrations that you love. Todd is currently hosting the PDF on his server, but I wanted to mirror it here (with Todd’s permission) in order to ensure that it has wider coverage.
I’m not yet at liberty to discuss the circumstances of the decision that I have been told about, but I strongly believe that Todd himself will do that some day. I would like to thank Todd for his years of generous service and for blazing a trail in the RF experimentation community.
Edit: Good news! Todd already has a new site up and running, this time in the form of a blog. It currently only has a test post, but I would bookmark it and put it in your feed reader if I were you. Go forth and visit Popcorn QRP. I’ve added it to the blogroll. Best wishes on the new endeavor, Todd!
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.
This evening I was browsing one of my favorite QRP homebrewing sites, QRP Homebuilder by VE7BPO, when I stumbed upon something that really struck a nerve. Todd does a really great job in documenting his projects and experiments, as well as walking you through his thought process in many of the design decisions that he made. There’s a wealth of information here, and you’d be hard pressed to find someone who does a better job.
In this article, VE7BPO gives a very nicely detailed explanation of his methodology for testing the stability of the LC-tuned VFO circuit which is the subject of the post. He mentions that you can always use a stable receiver with a narrow bandwitdth to listen for VFO frequency shifts by ear (a perfectly valid method for homebrewer testing, in my experience). Well apparently, that’s not good enough for some of the Internet experts:
I have taken some flack on the World Wide Web and by email for using my ears as a VFO stability tester. Perhaps this is well deserved criticism, as it does not quantify drift. However, the last time I checked, receivers are meant for listening to signals and almost any drifting oscillator beat note will not stay centered in a narrow IF pass band of a stable receiver. If a VFO stays put in a narrow pass band, I am pretty sure it is stable enough for use in a home-built transmitter or receiver. From my experience, albeit limited, any drift you can measure you can also hear. I sure wish some of my critics would publish their work so I wouldn’t have to perform so many experiments to try to improve my hobby projects! The target audience of this web site is people who want to have some fun and perhaps do not have hundreds of dollars worth of test equipment. It is okay to use a receiver as a piece of test equipment if you want to or don’t have anything better to use. Apart from digitization, miniaturization and the demise of HAM radio in general, I posit some of the other reasons that analog hobby electronics is dying is lack of mentorship, imagination and fear of failure. Every design or method generally has good points and bad points. This web site is truly for people who like to experiment with and enjoy building simple electronics circuits. This is the “popcorn” niche I aspire to. I have found that it is very easy to criticize, but far more difficult to contribute. Hopefully I am in the latter group! [Emphasis mine]
It really fries me to see people like Todd, who put such much time and energy into documenting their hobby for other people at no financial gain, get a raft of crap from online bullies. There’s not much to add to his statement, since he pretty much nails it, but I’ve had a small taste of the same thing. Fortunatly, most of the hams I’ve interacted with online have been at least cordial; but I know how frustrating it can be to put a lot of energy into a labor of love, only to have it criticized by the know-it-all do-nothings of the world.
I emphasized one sentence in that quote to give everyone something to think about. It’s quite tempting for someone who is freely giving their time to the hobby to throw in the towel on sharing their work when they are forced to put up with too much of this nonsense. So please, show support to your favorite authors and bloggers, they need all they can get.