Zero crossing detector and method of determining a zero crossing point

An ultrasonic flow meter comprising:

• an ultrasound pulse generator which emits an ultrasound pulse;
• an ultrasound receiver which receives the ultrasound emitted by said ultrasound pulse generator and which emits an output electrical signal corresponding thereto;
• a zero crossing detector comprising an analyzer supplied with a current input electrical pulse packet exhibiting multiple zero level crossings, said analyzer detecting one and only one of said zero level crossings of said current input electrical pulse packet occurring anywhere in said electrical pulse packet and emitting a trigger signal indicative of said occurrence, a pre-trigger unit, also supplied with said current input pulse packet, which monitors variations in an amplitude of said current input pulse packet to detect crossing of a pre-trigger level, having a variable amplitude, by said current input pulse packet, and a control unit, connected to said pre-trigger unit, and supplied with an amplitude signal derived from one of said current input electrical pulse packet and at least one previous input electrical pulse packet, for comparing said pre-trigger level to said amplitude signal and for automatically controlling the amplitude of said pre-trigger level to maintain a working difference therebetween to insure that said analyzer detects said one and only one of said zero level crossings;
• and
• an elapsed time indicator connected to said zero crossing detector, which receives said trigger signal from said zero crossing detector and which determines an elapsed time between generation of an ultrasound pulse and emission of said trigger signal.

A method for determining a zero crossing point of a current input electrical pulse packet comprising the steps of;

• establishing a pre-trigger level;
• monitoring variations in an amplitude of said current input electrical pulse packet relative to said pre-trigger level to determine when said amplitude crosses said pre-trigger level;
• determining a first occurrence of a zero level, crossing of said, current input electrical pulse packet after said crossing of said pre-trigger level by said current input electrical pulse packet;
• monitoring an amplitude signal derived from one of said current input electrical pulse packet and a previous input electrical pulse packet;
• and
• adjusting said pre-trigger level dependent on said amplitude signal.