Friday, October 5, 2012

Programmable Attenuator: Arduino Shields

I received the Arduino Mega shields back from OSH Park a few days ago.  I like they look and feel of the boards, and through some stroke of luck, it seems they actually work as designed.  Here are two of the boards, one partially populated with connectors that will be used to drive the Attenuators.

I soldered on the rest of the parts, stealing headers from an Arduino Mega proto-shield.  I put together another LCD Shield from Adafruit.  I don't yet have the front-panel board, so the LCD was wired directly into the headers.  When the front-panel board is ready, the 12-pin header should connect to it.  The front panel will connect to the two rotary encoders (one for menu selection, one for adjusting attenuator values) and the LCD panel.  Here's the new shield running, but without attenuators attached.

A few days later, I found time to install the new components in a new aluminium case.  Stamping the metal is quite costly, so I did it by hand with a hand punch, drill, and lots of quality time with a 'nibbler' to cut out the rectangular holes for LCD and USB plug.  I put the LCD panel hole too low, so it's been hacked a bit and is no longer level.   I need to move the attenuator modules closer to the panel mounting holes so that the cables don't have to stretch quite as far too.  Will be more careful measuring the next one!

With my new attenuator in hand, I set about trying to test it.  I set up two Atom based systems with Atheros 9380 3x3 MIMO wifi adapters.  These each have three antenna, and I connected on to each of the attenuator modules.  After lots of fiddling around, I had the station uploading to the AP at what I believe is near maximum speed, about 330Mbps UDP throughput.  The download speed from AP to Station is only about 230Mbps, but I have not had time to figure out why there is such a discrepancy.  I updated our LANforge software to graph the rx-signal and wifi link speed and ran some tests with the Attenuator.  The attenuation starts at 20 and is adjusted in increments of 5 up the the max of 95.5.  Due to RF leakage in the AP and Station systems, the signal is never completely extinguished, but it does go to about -82 dBm signal, with noise at about -93.  It is still able to pass traffic reliably (at lower rates) at this attenuation.  Here's a graph of the results.  Note that the signal quality is on the left-hand axis, so it is not to scale with the other graph lines.  Overall, it seems that the system handled the test well.

No comments:

Post a Comment