Function:

This routine generates an array of complex numbers from the swept sine information in a frame, and an overall calibration constant. Multiplying this array of complex numbers by (the FFT of) the raw IFO data yields the (FFT of the) differential displacement of the interferometer arms , in meters: . The units of are meters/ADC-count.

The arguments are:

`fri:`Input. Pointer to an array containing swept sine data. The format of this data is`fri[0]=`,`fri[1]=`,`fri[2]=`,`fri[3]=`,`fri[4]=`,`fri[5]=`,... and the total length of the array is`fri[0..frinum-1]`.`frinum:`Input. The number of entries in the array`fri[0..frinum-1]`. If this number is not divisible by three, something is wrong!`npoint:`Input. The number of points of IFO output which will be used to calculate an FFT for normalization. Must be an integer power of 2.`srate:`Input. The sample rate in Hz of the IFO output.`response:`Output. Pointer to an array`response[0..s]`with in which will be returned. By convention, so that`response[0]=response[1]=0`. Array elements`response[]`and`response[]`contain the real and imaginary parts of at frequency . The response at the Nyquist frequency`response[N]=0`and`response[N+1]=0`by convention.

The absolute normalization of the interferometer can be obtained from
the information in the swept sine file, and one other normalization
constant which we denote by . It is easy to understand how this
works. In the calibration process, one of the interferometer end
mirrors of mass is driven by a magnetic coil. The equation of
motion of the driven end mass is

(4.10.22) |

(4.10.23) |

Putting together these factors, the
properly normalized value of , in meters, may be obtained
from the information in the IFO output channel, the swept sine calibration information, and the
quantities given in Table by

The constant quantity indicated in the above equations has been calculated and documented in a series of calibration experiments carried out by Robert Spero. In these calibration experiments, the interferometer's servo was left open-loop, and the end mass was driven at a single frequency, hard enough to move the end mass one-half wavelength and shift the interferences fringes pattern over by one fringe. In this way, the coil voltage required to bring about a given length motion at a particular frequency was established, and from this information, the value of may be inferred. During the November 1994 runs the value of was given by

(4.10.25) |

- Author: Bruce Allen, ballen@dirac.phys.uwm.edu
- Comments: See comment for
`calibrate()`.