Guralp Systems CMG-DM24S12AMS - Manual (Page 56)

CMG-DM24S12AMS Operator's guide
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, you can synchronise the
microprocessor's time-base to an external reference such as GPS, or in
larger arrays to a centrally-transmitted time reference. Transmitting 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 band-width, even half-duplex 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 hours 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 main microprocessor board incorporates a battery-backed real-time
clock and RAM which is used to set the system's internal software clock
at start-up independent of the availability of the external time reference.
The RAM is used to store system parameters such as the optimum
control voltage setting for the system time-base and the system
configuration.
The microprocessor module includes the (multi-tasking) system
operating software in 64K EPROM. This module also has 512k of static
RAM for system workspace and data buffering depending on the
system requirements (number of data channels and sample rates).
The microprocessor serial port (19,200 Baud) provides an interactive
interface for system setup and configuration. This port is known as the
terminal port. Unlike the GPS serial port or the data port, the terminal
port is not optically coupled to the outside world. Care should be taken
not to run a terminal over very long RS232 cables.
The DSP software consists of 4 cascaded programmable
filter/decimation stages, which allow you to select multiple data output
rates simultaneously. The first stage is set to decimate the data by 10
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