Thursday, July 2, 2015

KiCAD

Since being laid off at Dynon Avionics, I have been looking around at schematic capture and PCB CAD packages.  Chris at Contextual Electronics has encouraged me to try out KiCAD.  I decided to give it a go and design a small 1 square inch microcontroller board with a full-on ATMega 328 processor and the minimum circuitry necessary to build an (almost) UNO clone.  The "almost" part is the fact that there will be no USB port on the board and it will need be programmed using the ISP connector.

The circuit diagramme I decided on looks like this from KiCAD:



Nothing here that doesn't need to be here for a minimal controller.  I have the entire I/O pin set that can be found on an UNO, plus since I use the TQFP package, I have two additional analogue input pins.

The board layout is pretty dense at one inch square.  Here is the board layout showing all of the layers from KiCAD.  I am not terribly impressed with KiCAD's copper pour capabilities, at least as far as I have explored them, but what do you want for free, eh?



Producing the Gerber files was trivial and I zipped up the lot of them and shipped them off to OSHPark to be manufactured.  Here is the screen shot of their rendering of the gerber files for the front and back of the board.



So, now we wait for the little purple envelope to arrive.  Meanwhile I will collect up the set of parts to build a few of these up.  This was an experiment to enable me to learn KiCAD and at the same time to produce a useful gadget for other projects.  Once I have verified the functionality is correct, I will publish the OSHPark boards so others can order them directly if desired.  At $5 for 3 of them, they are pretty cheap.

Overall, I found KiCAD relatively easy to learn, once I got a couple of key concepts down.  The main one was that hot keys apply to whatever your mouse is hovering over.  I found myself wanting to click to select something before hitting a hot key and this lead to all sorts of grief.  Moving objects once they are placed also needs a lot of work.  Rubber-banding of traces pretty much sucks.  Once I figured out how to drag an object, the best that KiCAD could do is straight line connect all the existing traces.  This ends up being pretty much worthless for anything other than adjustments of a few centimetres and you end up deleting all the traces and redrawing them anyway.

Selection of wires in schematics needs an easy way to select only a line segment or the entire line without having to draw box around it.  When you move the line, it needs to rubber-band the end points.  Straightening out a series of segments needs to collapse them into one segment and it needs to be able to easily break a line segment in two without disconnecting them.  All of these comments also apply to traces in the PCB layout editor.

I did like that KiCAD included the ability to view gerber files, a comprehensive footprint editor as well as building the notion of hierarchical schematics into the product as a key feature.  I cannot however get the bill of materials functionality to work, so I must be doing something wrong.  Always more to learn. 

For a free, open-source product, it is quite compehensive and there are a lot of part footprints available out-of-box and from 3rd parties.  Overall, I would continue to use it for hobby projects.  I would need to try and build at least a four layer board to have an opinion about suitability for commercial projects.

Thursday, June 25, 2015

Change of status

Today I accepted a Firmware Engineer position with LOUD Technologies in Woodinville, WA working with the Mackie Digital Audio Mixing team.  I will be starting mid-July, so my job hunt status appears to be over.  Yay!  Check out their web site when you get a chance.  http://loudtechinc.com/

Wednesday, June 3, 2015

Diode Ring Mixer - UPDATED

In an attempt to firstly melt more solder than I have of late, and secondly to try and actually understand the operation of a mixer, I have decided to build one up from discrete components, including (gasp) winding my own transformers.

The rather rough diagramme of the circuit I propose to build looks something like this which is based on the fine design by Pete Juliano N6QW found here:



I found a few 1N6263 Schottky diodes in my junk box and out of the lot was able to find four that matched on the forward voltage drop to the millivolt level at 0V300.  A quick look around found a bit of PCB and a 1/4 inch sheet metal hole punch allowed me to create simple Manhattan pads.  Here is the layout at the start of the build with just the diodes installed.



I wound coils on FT37-43 cores.  I seem to be out of enameled wire, so I am going to try this with 30 gauge insulated wire-wrap wire as the number of turns is small, though the coils are trifilar.  I wound the trfilar wires together using a drill motor and then wound the three wires onto the core.  Ten turns, both coils are identical.



