Hi! Welcome to the website of N7EKU -- documenting my homebrew projects and experiments with amateur radio. I hope that my work will help inspire you to experiment and build projects of your own too! Feel free to email me with questions or comments at the email address at the bottom of the page. I do plan to add a lot to the website, so please excuse if it changes a lot from visit to visit. 73 -- Mark
This is one of my current projects. Details of this CW transceiver, which was conceived by Roy Lewallen - W7EL, may be found on the ARRL website here. I had been looking around for a nice 40M QRP rig to build and was happy to find this one. It caught my eye because it seemed to be a well-thought out design with all of the main features that I wanted: low battery drain, zero-beat switch, RIT, mostly discrete parts, very nice design, and small size.
To ease testing and facilitate later customization, I divided the circuit into eight separate PCB's of varying size. I am building each plate in the 'dead bug' style on both etched-pattern and cut island boards, and using a mixture of SMD and traditional leaded parts. I am building the sections in an order that should facilitate testing each board as I go.
I plan to go at a pace of about one to two boards per week. The design uses fairly common parts that are easily available at Radio Shack and mail-order suppliers like Digikey, Mouser, and Arrow. I plan to detail the building process and testing of each plate as I go along, and will describe any particular trouble spots that I may experience. Please see below for picture galleries and building/testing details of each plate.
Anyone interested is heartily invited to follow along at home and build your own rig along with me! One wouldn't have to use any SMD parts if you don't want to -- I just chose to use some as they are faster for me to do. If built with all leaded parts, I think the boards would just need to be scaled up to about double the dimensions that I used.
Plate 0, Zener Diode/NPN Voltage Regulator This plate is 20 x 25 mm in size. As built, it provides ~9 V with a load of ~9 mA (a 1 kΩ resistor across the 9 V terminals) within a supply voltage range of 10-15 V. The efficiency is quite good, the regulation duties consuming only about 2 mA total. This was superior to the use of a 78ls05 adjusted for 9 V which required ~14 mA to do the same job. I am planning to mount the board to the solder lugs of two banana jacks spaced 3/4" on-center on the back panel of the rig's enclosure. Looking at the first picture in the gallery, the top left pad is the regulated positive output, the bottom left is positive input voltage, and the bottom right is ground. Once the board is mounted, I am planning to add a diode across the inputs for reverse voltage protection; if the power is connected incorrectly, the diode will shunt the input and blow the fuse of the power supply. This method adds some protection to the radio without causing the voltage drop experienced when a diode is connected in series.
Plate 1, RIT Circuit This was an interesting plate to construct. I originally was thinking of cutting some square pattern islands into the PCB as I did with the plate above. However when I studied the schematic more, I saw that there were some places in the circuit where there were many connections (these are described as "nodes" in engineering texts, and are simply connection points in a circuit that go to more than two places). I wanted to try using mostly SMD parts on this plate, but if I used a square-patterned PCB, each pad would only be directly adjacent to four other pads. Also, it would be difficult to solder an SMD transistor at a four corner point. So I instead decided to make a hexagonal patterned PCB so that I would end up having six places for direct connections on each pad. It also has, perhaps, the more important benefit of giving more flexibility in the layout and component connections due to the offset pattern of the hexagons. This worked out pretty well and made the layout on the PCB a bit simpler.
Figure 1 As you can see in the left hand square-patterned layout, the design restricts you in both the number of nodes available and also in direct connection routes. Looking at the center hex-patterned and the right hand rectangular offset-patterned layout, you can have nodes of up to six and also many numbers of connection routes.
I used mostly SMD parts on this plate and thus it ended up pretty small at 15 x 20 mm. With leaded parts, a square-pattern layout would also work quite well since the parts give you reach with the leads (I had to use the hex pattern because I wanted to try some SMD parts). It would also work well to make a rectangular patterned board with the rows of rectangles offset from each other. This would give good places to solder transistors and would also give six adjacent pads for direct connections.
Looking at the first picture in the gallery, the three hexagonal pads across the bottom are for the RIT potentiometer; bottom-left also for Vreg; bottom-middle for Vout; middle-right pad is ground; top-right is keying; top-left is +12V; the small component marked "P03" is the PNP transistor; and I didn't have an 18 kΩ SMD resistor so I used two 36 kΩ ones in parallel instead. I plan to mount this board on the back of the RIT pot.
