Building a KK7B R2 phasing receiver

Hi, I’m back! It was a rough week, with a death in the family. As it ended, I found some time for some “solder therapy”. There is something good for the soul in putting together electronics. I don’t know if it’s the scent of the rosin or the satisfaction of seeing a project come together. All I know is it’s good for me.

KK7B R2 receiver, top side

The target of my soldering was my KK7B R2 receiver kit. This is the companion to the T2 I wrote about a few weeks ago. To my surprise, the R2 was easier to put together despite the higher part count. The components are larger, forcing a less dense pin matrix, and the board is separated into sections: mixers and diplexers, audio phase shift, low-pass filter, and audio amplifier. Conveniently, the board uses jumper wires (not yet installed on mine) to connect the sections. The intent appears to have been to make it easier to extend the board by swapping in a different phase shift network or adding alternate filters for CW or contesting, but it will make the board easier to debug as well, since I can bring it up a section at a time.

Keep in mind that this is a vintage kit that is no longer available, but updated versions of the design are still produced by Kanga US.

The board surprised me with its heft. I work mostly with surface-mount digital boards where the fiberglass PCB is the heaviest component. This board, though, weighs 134 g (4.7 oz), which is a lot for 100 cm2 (16 in2). The shielded inductors are the culprits. Each black cylinder in the photo is the ferrite shield of an inductor. Each inductor is noticeably heavy for its size, and this board has 10 of them.

KK7B R2 receiver board, bottom side

Once I wash the flux off (it’s not as much fun as soldering), the next piece of the puzzle is to build or buy a VFO. Rick, KK7B, designed a companion VFO for these boards, but it is out of production at the moment. I’m thinking about one of the Si570 VFOs out there. This little chip offers a very low phase noise synthesizer with tuning in steps of a fraction of a Hertz. Having this on a single chip was a pipe dream when the R2 and T2 were designed.

After that, I’ll need to build a power amplifier to boost the T2’s milliwatts up to a reasonable level — at least a watt, maybe as much as 50 W. Finally, I will have to integrate all the pieces into a working rig. Professionally, I do a lot of microcontroller work, so it’s tempting to build a fancy digital control panel with circuitry to integrate every R2 and T2 option imaginable. Instead, I’m trying to keep myself to something simple: one band, SSB only, and a digital frequency readout as the only frill.

I still have to pick the initial band. I’m torn between 20m, my favorite for PSK31, and 40m, which I like for SSB. I’m after whatever DX I can land in either mode. What band would you recommend?

Making a coax feedthrough window for my shack

One problem every ham faces is how to get his signal from inside, where the radios are, to outside, where the antennas are. A while back, I prototyped replacing a screen in my basement windows with a feedthrough panel. The prototype used hobby polystyrene sheets, which were easy to work with but far too flexible for the job. As the weather cooled, I removed it and closed the window, for fear some mice or other critters would find it a convenient way into our nice, warm house. Without a feedthrough, I had no way to use my antenna, so it was time to build a permanent version.

The window I chose is a typical basement window made of glass block, with a small section that opens.

It didn’t take much work to remove the two screws that held the screen window.

My plan was to cut a piece of acrylic (plexiglass) to replace the screen, but a friend one-upped that by giving me a piece of polycarbonate. This stuff is tough. A thicker version is used in bullet-proof windows! It will work fine for the job.

I put the screen on the polycarbonate for size.

It’s perfect!

I marked the size I needed, along with the position of the screw holes, then used a scribing tool and a straightedge to scribe a deep line in the polycarbonate. The idea was to cut it like one cuts acrylic: by scribing it, then snapping it.

(This is what the plastic-scribing tool looks like. I found it in the window-repair section of my local hardware store.)

The polycarbonate was very hard to snap. I found out later that scribing and snapping is not the way to cut polycarbonate. In fact, it can shatter from this treatment. I should have cut it with a saw — I’m told table saws work particularly well.

