The project is moving closer to production. I made five of the latest atben and five of the latest atusb boards, plus three atusb-pgm. Then I set up my RF lab again and recorded the spectrum around the constant wave test signal. Finally, I started sending out samples.
I didn't take a picture of all the boards, but here are the last two I have left: http://downloads.qi-hardware.com/people/werner/wpan/tmp/2boards-20110305.jpg The others look quite the same, with a slightly more brilliant tin surface on the atusb boards. (See below.) These are atben-20110219 and atusb-20110214 boards. You can behold the measurement results in all their beauty here: http://downloads.qi-hardware.com/people/werner/wpan/20110303/ Click on the thumbnails to enlarge. atusb-sil is an older atusb board (atusb-20101229-3), still of the SiLabs-based design. Likewise, atben-20110115-1 and atben-20110123-1 are older designs I included for reference. atusb-20110214-* and atben-20110219-* are the latest batch. The 2 x 16 very similar-looking graphs on the right show the spectrum emitted by each board when sending a constant wave at +3 dBm (2 mW) at +/- 0.5 MHz of the center frequency of each IEEE 802.15.4 channel. The y axis is normalized such that the peak is at 0 dB. Other information included in the graphs is the center frequency, the difference of the peak from the center, and the absolute magnitude of the peak. The frequency difference can also be seen as a slight shift of the peak from the center. Frequency differences are caused by variations in the crystals in the atben/atusb boards and also by any error in my USRP2's oscillator. IEEE 802.15.4-2003/2006 requires an accuracy of +/- 40 ppm. The USRP2 has about 20 ppm [1] (+/-, I supposed), which leaves a range of +/- 20 ppm for the measured tolerance. The crystal I used has a nominal tolerance of +/- 15 ppm. [1] http://gnuradio.org/redmine/wiki/1/USRP2GenFAQ#What-is-the-USRP2-reference-clock-stability As one can see from the data, all clocks are well within specification. The magnitude of the peak is shown as a number and also as a downward-growing green or red bar a bit left of the center. More is better here. Returning to the table, the leftmost column shows a summary of the spectra across all transmit frequencies and below a graph with the peak power for each frequency. In the summary, the green line shows the maximum power, the blue line the average power, and the red line the minimum power. The averaging algorithm I used isn't too great, so its role is more a decorative one. The peaks show us if there are any gremlins. Note that the center frequency of the USRP2 was set to +0.5 MHz off the transmit frequency. This causes artefacts at +0.5 MHz and +1.0 MHz from the peak. The other peaks should be for real. Some of the spectra look a bit like porcupines, with spikes of up to -30 dB in 0.25 MHz intervals. They may be indicative of the build quality of the respective board, and may also indicate poor antenna matching. I didn't test yet whether they cause a real degradation of performance. It may be possible to reduce them by adding a small capacitor. The design already provides a place for this option. IEEE 802.15.4 limits emissions outside +/- 3.5 MHz off-center to -20 dB from the peak, so there's no problem with meeting the standard. Last but not least, the power-over-transmit-frequency graph shows how the system performs at different frequencies within the 2.4-2.5 MHz band. My test equipment is not calibrated, so some distortions will also come from there. The ideal result would be a flat line, as high a possible (relative to boards tested with the same setup - absolute values are not very meaningful.) The atbens look pretty good. The atusbs are also reasonably flat but seem to favour higher frequencies. This may also be an artefact of the test setup. (In the case of atusb, also the PC it is plugged into becomes part of the RF system. I used an OQO 01+, which is basically a small metal brick.) All things considered, performance is quite smooth over the whole range and the individual devices from the latest run don't vary too much among each other. Next, there are the mechanical characteristcs. My production has somewhat variable tolerances, and they show in board dimensions: Board Length Width Comment Dest. ---------------- ------------- ------------- -------------- ------- nominal 37.6 16.3 atusb-20110214-3 37.0 -1.6% 15.8 -3.1% bad tin richard atusb-20110214-4 37.1 -1.3% 16.1 -1.2% bad tin werner atusb-20110214-5 37.5 -0.3% 16.2 -0.6% roh atusb-20110214-6 37.0 -1.6% 15.9 -2.5% adam atusb-20110214-7 37.5 -0.3% 16.2 -0.6% DNR C11/C13 adam Board Length outside Length inside Width outside Dest. ---------------- -------------- ------------- ------------- ----- nominal 35.5 26.3 9.9 atben-20110219-1 35.5 - 26.3 - 9.9 - adam atben-20110219-2 35.5 - 26.3 - 9.9 - roh atben-20110219-3 35.4 -0.3% 26.3 - 9.9 - richard atben-20110219-4 35.5 - 26.2 -0.4% 9.9 - adam atben-20110219-5 35.4 -0.3% 26.3 - 9.9 - werner All dimensions are in mm. The atbens are all quite good while the atusbs still suffer some small variations. I tried a different PCB cleaning process for atusb boards 3 and 4, which turned out to chemically alter the tin, making it quite difficult to solder. Also, atusb board 7 suffered copper delamination at C11/13 (RF power bypassing) during some rework, so these capacitors should not be reworked further. In general, I had a hard time making this last batch of atusb boards. The large ground areas, the number of vias, and also some issues with layer registration (well, that's just my clumsiness) all contribute to making the boards difficult to solder. Luckily, most of these problems disappear with industrially made PCBs. The atben were fun to make. As indicated above, I've already assigned boards for all the recipients of samples. The boards for Adam and Joachim are already on their way, the ones for Richard will follow next week. - Werner _______________________________________________ Qi Hardware Discussion List Mail to list (members only): [email protected] Subscribe or Unsubscribe: http://lists.en.qi-hardware.com/mailman/listinfo/discussion
