Active Filters: Band Pass

• ULTRA PURE 125KHz SINE WAVE SIGNAL SOURCE
  For some RFID systems operating at 125KHz, a very low distortion signal source reference is needed.  The circuit shown on this page produces a 10-volt peak-to-peak signal into a 50-ohm load, with a distortion of only 0.01%. 

Active Filters: High Pass

• HIGH PASS ACTIVE FILTER COLLECTION
  This is a collection of inverting and non-inverting active high pass filter circuits.  I included one, two three, four and six pole filter circuits.  You can change the component value ratios shown to achive any frequency cut-off you may need. The circuit does not specify an operational amplifier.  The circuits should be used to select the needed resistor and capacitor components for a particular frequency knee and  slope.

Active Filters: Low Pass

• 3KHz LOW PASS FILTER PLUS AUDIO AMP
  This circuit uses a switched capacitor filter IC from National Semiconductor to filter signals with frequencies higher than the 3KHz needed for voice audio. The schematic includes an audio amplifier that is designed to drive a standard audio head phone. The circuit is described in more detail in the receiver section of Dave Johnson's Handbook of Optical Through the Air Communications. (this link is off-site)

• AUDIO AMP + 3KHz FILTER
  This circuit is the audio amp section for a complete optical transmitter. The circuit amplifies and filters the voice audio signals from an electret microphone. The circuit is described in more detail in the transmitter circuit section of Dave Johnson's Handbook of Optical Through the Air Communications. (this link is off-site)

• ELECTRIC FIELD DISTURBANCE MONITOR
  This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor. The system can detect human and animal motion by the electric fields they disturb.

• LOW PASS ACTIVE FILTER DESIGN COLLECTION
  This is a collection of inverting and non-inverting active low pass filter circuits.  I included one, two, three, four and six pole filter circuits.  You can change the component value ratios shown, to achieve any frequency cut-off you may need.  The circuit does not specify an operational amplifier.  The circuits should be used to select the needed resistor and capacitor components for a particular frequency knee and roll-off slope.

Active Filters:  Notch

• ELECTRIC FIELD DISTURBANCE MONITOR
  This schematic is the power supply and front-end sections of the field monitor that is discussed in more detail at Electric Field Disturbance Monitor. The system can detect human and animal motion by the electric fields they disturb.