Homodyne impulse radar hidden object locator

An electromagnetic detector for locating a cavity within a separator, comprising:

• a signal generator for generating a sequence of pulses having a predetermined shape and frequency;
• a step generator connected to said signal generator for producing a sequence of transmit pulses:
• a homodyne oscillator connected to said step generator for modulating said sequence of transmit pulses at a homodyne modulation frequency;
• a transmit antenna connected to said step generator for transmitting said modulated sequence of transmit pulses through said separator;
• a receive antenna for receiving reflected pulses from the cavity;
• an averaging sample and hold (S/H) circuit connected to the receive antenna for sampling a plurality of reflected pulses to produce an average reflected signal;
• a gating switch connected between the receive antenna and the S/H circuit;
• and to the signal generator so that only reflected pulses from a selected range are sampled by the S/H circuit:
• a DC bias rejection filter for significantly reducing near wall proximity effect on said receive antenna;
• said filter comprising an AC coupled amplifier connected to the output of the S/H circuit for filtering out DC bias level shifts in said receive antenna;
• and
• a rectifier circuit connected to said homodyne oscillator and to said AC coupled amplifier, for synchronously rectifying said reflected pulses to remove the homodyne modulation frequency.

An automated tool arrangement for use relative to an object behind a separator comprising in combination:

• two or more generally identical detectors, mounted on a tool housing;
• said detectors being distally separated from each other;
• and
• each of said detectors comprising:
• a signal generator for generating a sequence of pulses having a predetermined shape and frequency;
• a step generator connected to said signal generator for producing a sequence of transmit pulses:
• a homodyne oscillator connected to said step generator for modulating said sequence of transmit pulses at a homodyne modulation frequency;
• a transmit antenna connected to said step generator for transmitting said modulated sequence of transmit pulses through said separator;
• a receive antenna for receiving reflected pulses from the object;
• an averaging sample and hold (S/H) circuit connected to the receive antenna for sampling a plurality of reflected pulses to produce an average reflected signal:
• a gating switch connected between the receive antenna and the S/H circuit and to the signal generator so that only reflected pulses from a selected range are sampled by the S/H circuit;
• a DC bias rejection filter connected to the output of the S/H circuit for significantly reducing near wall proximity effect on said receive antenna;
• a demodulator connected to the output of the DC bias rejection filter and to the homodyne oscillator to remove the homodyne modulation frequency from the reflected signal.

An automated tool arrangement comprising in combination:

• a transmitter unit;
• two separate receiver units disposed generally equidistally on either side of said transmitter unit;
• said transmitter unit and receiver units being mounted on a tool;
• said transmitter unit comprising:
• a PRF generator for producing a sequence of pulses;
• a transmit pulse step generator connected to the PRF generator for generating a sequence of transmit pulses;
• a homodyne oscillator connected to the transmit pulse step generator to modulate the sequence of transmit pulses;
• a transmit antenna connected to the transmit pulse step generator for transmitting said modulated sequence of transmit pulses;
• an adjustable delay connected to the PRF generator to produce a delayed sequence of pulses;
• a gate pulse step generator connected to the adjustable delay;
• a pulse differentiator connected to the gate pulse step generator to produce a sequence of gate pulses;
• each receiver unit comprising:
• a receive antenna for receiving reflected pulses from an object;
• a sample and hold (S/H) circuit connected to the receive antenna for sampling a plurality of reflected pulses to produce an averaged detected signal;
• an AC coupled amplifier connected to the output of the S/H circuit to subtract DC bias level shifts caused by near wall proximity effects on the receive antenna;
• a synchronous rectifier connected to the output of the AC coupled amplifier and to the homodyne oscillator for demodulating the averaged detected signal:
• a DC coupled amplifier connected to the output of the rectifier for producing an amplified demodulated detected signal;
• a display connected to the DC coupled amplifier for displaying changes in the amplified demodulated detected signal;
• wherein said transmit unit transmits a common gate pulse to both receiver units.

A method for detecting an object or cavity behind a separator, comprising:

• generating a sequence of transmit pulses;
• modulating said sequence of transmit pulses with a homodyne continuous wave (CW) signal at a homodyne modulation frequency;
• transmitting said modulated sequence of transmit pulses in the direction of the separator;
• receiving reflected pulses from the object or cavity;
• averaging a plurality of reflected pulses to produce an averaged detected signal;
• subtracting DC level shifts caused by near wall proximity effects on the receive antenna from the averaged detected signal;
• demodulating the averaged detected signal to remove the homodyne modulation frequency.