An animal confinement arrangement including a home base for transmitting a modulated signal and a receiver for receiving the modulated signal. The received modulated signal is demodulated using a demodulator. The demodulated signal is compared to a reference signal to produce a comparator...http://www.google.com.au/patents/US5781113?utm_source=gb-gplus-sharePatent US5781113 - Electronic confinement system for animals using modulated radio waves
