ALOHA Cabled Observatory
|
ALOHA Cabled Observatory
|
|
|
Aloha Cabled Observatory Engineering Documentation Digiquartz Rabbit Interface
DigiQuartz interface The DigiQuartz card consists primarily of 32-bit counters used as period counts and running (rollover) counts. Using the manufacturer-supplied calibration curves, the counts and be used to measure the ocean depth to a resolution of a few centimeters at depths of kilometers. The inputs from the DigiQuartz go to a front end on the board to square up the signals to clean TTL and go to the counters. The counters are then read by the Rabbit 3000 Controller (9).
Since the DigiQuartz pressure sensor is a frequency-output transducer, the absolute accuracy would be degraded by drift in the local oscillator used to measure that frequency. The decision was made to send back raw counts to shore. In this way the DQ frequency can be measured against a GPS clock. The other advantage is that any desired filtering can be applied in post-processing. One minute, one hour and one day averages can all be performed depending on the precision and time resolution desired. With a frequency-output device there is always a trade-off between sample rate and resolution. The nominal frequency is around 30 kHz, with a 10% change representing full-scale pressure. With a 3 kHz delta, at least a minute count is required to get down to 3 centimeter resolution. For fast sample rates, where the resolution becomes poor, a reciprocal n-period measurement with a 40 MHz clock provides the solution. Although this measurement is affected by drift of the local oscillator, this error can be corrected by reference to the long frequency counts. Although we will certainly do this initially to remove the discrepancy between the two readings, it is probably not necessary to do so on a continuous basis, since this channel can be considered as an “AC coupled” or delta channel. If the need arises, there is a way to do that correction -- even retroactively. This high-rate data should be excellent for detection of tsunami.
Rev: 08/10/2007
|
|
|
