Abstract: A radio transmission system for a touch trigger probe for machine tools or coordinate measuring machines. In the transmitter shown in FIG. 1, a 1 kHz oscillator produces a signal which is phase modulated by an electronic switch and an inverter, when electrical contacts within the probe change state. This phase modulated signal is mixed in a mixer with an unmodulated 500 Hz reference signal, produced in a divider from the oscillator, so that there is a precise phase relationship between the two signals. The receiver shown in FIG. 2 has filters which separate out the 500 Hz reference signal and the phase modulated 1 kHz signal. A decoder phase demodulates the 1 kHz signal, using the 500 Hz signal as a reference in order to increase the reliability of the resulting signal.

Abstract: A touch trigger probe has electrical contacts. A comparator compares the resistance of the contacts with a threshold, in order to generate a repeatable trigger signal when the contacts open. To ensure that the trigger signal can be generated reliably even if the resistance of the contacts when closed should increase, the threshold set is variable. The threshold can, for example, be set by a sample and hold circuit which tracks the resistance of the contacts when closed.