Permeability detection apparatus, image forming apparatus or digital copier using the same, toner concentration detection apparatus, and electric conductivity detection apparatus

A permeability detection apparatus, comprising:

• a detection circuit including a circuit element which has a coil arranged in a region near a magnetic material whose permeability is to be detected, a capacitor connected to said coil in series or in parallel, and a resistance connected in series to said coil and said capacitor;
• an oscillation circuit that oscillates a natural frequency to drive said detection circuit;
• and
• a phase comparison circuit that extracts signals from an input side of said detection circuit and an end side of said circuit element to compare phases thereon
• said resistance having a function to allow said oscillation circuit to eliminate influence from said detection circuit and to oscillate said natural frequency continuously.

A permeability detection apparatus, comprising:

• a detection circuit having a coil that is arranged in a region near a magnetic material whose permeability is to be detected and that essentially consists of a first coil of a first resonant circuit and a second coil of a second resonant circuit, which is magnetically coupled with said first coil, first and second capacitors respectively connected to said first and second coils in series or in parallel, and a resistance connected to said first resonant circuit;
• an oscillation circuit that oscillates a natural frequency to drive said detection circuit;
• and
• a phase comparison circuit that extracts signals from an input side of said detection circuit and an output side of said second resonant circuit to perform phase comparison,
• resonant frequencies of said first and second resonant circuits being set approximately to the same value and said resonant frequencies are set approximately equal to said natural frequency,
• said resistance having a function to allow said oscillation circuit to eliminate influence from said detection circuit and to oscillate said natural frequency continuously.