Separating the coil winding ends, I check continuity on them and mark them with a Sharpie (why is a dull felt pen called a Sharpie?) on each end of the wire with no mark, a single mark or a double mark.  The no mark wire will be the single coil while the one and two mark wires will be hooked in series (observing phasing).  The left side as drawn in the circuit above, will have a 100 ohm variable resistor at the centre tap of the transformer secondary to allow small balance adjustments.  As suggested by Pete Juliano N6QW, but not drawn above, I plan to also include a 39 ohm resistor that is switchable to ground from the centre tap in order to intentionally unbalance the mixer.  This is useful for injecting carrier for tune-up purposes for example.

The final build, minus the output filter can be seen below.  I hope to get out of interview preparation mode soon so that I can have some time to check this out.  The trim pot is a 100 ohm multi-turn pot to allow fine tuning of the balance.  Connecting the top of the 39 ohm resistor to ground will deliberately unbalance the mixer.



Finally got a chance to see if this little guy is mixing or not.  I set the 100 ohm multi-turn pot to the middle and attached the output of a 20 MHz crystal oscillator to it along with a signal generator output and had a look at the output on the spectrum analyzer.  Firstly, we needed some connectors.


I gave it 20 MHz on one port, 1 MHz on the other, both at 7 dBm and obtained the following output.  Both the sum and difference were within 0.08 dBm of each other.


So, we seem to be mixing and with proper IF filtering following, should be good to go.  Many thanks to Wayne NB6M for helping with the testing.



Tuesday, May 26, 2015

Version 1.0 of Si5351 Signal Generator

I have completed version 1.0 of my signal generator.  There will definitely be a revision as there are a number of things still to straighten out, but overall I am pleased with the result.

The 3D printed box with 20% fill on the box walls, was easy to drill, though I recommend a drill press.  I made the top cover thick enough that my controls (rotary encoder and switch) could be threaded into the plastic and no surface nuts required.  This seems to work well, though it remains to be seen how it holds up over time.


The point-to-point wiring inside is a pain in the tush.  Version B will definitely have a single PCB with all the components (display, power switch, rotary encoder, Trinket PRO, Si5351 and battery) plugged into it and mounted on stand-off spacers to the front panel.  I also definitely need to screw down the display.  The friction fit to the panel is good, but not good enough.  The picture below was taken before the Si5351 was installed.


So, overall I am pleased and look forward to using this new addition to my tools.

Monday, May 25, 2015

So, what do you think?

The second round of 3D printing I think is very close to what I need for this simple Si5351 signal generator.  The new mounting method for the display works well.  A little bit of plastic needed to be trimmed in order for the display to fit into place, but not bad.


The display fits very snugly so I will let it be for now.  I will likely drill out the corner holes and install screws to keep things in place.


The rotary encoder will  be mounted to the right of the display.  In reality, with a single line of code I can flip the rotation of the display by 180 degrees so it is easily adaptable to left or right handed operation.

I have been considering how to implement a single control interface using the rotary encoder and its built in push button.  I think that I will use a single press of the encoder knob to cycle between the three clocks.  The active clock is the one displayed in yellow.

Turning the rotary encoder will change the currently selected digit of the active frequency.  To change the tuning rate, I am considering rotating the encoder while holding the knob depressed to select which digit is changed by the encoder.

Otherwise, there will be a power switch and the three clock SMA output connectors.  I am unsure if I will mount them on the face along side of the display or if I will mount them on the side of the case.  I plan to use a small LiPo battery to power the entire device.

Overall, I am pleased with 3D printing as a medium for prototyping things like this case.  The aesthetics are not the greatest, but the strength of the case and the amount of accuracy of the print to the object design is quite good.  It will be interesting to see if my son's calibration efforts on the printer will have any effect on the aesthetics.

Si5351 Signal Generator (continued)

My first attempt at 3D printing a case for my signal generator project was, shall we say a good learning process.  The printer printed what I created, but a number of rather fatal flaws in the design indicated the need to scrap it and try again.

Meanwhile, I have decided on the controller that will be dedicated to this project.  Given that the Si5351 and my TFT display are both 3V3 devices, I decided to utilize the Adafruit PRO Trinket 3V3 device.  This is essentially a 3V3, 12 Mhz Arduino Uno without the USB controller and with a couple of I/O pins being dedicated to other purposes and therefore unavailable (pins 2 and 7).



