IF Amp & Product Detector from CC-20 Beta 1

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.

G3UUR Crystal Checker

I decided to make my initial Project X prototype PCBs at home using the old tried-and-true method of toner transfer (via Pulsar Professional paper and foil). Since I'm a novice at PCB layout, I didn't feel comfortable paying the money for a few proto PCBs from a board house, then finding out that I did something wrong and flushing that money down the drain. Instead of buying Pulsar's starter kit, I just purchased a pack of the transfer paper and a roll of the green foil. I also got the required GBC laminator from Amazon instead of paying significantly more for it from Pulsar.

Last night, it was time to give the process a whirl, so I decided to make a PCB of the G3UUR crystal checker circuit that was printed in the Fall 2010 QRP Quarterly (an excellent article, by the way). The instructions seemed clear enough, but I had about five failures before I finally figured out how to make the process work correctly. I was just about ready to chuck the whole thing in the trash bin, but I managed to keep my wits and persevere through it. For those who might be new to the process, let me help you to avoid some of the problems that I had:

• Give yourself at least 0.5 inches of copper clad clearance on each margin of the final board edge in order to give the laminator good purchase on the board and transfer paper. Putting the toner traces too close to the edge will result in those edges failing to adhere to the board. There's just not enough heat and pressure to do the job properly at the edge.
• When passing the copper clad plus transfer paper through the GBC laminator, I found that it worked better with four passes. Pass the board once, turn it 90°, pass it again, etc., until all four edges have been the leading edge through the laminator.
• I didn't see this mentioned in the instructions, but after you apply the foil on top of the toner traces, you must let everything cool down to room temperature before attempting to peel the foil off the board. Failing to do so will rip most of your toner off of the board!

Once I got the bugs worked out, I was quite happy with the end result. I also decided to try out a new etching method. Instead of using ferric chloride, I used the hydrogen peroxide/hydrochloric acid recipe that I've seen touted on the Internet. Let me just say that it worked out beautifully and is waaaaay cheaper than buying ferric chloride. It only took about 3 minutes to etch my small board in a Ziploc baggie. No need to mess with expensive equipment or chemicals!

The etched board turned out very well. There are a few places with very close traces, as you can see in the photo above. These etched out perfectly, no problems at all. You might notice some bad copper on the bounding rectangle on my board, but that was because of the close clearance between that trace and the board edges. I know how to avoid that in the future.

Tonight, I got the board all soldered together and it worked perfectly on first power-up! That's always an extremely satisfying feeling. Now that I've got a handle on the process, I feel comfortable using it on the Project X prototype. Stay tuned for more progress on the new radio!

G3UUR Crystal Checker - Bottom

G3UUR Crystal Checker - Top

AmScope SM-4T

I'm starting to build up the gear for the kitbiz to get off the ground, and the first large investment just arrived at the shack. Thanks to some blog posts from Eldon WA0UWH, I found AmScope.com, who sells all manner of microscopes for low prices. I ended up finding a scope that was very similar to the one that I worked with at Tek; the SM-4T. It's a 7x-45x trinocular stereo zoom scope with a double-arm boom. I also ordered a 80-LED ring light with a variable brightness control. The beast barely fits on my tiny construction bench, but I'll have to deal with the cramped space for now.

Here's some of my impressions after assembling and using it. First off, the base is massively heavy; which is great for stability but no fun when transporting it. I thought the poor UPS drive was going to get a hernia getting it off of his truck. The fit-and-finish is pretty good. Not quite the same as the high-quality brand name microscope at Tek, but I felt like it was good for the price I paid. The manual is terrible. I'm still not sure that they sent me the right manual, since the diagrams in the once I received show a completely different model. But the back of the manual does show the parts for my microscope, so who knows, maybe you just get some generic manual for all of the different models. The Engrish factor is pretty high, as you might expect. Fortunately, I was still able to easily assemble the microscope by referencing the photos on the AmScope website.

The image quality is quite good, although the field of view is a bit smaller than my Tek microscope. The interpupillary distance and diopter setting is fully adjustable. Once I got them set correctly, the microscope was a pleasure to use. The 80-LED light did a good job of providing bright white illumination for the circuit boards that I examined. My first impression is that I've received a good value for my money. AmScope claims that their production line is the same one that manufactures microscope for Zeiss, Leica, Nikon, and Olympus. Based on the quality, that certainly seems like a credible claim. It's a bit spendy for the average homebrewer to purchase, but if you are serious about doing a lot of SMT work, I think that these microscopes would be an excellent choice for the shack.

Clackamas Schematic

Since FDIM 2010 is in the history books, it is my pleasure to finally publicly release my entry into the FDIM 2010 QRP Challenge: The Clackamas 40 Meter Transceiver.

