Guralp Systems CMG-CD24 - Manual (Page 49)

Operator's Guide
4 Calibrating the CD24
4.1 Calibration methods
Sensors attached to the CD24 can be calibrated using the built-in
signal generator. There are three common calibration techniques
used.
•
Injecting a step current allows the system response to be
determined in the time domain. The amplitude and phase
response can then be calculated using a Fourier transform.
Because the input signal has predominantly low-frequency
components, this method generally gives poor results. However,
it is simple enough to be performed daily.
•
Injecting a sinusoidal current of known amplitude and
frequency allows the system response to be determined at a spot
frequency. However, before the calibration measurement can be
made the system must be allowed to reach a steady state; for low
frequencies, this may take a long time. In addition, several
measurements must be made to determine the response over the
full frequency spectrum.
•
Injecting white noise into the calibration coil gives the response
of the whole system, which can be measured using a spectrum
analyser.
Further information about calibration is available on the Güralp
Systems Web site.
4.2 Noise calibration with Scream!
A connected instrument can be calibrated using the digitiser's pseudo-
random broadband noise generator, along with Scream!'s noise
calibration extension. The extension is part of the standard
distribution of Scream! and contains all the algorithms needed to
determine the complete sensor response in a single experiment.
1. In Scream!'s main window, right-click on the digitiser's icon and
select Control.... Open the Calibration pane.
2. Select the calibration channel corresponding to the instrument,
and choose Broadband Noise. Select the component you wish
to calibrate, together with a suitable duration and amplitude,
and click Inject now. A new data stream ending Cn (n = 0 – 7)
March 2011 49