Carlos Camargo wrote: > I prefer read notes about SW and HW development,
More of that would indeed be welcome. Well, here's a bit from my side: during the last few weeks, I've been fighting with the antennas. What I was looking for was the correct size for the antenna. What I found was a little more ... The antenna size is an issue because the design I'm using as a basis is for a 1 mm board while I have a 0.8 mm board. Alas, the information I was able to find on adjusting the antenna size was little more than "make it slightly longer", without being too specific on the "longer" part. Therefore, I made a set of antenna boards, ranging from 80% of the original size to 125%, and tested their performance. Here's a picture of them: http://downloads.qi-hardware.com/people/werner/tmp/antfarm.jpg My RF test equipment consists mainly of an USRP2 with an XCVR2450 board. One issue I didn't anticipate was that the XCVR2450 can't send and transmit at the same time (some of the transceiver boards can). So instead of connecting the test antenna and a reference antenna to the TX and RX side of the USRP2, as I had planned, I had to use one of the WPAN boards I've made so far as the sender, with the USRP2 only receiving. (I wanted to have the USRP2's much larger resolution - 14 bits vs. 5 bits - at the receiver side.) The AT86RF230 transceiver chip is capable of emitting a constant test signal if its TST pin is pulled high. For space reasons, the atusd boards don't have this option, but the atusb boards can be reworked to have it. So I did that and adapted firmware and user-space tools accordingly. Now, gnuradio - the software that talks to the USRP2 - comes with a great many features, but it isn't so easy to wrap this into something suitable for automated testing. With a total of 13 antennas 16 channels to test, several successive runs per configuration (to combat noise), and at least two passes of the whole process (for drift), this clearly needed automation. What I did then was write a script that would set up the sender, record a few thousand samples to a file, remove all sorts of noise and interferences, and finally extract the received signal strength. The gory details of this process can be found here: http://projects.qi-hardware.com/index.php/p/ben-wpan/source/tree/master/usrp/README One problem I encountered, and that was also one of the reasons for the complexity of the filtering, was that the signal strength my calculations produced was surprisingly weak. The spectrum looked right and I could also verify with the gnuradio program that provides an FFT display that things appeared reasonable (and the signal was stronger). Well, to make a long story short, it turns out that, when I run a sufficiently large number of receptions, the USRP2 enters a mode where it massively attenuates the signal. I haven't tried to find out what exactly is happening, but power-cycling it before each test run made the problem go away. Here are some of the results I got: http://downloads.qi-hardware.com/people/werner/wpan/three-ants.png The green curve (ant-120B0) is one of the attenuated signals that resulted from the USRP2 switching to that "bad" mode. The "good" signals are red and blue (ant-120A0, ant-120B2). Now, there's a little problem with them: they should show more or less flat lines, but instead, there's a deep drop right in the middle of the frequency band. When I compared with a regular WLAN antenna, I still saw some problems in the middle, but nothing like this abyss. First, I believed in yet another artefact of my measurement setup, but I couldn't find anything. I then went to search for material on PCB antennas I hadn't found before, hoping that there may be something I had overlooked or misunderstood in the description of TI's design. Eventually, I came across a paper describing a similar PCB antenna, which recommended to place the vias 50 mil apart. The formula I had used earlier suggested that 5 mm would be sufficient. Details are in http://projects.qi-hardware.com/index.php/p/ben-wpan/source/tree/master/ecn/ecn0007.txt I reworked the ant-120B board, now calling it ant-120C, and this antenna produced the purple curve. This is clearly much better than anything I had before. The signal is still weak in the middle region, but we can probably blame the sending antenna for this, which also has the large via spacing (and only one ground plane, so I couldn't simply rework it and try.) Another result of all these experiments was that enlarging the antenna to about 110% of the original design seems to produce the best results, with 105% and 115% showing similar performance. The plan is now to make an atusb board that incorporates all those findings and repeat the tests with it. With a little luck, no major tuning will be needed after this. So far, I've drilled all my vias manually, with the inaccuracies and the broken drill bits that come with this. Since the atusb board now has quite a lot of vias (37 at last count), it's time to automate this part of the process as well. Thus, I'll first extend my collection of CAM tools to extract the drilling data from Kicad-generated Gerber files and then try which of drilling-before-etching or etching-before-drilling yields better results. - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