The plate tested successfully on May 8th. At the extremes of the RIT potentiometer, Vout goes from 3.67V to 9.08V; on grounding the keying line, Vout goes to 9.04V (regardless of RIT setting).
Plate 2, Side Tone Oscillator Double, double, toil and trouble... First I had trouble from a tough to figure out construction problem. Second, I had trouble when I decided to try a Twin-T AF oscillator design instead of the astable-multivibrator circuit used in the original design.
My first trouble came from an unexpected direction, but was good to figure out in the end! I typically use a fiberglass reinforced plastic-jawed Panavise to hold my little PCB's while I solder on the parts. The problem I had with this was that the jaws on this vise (being made of plastic and all) tend to flex a little bit, so when I clamp the PCB's I have to tighten the vise pretty firmly in order to keep the board from moving around when I solder. This was causing the board to bow a bit, and when unclamped the PCB would flatten out. Since the SMD parts don't have leads (which would absorb the flexing) they were stressed by the flattening and would imperceptibly crack internally. The SMD resistors seemed pretty tough and didn't have any problem, but when I measured the SMD capacitors they showed widely varying values. I think the SMD capacitors are not as tough as the SMD resistors because they are multi-layered so it's kind of like quartz versus shale. Now I am, instead, holding my PCB's for soldering by taping them to a work surface with masking tape. This actually turns out to be a lot better anyway, because now I can quickly reposition the board by simply untaping it and turning it exactly in the position that I want. It only takes a small piece of tape to hold it and, if you buy the good 3M brand kind, it lasts a long time.
My second brew of trouble came from my (and it seems many others!) unfamiliarity with the Twin-T oscillator design (see: The Misunderstood Twin-T Oscillator). In the original transceiver schematic, the author uses an op-amp to contruct a simple astable mulitvibrator AF oscillator (fancy name for an audio square wave generator!) So I decided to do a switch to a sine wave generator as those are a lot more pleasing to hear. But to quote from the article, "Take your favorite electronics book and look at what it says about twin-T oscillators. Probably nothing. Now try another book or handbook; you will soon find out that the authors show a lot of prudence on this subject. But you might be lucky and finally discover an evasive paragraph on the principle of operation of such oscillators." So you can imagine where our trouble comes from when we try to design our own Twin-T oscillator! The main thing is that the twin-t gets it's name from the twin RC filter circuits which make up the feedback part of the oscillator. But if you make those filters too perfect (like I did) then you need a perfect amplifier (with infinite gain) to make it work! The key can be seen in the Wikipedia article about it: make sure you have your ratios of the large/small resistance and large/small capacitance correct (greater than a factor of two). Once I did that all was fine -- all I can say is thank goodness for the internet and for breadboards!
Construction details: As you can see in the picture gallery, this plate was all SMD parts except for the transistor. The plate dimensions were 20 x 18 mm with knife-cut islands. Looking at the scan of my notebook page, there are the parts I used in a column on the left, the parts layout in the middle, and a schematic on the right. The schematic was from and earlier test I did, but it is essentialy the same with different values (also the R3 and R4 potentiometers have been replaced by fixed values). As you can see I used a ratio of ~3 (25kΩ/8.2kΩ and 33nF/10nF) for my large/small resistance and capacitance choices. The transistor I used was just a generic house-numbered NPN.
For testing (May 10th), I used an electronics breadboard and, looking at the schematic of the rig, hooked up just the section of the audio amp to which the side-tone oscillator injects its signal. Then I powered up both the amp and side-tone oscillator and listened with a pair of headphones. The sound was good, but a little low in volume. The article states though that this can be adjusted by changing the value of the 100kΩ resistor that goes to pin 6 of the audio amp if needed, so I will most likely put a trim pot in place of that resistor on the audio amp. For another test, I checked the frequency, amplitude, and wave shape with an oscilloscope. The results were 660 Hz, 740 mVpp, and a very nice sine-shaped wave.
Plate 3, Audio Amplifier
Plate 4, Active Audio Filter
Plate 5, VFO
Plate 6, Mixer
Plate 7, Transmitter
Below I will show the methods I have used in constructing homebrew projects. Many of these I have learned from researching on the web, some others I have thought of during the process of construction. I hope they will prove useful to other electronic homebrewers.
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I sure hope you enjoyed my site. 73!
Comments and questions may be sent to my email address: my callsign @ this website (without the w's).