Search algorithms for continuous gravitational waves are best tested by injecting simulated signals into the detector hardware. These signals need high phase and amplitude stability for reliable calibration. At GEO 600 we have developed an instrument to inject a simulated pulsar signal with a phase error less than 1% of 2π from the correct signal phase. The injection can run over several months and can recover from interruptions. A key component of the instrument is an analogue electronic

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... that can be applied to an actuator of one of the test masses of the interferometer. For the generation of this signal a microcontroller is operated as a direct digital frequency synthesiser (DDS). A digital phase locked loop (PLL) on a control computer controls the phase accumulator of the microcontroller via its phase increment register. The signal to be injected is calculated within the LIGO/LSC algorithm library on the control computer.