In my ignorance, I scribed it deeper and deeper, trying to snap it over and over. Finally, it snapped. The material split a tiny bit along one edge (where the scriber had split and I had two parallel lines in the plastic), but in general the edge was clean and straight. I had scribed more than halfway through the sheet before it was ready to snap. Maybe I effectively sawed it after all…

The next step was to make a hole for a BNC bulkhead feedthrough. The tool for this job was a jeweler’s saw with a spiral saw blade. These nifty blades are round and can cut in any direction without turning.

As a template, I used an electrical-box faceplate that had two holes for BNC feedthroughs like this. It came from the trash when an old coax-based (“thinwire”) Ethernet installation was upgraded to twisted pair.

I drilled the hole with my little Dremel drill press.  Some Googling of polycarbonate turned up recommendations against using a hand drill with it, because it can break bits if they aren’t held straight. I wish I had a real drill press, but this attachment for my Dremel tool seemed a good choice when I wanted to drill holes in homemade PCBs. It worked great for this job, too. A 1/8″ hole, the largest I had a collet for, was more than large enough to slip the saw blade through.

I also used the drill press to make two holes for the screws that will hold the panel in place.

As I sawed, I took care not to run the blade right against the template. It is stainless steel and would have dulled the blade much faster than the polycarbonate. In any event, I also had trouble turning one corner and needed to clean that up as well.

I set up the Dremel with my favorite burr bit, which looks like it needs to be replaced soon. It went through the polycarbonate like butter, quickly bringing the edges of the hole out to match the template.

I removed the template and tried the BNC bulkhead feedthrough for size. It fit perfectly! My past includes many a panel with misshapen holes that barely fit their intended connector, so this was surprising but welcome. I guess that’s what I get for never using a template before. The feedthrough I used was the former resident of the faceplate-turned-template.

Now, one thing to mention here… I took care to put the nut on the inside of the panel. I used Coax-Seal on the outside connection, to prevent water ingress, and that stuff is messy and a bother to remove and replace. By putting the nut on the inside, I can remove and replace the feedthrough window without disturbing the Coax-Seal. That might be handy when I cut a hole for another connection.

Finally, I put it all together. I pulled the protective covering off of the polycarbonate, installed the feedthrough for real, and screwed it in place of the screen window. It was a little dusty, so I cleaned it before I installed it. Now it is so clear that the coax looks like it floats in space.

The clearance between the internal, movable window and the feedthrough window is enough that I can close the window after disconnecting the internal coax. (I figured that out with the polystyrene prototype.) That’s good for energy efficiency. The panel fits tightly enough that I don’t feel a draft, and I could always add weather stripping, but it’s still only a single-paned window. The moveable window behind it is double-paned.

The picture above shows the Coax-Seal, too. My technique for that, which I learned from the hams at the Case Amateur Radio Club, is to first wrap the connection with electrical tape, with the outer end folded over a bit to make a pull tab. Cover the tape with Coax-Seal, with the Coax-Seal extending a bit past the tape on each end for water-tightness. When you want to remove the connection, you can cut almost all the way through the Coax-Seal with a sharp knife, split the rest open like peeling an orange, and remove it down to the electrical tape. Next, unwrap the tape started at the pull-tab. The tape will take the perpetually sticky Coax-Seal residue with it, leaving a clean pair of connectors ready for reuse.

Jay Eiger was a font of wisdom for this project and also gave me the polycarbonate. Thanks, Jay!

Itead Studio’s Open PCB exchange: how it worked out

The boards I ordered last month from Itead Studio arrived with something extra: someone else’s boards! No, it was not a mistake, but a 10-cent option that I could not resist: the Open PCB service. For 10 cents above the cost of a prototype PCB order, Itead fabbed two extra boards of my design. Those boards went into a pool of boards from the other Open PCB participants, then Itead sent each of us two random boards from the pool. All participating boards are supposed to be open source. Sure, there is no guarantee that the boards will be at all useful to the recipient, but who knows, maybe something nifty will arrive!

