Guralp Systems CMG-DCM - Manual (Page 107)

Operator's guide
GPS Power Cycle : If you have selected Garmin GPS as the timing
source, above, this setting determines how often the attached
instrument will power up the GPS receiver to obtain an accurate
timing signal. Between timing fixes the instrument will run on its
internal clock, saving power at a small expense in accuracy. If your
instrument has ready access to a power source, you should select
always on.
7.2 Digitizer output control
The analogue-to-digital converters on a DM24 output data sampled at
2000 Hz, which is then filtered and reduced to a lower rate
( decimated) using an on-board digital signal processing (DSP) unit.
The DSP has four filtering-decimation stages, which run one after the
other. Each can be programmed to reduce the sampling rate by a factor
between 1 and 10. The output of each stage is called a
tap.
Each of the taps may be configured for a different decimation factor by
choosing values from the drop-down menus on the left. If you are
using a mouse wheel to select values from a drop-down menu, ensure
you remove the focus from the drop-down menu before scrolling the
window, or you may inadvertently change the setting.
Not all digitizers support the full range of taps and decimation factors.
For example, the Güralp DM24 allows you to select decimation factors
of 2, 4, 5, 8, and 10 only, and does not allow the decimation factor of
Tap 0 to be altered from its default setting of 10. In addition, no
combination of decimation factors may be used which produces a non-
integer data rate (in Hz). A full list of possible tap combinations for the
DM24 is given in Section 7.3, below.
To the right of each decimation factor menu is a grid of six check-
boxes marked Z, N, and E. These boxes mark which streams of data to
record at each sample rate. Three streams of data are measured by the
seismometer, corresponding to movement along each of three
perpendicular axes. Although all the streams are decimated by the
same set of successive scale factors, you can decide at which stage(s) of
processing each stream outputs data. A tick in one of the check boxes
will produce an output for a particular channel (column) at the
corresponding sample rate (grid).
Each grid also has two rows, which differentiate between constant and
triggered output. If a box in the upper row is ticked, that stream will
produce output constantly at the corresponding sample rate. If the box
below it is ticked, that stream will only produce output at that rate if a
particular set of trigger criteria are also met. If the constant-output
check box is ticked, the other will be ignored.
December 2006 107