Guralp Systems CMG-DM24 - Manual (Page 104)

CMG-DM24
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
digitizer 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 any or all of
these rates. For example, a system can be configured to provide data at
200, 50, and 10 samples/sec, 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 primary digital interface is the multiple serial port card, which
allows the DM24 to support up to 8 serial ports using the ARM's on-
board UARTs (Universal Asynchronous Receiver/Transmitters). On a
DM24 with analogue inputs, one serial port is usually configured to
send the data packets to a local SAM or DCM unit for storage or, via a
modem or radio link, to a central recording station. A second serial
port is often used with a local GPS receiver for time synchronization,
or, alternatively, the first (data) port can be used for time
synchronization from a central station.
The UARTs and serial port module are optically isolated to avoid any
ground loops that could degrade the performance of the ADC’s. The
serial port module also includes 32k of RAM for data buffering and
formatting by the transmission/reception process.
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