The rig is a VXO-tuned superhet that operates around 7.030 MHz. The heart of the design is the BF998 dual-gate MOSFET (which was popularized by W7ZOI on his website and in EMRFD). The BF998 is used as the front-end mixer and as a combination product detector/BFO. My new favorite AF amp, the TDA7052, is my choice for the single allowed IC. The VXO signal is mixed with a carrier oscillator in a JFET mixer, which is then bandpass filtered and fed to a BS170 power amplifier.

Please download my contest writeup for full details of the design. I'll dissect the design in further detail in future posts.

LogiKlipper Prototype

I recently received a very interesting e-mail from Dave W8NF regarding a very spiffy looking labeling technique that he successfully used for his latest LogiKlipper prototype (BTW, LogiKlipper is going to kick the butt of the RF clipper manufactured by that other company in the South...). Here's the details on how he did it:

I printed out what I proposed as the "final" lettering artwork on the transparency film.  I included four alignment marks for the corners.  I laid the panel down on newspaper and sprayed the panel with "Elmer's Multi-Purpose Spray Adhesive" - I got it from Home Depot a year or so ago for another project.

I moved the panel to on top of a magazine that had not been sprayed, and carefully positioned the transparency film over it.  The fact that the film is transparent helped a lot...this would be difficult with an opaque film.

I was worried about bubbles or smudges in the adhesive, and they do indeed exist.  They don't look as visible as I worried about, though.  They provide a bit of texture, even.

The film is pulling up at the edges...since this was more of an alignment test than a finished piece, I didn't really work hard to press the film into the panel.  Also, I used a pair of scissors to trim the edges, and that pulled the film up...an X-Acto, bearing against the panel edge, would have avoided this problem.

I X-Acto'd through all the holes, bearing the blade against the metalwork to form scissors.  The only problem points were the countersunk holes.  For a homebrew project, I probably would not countersink.  Or, if I did, I'd have those screws in place before the film was attached, and just glue the film right over them.  The flathead screws, when they went in, wrinkled up the film around them.

I think I'll try the technique for my next homebrew project.  It certainly serves for what I needed to demonstrate this time.

A very nice looking front panel, Dave! I'm going to have to give this a shot. I'm a lousy mechanical engineer, but I think even I could pull this off respectably.

After dinner I managed to slip into the shack after Jennifer laid down to rest and I bribed Baxter to leave me alone with a Kong full of treats. I thought about trying to self-spot in order to scare up a QSO, but that didn't seem right. I wanted the first QSO to stand on the rig's own merits, not because I asked someone to listen for me. So I parked somewhere near 7030 kHz and started banging out a straight key CQ with 1 watt output. After about 5 rounds of CQ, I started getting the sneaking suspicion that I had screwed something up, but I trusted that my pre-QSO checks on the rig were correct. So I kept at it.

After a few more CQs, I finally got my reward. A huge signal about blasted the phones off of my ears! And it was saying my callsign! I got a reply from W7MDK in Peck, ID. I was so excited that the QSO didn't go so great, but we got all of the pertinent information across. He was going a bit faster than I normally copy, and I think my speed was impaired even further because of my extreme excitement. Dick gave my 1 watt a 579, while he was easily 40 dB over S9 here. I cut the QSO short to stop me from embarrassing myself further, but I got the first one in the log!

It's said that there isn't much better for the homebrewer than to turn on that newly created receiver and hearing signals off the air for the first time. It's hard to argue with that, but I think that making a complete first QSO with a rig you designed and built yourself has to top even that experience. Sorry to toot my own horn so shamelessly, but I'm just thrilled with the love of radio right now.

The next steps are to get going on the documentation for the judges, get the prototype into an enclosure, and start working on a second copy of the rig to make sure I can duplicate it from the schematic. I can't wait to share the design details with everyone, but that's going to have to wait for another month or so. In order to get the rig within the required parts count, I had to trim the receiver down to 34 parts. The transmitter ended up being 30 parts and the VXO came in at 8 parts. However, I was able to make the Clackamas a true transceiver, not just a trans-receiver.

Stay tuned for additional details as I can release them!

Update: Just worked JF2QNM in the JIDX contest. 1 watt spans the Pacific! Of course, all of the credit goes to the op on the other end, but it's still really gratifying to have your HB 1 watt signal make a nice hop like that.

KC2UHB with EFHW Tuner

Our favorite fashion hacker/ham, Diana Eng, KC2UHB, has posted a new article on the Make Magazine blog with instructions on how to build an End-Fed Half Wave antenna tuner and deploy it with an antenna. (BTW, did you see that she was recently named to the ARRL Public Relations Committee? An excellent choice the League.)