Since I developed the code on the ATMega2560, it seemed that my next task would be to convert the code to run on the ATMega328 processor found in this device and to make some final I/O pin assignments with this device in mind.  The conversion was a little more complicated than I first anticipated due to my sloppy coding on the first go-around.

I didn't want to put header pins on this board for this project, so I used my Arduino Uno to verify the code changes (and to clean up my sloppiness).  I have wired everything using the 3V3 source, but since the I/O pins are 5V on the Uno, I used an NTE4050B buffer to do the level conversion to 3V3 on the pins going to the display.  This will not be necessary once I wire up the intended controller as it is a 3V3 device.  The Si5351 is not shown in this image.



The setup of the 1.6.4 IDE to support the Trinket PRO was fairly straight forward, but here still be dragons.  The Adafruit web site recommends just downloading the IDE with the Adafruit board support definitions already installed.  This is convenient, but creates a set of additional problems which I will likely make the subject of a separate post.

The other thing I learned is that if a class name defined in your code happens to collide with a class name in an existing library in your installation, even if that library is not used in your project, that library will still be compiled and can cause duplicate symbol errors or can surface dependent library missing errors.  I ran into this because I have in my project a file that defines a class for the Si5351, but I also have installed the Adafruit_Si5351 library into the IDE which also defines a class with that name, though in a different name space.  Even though I am not using it, the Adafruit_Si5351 library gets compiled by my project, but since that library depends on the Adafruit_Unified_Sensor library which I had not installed, I got a compile error.  Had the compiler error not occurred, I would have likely gotten link errors.  I need to dig into this in more detail as I believe this to be a bug in the current 1.6.4 IDE from Arduino.cc but need to simplify the reproduction of the issue before filing a bug report.  A lot of work has gone into this version's handling of libraries, but here still be dragons.  My workaround was to delete the Adafruit_Si5351 library from my installation and able to recompile without errors.

So, now I need to make some software changes to allow me to eliminate the extra push button in favor of just the rotary encoder, its push button and the display being the entire user interface.  I also want to provide a mechanism to enable/disable each clock as desired.

My son meanwhile has been busy printing my latest attempt at a case for this project.  I am certain this will not be the final print, but the changes made were to eliminate the plastic posts to hold the display and to make the front panel thicker to give it more rigidity.  I increased the lip size and provided a recess for the display to sit in.  Hopefully it will be a nice friction fit, but if not, I will drill the case and install plastic screws to hold it in place at the corners.





3x4x1.5 inch project box
My son is in the process of dialing in his 3D printer.  This one looks a little rough as he has sped up the head speed considerably in order to cut down on print time.  His extrusion feed rate is a little high and his X vs. Y axis accuracy in this print is about 0.06 mm (about 2.5 mil).  After calibration his accuracy is now about 0.02 mm (about 0.78 mil) so the next print should be a lot cleaner.  His extrusion feed rate causes his interior dimensions to suffer a bit as he is putting down too much material.  He should have that dialed in before he reprints this for me.  We also went to a 20% fill rather than solid, so it will be interesting to see how well that can be drilled.  The top is a tight fit so I will likely need to cut a thumbnail slot along the edge so I can get it off the case once the display fills the hole.  Once he has the new calibrations in place and I have verified my geometry will work with the parts I have in hand, we will print one more and it should look a lot better.

More to come...

Tuesday, May 19, 2015

Si5351 Signal Generator - 3D print of case

Now that the software is in pretty good shape (with a couple lingering issues) I turned my attention to building a case for this project.  Here is a simple front panel design I whipped up (ok, painfully slowly created after many mistakes...) using the online CAD software at www.onshape.com.  They have a pretty cool product offering there and it is certainly sufficient for these kinds of simple projects.



My little display and rotary encoder should fit nicely onto this panel.  As soon as I decide which Arduino board I am going to put into this project, I will work up a nice case back that this will snap into.  Yeah, yeah, this is my first CAD project, so I am 100% certain I will need to redesign it in some fashion.  Like for example, I suspect that I will have to put mounting screws through the panel into posts.  Don't beat me up too badly about my lack of CAD skills.


At this point it is pretty handy to have a son with a 3D printer.  Here it is printing this little guy.  I will go pick it up tomorrow and see if my display will fit into it.



Here it is, finished in 56 minutes.



More to come...