Sensor node 5 with the CC2500 transceiver board serial 01042 was mounted on an outdoor location on 7 November 2012 and unmounted on 18 November 2013. Before it left my office and after it returned, I measured the relationship between power at the antenna interface indicated by CC2500 RSSI register and signal power, as reported by a calibrated Rohde & Schwarz SMBV vector signal generator. In both cases +9.50 dB has been added to the indicated power, which was the calibration value for this transceiver board. According to my testing, RSSI offsets between +9.50 and +11.50 dB are typical for this series of boards.
You can see the difference between these two measurements on the graph below. The sensitivity of the receiver has dropped by nearly 40 dB sometime during the last year. This directly contradicts the theory that bad sensitivity was due to overheating during reflow soldering.
Another hint at what happened can be seen from the following log of in-situ RSSI measurements. The graph shows the received signal strength at a neighboring node 3, listening to transmissions using this transceiver board. Obviously something happened to this board in the first week of February that drastically decreased its transmission power (while it can't be seen from the graph above, transmission power has also similarly decreased between the two times I had this node on my desk).
Curiously, similar measurement data for the reverse direction (so in the case where transceiver 01042 would listen to transmissions from its neighbor) stops at the same day of February, due to what looks like SD card failure on the sensor node 5.
So, from this new evidence I can conclude that at least in this one case:
- The failure appeared while the node was mounted on a light pole,
- change was not gradual but instantaneous,
- it likely happened together with failure of other components.
As Iggy commented on my last post on this topic, a humidity problem would still fit this description (as water could easily break the SD card interface). Unfortunately we don't seem to have any suitable ovens available at the department to try baking the bad boards to see if this change is reversible (I'm a bit partial to trying this in my kitchen). As I am currently focused on UHF receivers this problem is not high enough on my priority list to try and coordinate such an experiment with some other institution. Unless of course somebody else is interested in investigating this issue - in that case I would be happy to provide a pile of bad transceiver boards.
I might actually just try the reverse and plop a few good boards into a glass of water over night and see if that has a similar effect on them.