Her BOM calls out an air variable cap from MFJ and a T94-2 'roid, so I suspect that her tuner could handle a bit more than QRP power. In typical ham fashion, it appears that she was too eager to get it on the air to worry about little things like putting it in an enclosure. FB with that Diana, I think most of us can relate.

It sounds like she had great success using this setup with her FT-817 for some SSB QRP fun:

Setting up my antenna for 20m the first time took about half an hour, and I was able to get very close to 1:1 SWR using the 9:1 input with a vertical wire supported by a tree. Over the next hour or so, operating from a park bench in Brooklyn, using 5 watts on sideband, I made contact with stations in the US, Europe, South America, the Caribbean, and Hawaii. The furthest station was about 5000 miles away, which means I was getting 1000 miles per watt out of this setup. I'm sure it was a big contest station and not another QRP operator sitting on a park bench with a wire antenna, but it was still fun.

I'll take this opportunity to shamelessly promote my own EFHW tuner, which I've managed to get back up on my new website. Don't forget to compare to AA5TB's design, which features probably the best page on this subject matter which I've seen.

Audio Amplifier Guts

As I mentioned in a previous post a few days ago, I found a nice 2 watt mono audio amplifier board that's a perfect addition to give speaker capability to a receiver with headphone output. I purchased four of these bad boys since they were so inexpensive (and I was paying for shipping from Hong Kong, so might as well order in bulk). I figured a good test would be to build up a bench amplifier so I could have the convenience of listening to any of my homebrewed phones-only receivers/transceivers (most of them) on speaker.

To the right, you can see the pleasing results of this little project. I had a nice Ten-Tec TPC-17 that turned out to be a perfect enclosure. The 1/8" input connector is on the left, a power LED in the middle, and a mute switch on the right (the amplifier board has an active low mute line which works extremely well). I created a speaker grille drill template in Inkscape for the 3" speaker bolted to the top lid of the enclosure (if there's any demand for this file, I can post it here). The actual 1" x 1" audio amp board is on the left and a small 7805 voltage regulator board on the right.

AF Amp on the Bench

Here's the enclosure all buttoned up for use. You can see that the speaker grille is a bit ugly, but it works well enough. There is no power switch, since the quiescent current is so low that there's no problem in leaving it powered up. Tonight I hooked it up to my VRX-1 (with a rock on 7030) and listened to the wide-open band with the DX contest in full swing. The audio quality was quite nice; better than I remember it when testing it out in the open on the workbench. I'm guessing this is because the speaker was properly mounted instead of having the magnet rattling against the bench. Even though it's a simple project, it's a great addition to the shack that makes me wonder why I didn't do this earlier. Yeah, I could have purchased a set of amplified PC speakers, but that just didn't seem appropriate. Besides, the amp in the Ten-Tec enclosure looks spiffy next to all of the other similar looking equipment in my shack.

Update

I got a bit bored tonight, so I took a video to demonstrate the amplifier working with the VRX-1. Yes, I know there's nothing to really look at, but the audio is a bit interesting.

Sure Electronics EUA2005 Audio Amplifer Lash-Up

Lately, I've found that eBay has been one of the best ways to beef up my junk box with inexpensive electronics components (eBay Stores are my preference, the auctions often aren't that great). I purchased a nice SMD resistor/cap assortment from a company called Sure Electronics and found that they have some other goodies that might be of interest to the homebrewer. One of those is a tiny little (about 1" x 1") 2 watt mono audio amplifier based on the EUA2005 IC. The spec sheet states that the IC output power is 1.5 watts, so the manufacturer might be fibbing a bit on their specs. Regardless, it's enough power to drive a 3" speaker to a decent volume for shack listening. The IC has a maximum V_CC of 6 V, so I had to create voltage regulator to step my 12 V bench supply down to 5 V using a 7805. The audio quality is definitely what I would call "communications" grade, but the price is great: $2.22 for a 2-pack. One word of warning: your order is shipped from Hong Kong, so expect about 2-3 weeks for your order to arrive.

For those working on W8DIZ's frequency counter project in QQ, they also have a Atmel STK500 in-circuit programmer clone for a very inexpensive price. I haven't had a chance to try mine out yet, but if it works as advertised it's well worth the price (I have no reason to believe that it won't work, but if doesn't, I will update this post).

The usual disclaimer applies: I'm not associated with the company in any way, just a satisfied customer.

Subharmonic SDR Schematic

Today in my feed reader, I caught wind of a really neat topology for a minimal parts count SDR, courtesy of Joachim, DL1GSJ. His receiver uses a simple phase shift network to create the I/Q LO signals to drive a pair of Polyakov (subharmonic) mixers. It looks dead simple to breadboard and could be an easy way to try SDR without having to order a SoftRock kit. Get over to Joachim's blog to get the full details of this fascinating design.