Friday, October 5, 2012

Receiver v2 hardware design & build

As of this writing the receiver v2 is still somewhat of a work in progress, but the design of the main hardware is mostly done. In this new revision I wanted to address a couple of issues that I had with the old receiver, mainly that the old receiver was large, heavy, fragile and complex. The new receiver was thus supposed to be small, light, sturdy and simple :-). I also wanted to finally implement antenna diversity, which I never got around to do in the old receiver. In addition, I decided to dump the analog inputs altogether in favor of an external battery management board connected via I2C and to only have 8 servo channels.

The target size was 2 x 1 inches, which is about the footprint of the regular commercially available receivers. I originally hoped to get everything to fit on one board of that size, but it soon became apparent that if I were to manufacture the board myself using photoresist methods (two-sided this time, however), I would need to split the design in two. I might still do a single board version of the receiver, which could be manufactured by a commercial board manufacturer, for instance through ITEAD studio and their cheap prototyping service. Anyway, the boards of this new design are simply
  1. Connector board (schematic, layout)
  2. Main board (schematic, layout top, bottom, both)
The connector board is completely passive. As its name suggests, it only contains connectors. It mounts on the main board through a pin header and breaks out the servo connectors as well as I2C and UART.

Like before, I didn't want to buy too many parts specifically. So, many of the choices of components were dictated by what I had in my collection. In the end, I only had to order the AS169 antenna selector switch. A local electronics store Partco offers a quasi-cheap way to order a low quantity of components from wholesalers like Farnell. Normally for small orders Farnell would charge a fixed price for shipping and handling, which is in the order of tens of euros. Partco makes larger orders once per week, which make the handling fees more reasonable per item. It still ends up costing me around 25-50% more per component than if I ordered in bulk, but as we're talking about a couple of 0.50€ components for prototyping, it doesn't really matter.

For the RF design, as I am working with 868 MHz (~345mm) and the RF signal path lengths on the board are in the order of 20 millimeters, I was confident I didn't need to worry about impedance matching. Assuming perfect ground, the characteristic impedance of the signal paths would be around 75 ohm, which is way off the target of 50 ohm.

I took some photos along the way, as I was assembling the main board

Bare board, bottom side. Right after drilling and removing the photoresist.
The board looks covered in holes, but those are actually drops of water.
This board was my second ever two-sided board. The first was a success, so I was not too worried about this one.

Top side, with vias soldered in place.
Getting via holes copperized is something I don't know how to do at home. I understand that in commercial fabrication they actually begin (I mean even before applying the photoresist) by drilling the holes and then building a copper layer inside the holes by electrochemical methods. What I do is I push a copper wire through the via hole and cut it almost flush with the board. It leaves quite a bump on the board after soldering, so I can't have vias under components. This makes the board quite a deal bigger than what it could be.

Top side. Much of the components soldered.

Bottom side. Everything except the pin header and output series resistors in place.
As I got most of the stuff in place I tested that I could talk with the processor. This was just to make sure everything was working correctly before attaching the radio module, which is the most expensive component in the board by a factor of two. In the pictures, along with the radio module the top side is also missing the antenna selector switch, which I had to order specifically (I didn't have a hand-solder friendly version in my junk box). The bottom side is still missing the large female header as well as output channel protection resistors. I didn't want to end up killing the receiver if I accidentally shorted the servo connectors or if I plugged them the wrong way around. They limit the short-circuit current to the absolute maximum rating of the processor GPIOs. None of the servos I am using seem to have any ill effects due to the resistors or due to the fact that the signal fed to them is only 3.3V high. Getting rid of the resistors and instead being really careful, or using 0603 size instead of 0805 could help make this board still smaller, if I ever go for that.

Top side. Everything except for the antenna selector switch soldered.

Bottom side. Everything in place.
Here the board is otherwise complete, but I'm still waiting for my order of the antenna selector to arrive. I also haven't yet attached the antennas.

While waiting for the antenna selector, I assembled the connector board and tried it on. The two white connectors in the picture below are (from left to right) the I2C interface and the UART interface. The I2C is for connection to a battery management board, which will contain a high side battery current and voltage measurement IC (I found a suitable one from Texas Instruments, but haven't yet ordered it). In the current plan, the UART will be used to interface a navigation board that infers the attitude and the position of the plane (or other platform) from inertial measurements (gyroscopes), Earth's magnetic field (magnetometers) and GPS. More on that in a later post.

Connector board is completed and tested on.

View from the back.

The receiver is looking pretty good. I'm a bit worried about the strength of the connection between the main board and the connector board. My plan is to stick the connector board down to the main board with double sided foam tape. That way there is less stress on the header.

A close up, now with the antenna selector and the antennas connected.

The AS169 comes in a SOT-23-6 package,
which is among the largest cases for these things.

The antennas are just RG-174 coax, which have ~1/4 wavelength (82 millimeters) of the shield stripped at the end. These are by no means optimal antennas, and it is something I still need to look into. The idea anyway is to have one of the antennas vertically polarized and the other horizontal. This way I'm hopefully less likely to lose the connection due to orientation, or at least have less variation in the received signal strength. However, there still will be an orientation at which both of the antenna polarizations are orthogonal to the transmitter antenna. This is probably just a theoretical problem of ideal antennas, since with my old receiver I've been flying with just one vertically polarized antenna at a distance of 1.5km without any problems. Another choice would be to go with circular polarized antennas, but the size of them at 868 MHz seems a bit intimidating.

The bounding box of the entire assembly (not including the antennas) is approx. 51 x 26 x 22 millimeters. A plastic case would be very nice. I'll probably investigate getting one printed at Aalto University's fablab in the near future.

No comments:

Post a Comment