I ordered the Open PCB option with my AK5388 ADC board. Along with my 8 copies of the board, I received two boards from strangers. Both are 5 cm x 5 cm, which is likely a popular size for Itead because it’s the maximum size for their cheapest PCB fab deals.

Boston University Rocket Team thermocouple digitizer PCB, top side Rocket team's thermocouple digitizer PCB, bottom side

The first board is a thermocouple digitizer from the Boston University Rocket Team. The team has posted the schematics, layout, and Gerbers online on GitHub. The board was clearly labelled, making it easy to find the documentation in Google. It even had a QR code. though the pixels were blurred by the silkscreen and my phone was unable to read it. It’s a great idea for open source hardware, though, and would probably work if it were a little bigger.

The design uses a single MAX31855 as a thermocouple-to-digital converter. This is a neat chip that contains a thermocouple amplifier, cold-junction compensation, and a 14-bit ADC all in an 8-pin SOIC. That’s a ton of analog circuitry condensed into a single chip! It can cover temperatures from near absolute zero to molten metal, with quite respectable accuracy and resolution.  The board runs the chip’s Serial Peripheral Interface (SPI) to a USB 3 connector, wired in a non-standard way that carries power (12V, 5V, and 3.3V) and an SPI bus.

The Rocket Team has chosen an interesting mission. They don’t fly rockets, but rather research the design and performance of hybrid rocket motors, including firing them on a static test stand. They build their own instrumentation, all open source hardware, and this board is part of that package. I can see why they would be interested in accurately measuing the temperature of very cold and very hot things!

The board actually has some potential to be useful to me. I don’t need a thermocouple interface right now, but I can imagine using for one down the road to monitor a reflow oven or to manage the heatsink temperature in a linear amp.

Ville K's board, top side  Ville K's flash power control board, bottom side

The second board is a bit of a mystery.

On first inspection, I was puzzled by the single-row header right across the middle and the smaller row of holes at the upper-left side. Eventually I noticed that there are no traces running to either, so it’s likely that they are perforations to simplify cutting the board into three pieces.

The bottom portion is the least obscure. It bears the labels “Flash power control” and “X-SYNC”, so it must have something to do with photo flash. Beyond that, I’m stumped. A two-pin header for an IGBT (a three-terminal device) particularly leaves me scratching my head. The designer did a nice job of bonding his top-side ground pour to the bottom-side ground plane with plenty of vias, including all around the edge of the board.

On the upper right, there are two copies of a circuit, separated by a row of holes to aid breaking them apart. The circuit has a transistor in SOT-23, a diode, a few capacitors, a resistor, and what is likely an IC in a small 5-pin package. Looking at the topology, I think the circuit is a boost converter, at least if the unlabeled two-pad component on the center left is an inductor.

The patterns in the upper right corner are even harder to understand. They look like series chains of something, maybe resistors or LEDs. The vias in the pads and the wide traces indicate that the designer was concerned about resistance, inductance, or heat dissipation. Since the three-device chain (upper center of the board) has the triple vias to back-side copper, but does not use the copper to interconnect, I would guess heat sinking is the concern. It could be a challenge to reflow the board with the open vias in the pads, but it’s probably meant to be hand-soldered. When hand-soldering, one can keep feeding solder until the holes have wicked up their fill.

I sent some e-mail to the address in the silkscreen but got no reply. Google searches on other likely terms turned up nothing. I’m left with a board and guesses.

The Open PCB  exchange is a great idea, and I’ll happily participate again in the future. The thermocouple board is an example of how it can go right. I got a well-documented board that led me to find out about the Rocket Team’s interesting work. In contrast, the Flash Power Control board is an example of what can go wrong. There is nothing to stop someone from entering an undocumented PCB in the exchange, getting documented and interesting boards but failing to repay the favor. Still, I like seeing what other people are doing and hopefully two other people enjoyed seeing what I’m up to. For 10 cents, less than a 1% increment on the cost of a PCB order, it’s worth it.

Have you tried Open PCB, and how did it work out? Are you able to shed any light on the mystery board?  As always, comments are welcome!