1. Hardware injection of continuous gravitational wave signals at GEO600 U. Weiland, G. Heinzel and the GEO600 team References P. Jaranowski, A. Królak, B. F. Schutz, Phys. Rev D 58, 063001 (1998) http://www.lsc-group.phys.uwm.edu/lal/ B. Willke et. al., Class. Quantum Grav. 19, 1377-1387 (2003) M. Hewitson, G. Heinzel, H. Grote, H. Ward, K. Strain, U. Weiland, Rev. of Scien. Instr., (September 2003) http://www.geo-600.uni-hannover.de/ Max-Planck-Institut für Gravitationsphysik Mapping the amplitude Due to the definition of the amplitude as a nonnegative number there are jumps of  in the phase. These phase jumps are converted into a change of the sign of the amplitude. For the production of the amplitude the following steps are taken: amplitude maximum during injection time is determined phase jumps are converted into a change of the sign of the amplitude amplitude is mapped onto an 8bit DA converter As the resulting amplitude is usually not symmetric about zero but the DA converter is chosen to be symmetric about zero, between 60% to 100% dynamic range of the DA converter is exploited, depending on the neutron star parameters. The amplitude error due to digitisation of the DA converter for 60% dynamic range is below 0.7%, and hence better than the accuracy of the injection hardware. Institut für Atom- und Molekülphysik The left graph shows the amplitude of a neutron star for GEO600 with h 0 =1 in arbitrary units, signal frequency=1 kHz, ascension=0, declination=0,  =  /2,  =0, distance 3600 parsec, and spindown 0 for January 2nd 2002. The magenta parts are times at which the amplitude is given a negative sign to take account of phase jumps of . This signal amplitude is mapped onto the 8 bit DA converter into bins 81-254, thus exploiting 68% of the dynamic range. The right graph shows the amplitude of a neutron star with the above signal parameters for the first 30 minutes of January 2nd 2002. Plotted are the numerical values of the amplitude determined with LAL and their mapping to the DA converter. For this signal the time period over which the amplitude is set to the same bit value is 60 s at the steepest slope and 2090 s at the shallowest slope. A sample rate of 0.1 Hz was used. The microcontroller's phase register is phase locked by a digital phase locked loop (PLL – a control loop) to the correct signal phase. The difference between the phase register and the correct signal phase provides the error signal for the control loop. A digital filter is used in the servo to produce a suitable open loop gain. As the ephemeris data for the correct position of the detector in the simulation with LAL is only updated every four hours, the maximum error in travel time  for the signal from the solar system barycenter to the detector can accumulate up to  0.5  s. This leads to a phase error of  = 3.14 mrad  [ f / 1kHz ]  [  / 0.5  s], where f is the frequency of the gravitational wave. For the above signal parameters the envelope of the phase difference due to the used ephemeris data is plotted in the graph to the right for the year 2002. The left graph shows the difference between the injected signal phase and the correct signal phase of a neutron star for GEO600 with signal frequency=1 kHz, ascension=0, declination=0,  =  /2,  =0, and spindown 0 for January 2nd 2002. The peaks every four hours are due to the limited accuracy of the ephemeris data used in LAL for the simulation of the signal phase. In simulations stable filters with sample rates down to 0.1 Hz have been found. In the graphs further down a second order filter with sample rate 0.1 Hz and unity gain 0.034 Hz has been used. In the picture on the right side the microcontroller with the interface to the data acquistion system, the remote computer and the hardware for injection is displayed. Abstract The Signal T he gravitational waves emitted by a non-axisymmetrical neutron star with a wobble angle of  /2 at twice its rotation frequency have the well known form Within the LIGO/LSC Algorithm Library (LAL) this signal can be produced numerically. The resulting analog signal is converted into a force that acts upon a mirror of the interferometer either via a coil magnet pair electrostatic drive photon drive test mass electrostatic drive coil magnet pair LASER radiation pressure triple pendulum ● The amplitude and the oscillatory part of the artificial signal are produced electronically independently of each other and then combined by analog electronic multiplication. ● A microcontroller with two DA converters is used, one provides the amplitude of the signal and the other the oscillatory part. The parameters for the DA converters are set by a remote computer via a serial link to the microcontroller. ● The absolute timing of the signal injection is controlled by synchronising an internal counter of the microcontroller to a 1 Hz signal of the data acquistion system, which provides the current GPS second. ● To produce the oscillatory part the microcontroller is operated as a direct digital frequency synthesiser (DDS). To obtain high phase stability the clock that increments the phase register is derived from a GPS-locked 4 MHz signal of the data acquisiton system. LAL Microcontroller multiplication phase register amplitude phase increment modulo 2  sine table GPS time remote computer data acquistion system GPS antenna 4 MHz 1 PPS DAC oscillatory part DAC amplitude to inter- ferometer sign digital PLL divider optical coupler clock synchronisation Schematic of the PLL Phaseregister Filter phase increment correct signal phase  phase difference stays below 10 -6 rad  PLL stable over several months  64 bit phase increment and phase register  50 kHz clock frequency Controlling the phase Hardware injection The gravitational wave signal can be rewritten with one amplitude and one oscillatory part only: hardware injection of pulsar signals with phase errors < 1% over several months to test and calibrate search algorithms self built microprocessor based frequency synthesiser using a PLL to control the signal phase injection scheme recoverable from interruptions commercial frequency synthesisers unsuitable, as phase register cannot be read out and directly controlled