Inside the DM24
from the ARM processor. The DSP can control all 5 ADCs and process the data in real
time.
An important feature of the system design is its ability to synchronise the sampling
of the analogue to digital converter to an external time reference. This way, data
samples are accurately time stamped at source. To keep sampling accurately in step
with UTC, the microprocessor's time-base is synchronized to an external reference,
derived from GPS or, in larger arrays, to a centrally-transmitted time reference.
Sharing a time reference avoids the cost and power consumption of multiple GPS
receivers and, since it only involves sending 2 characters per second, it can utilise a
low bandwidth link.
To achieve the high degree of timing precision required for a 24-bit digitiser system,
the microprocessor time-base is run from a precision voltage controlled oscillator.
On-board software keeps this oscillator tuned to the external reference so that its
frequency is accurately set and maintained through changes in temperature or
ageing. Once the system has stabilised, the control is sufficiently accurate to
maintain precision sampling for several days without an external reference. The
system also automatically compensates for the pure time delay introduced by the
digital filtering/decimation processes in the DSP.
The DSP software consists of 6 cascaded programmable filter/decimation stages,
which allow you to select multiple data output rates simultaneously. Each stage can
be set individually for decimation factors of 2, 4, or 5. Data can be output at up to four
different rates. For example, a system can be configured to provide data at 200, 50,
and 10 samples per second, covering the whole of the seismological broad band
range. The configuration of the DSP is programmable in the field
via
the host ARM
microprocessor.
The DM24's interface to the world is via its serial ports. Port 0 is used for GCF data
output. Port 1 is the NMEA input from the GPS receiver and Port 2 is the system's
console. The console can be used as a command line interface without interrupting
data flow via Port 0.
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Issue U - December, 2021
