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Loop
Output
Detailed Description of Operation
When power is applied to the sensor unit, the microcontroller first checks the condition of the
front panel switches to determine that the channel is enabled, what sensitivity level is set and
what operational mode is required. Then the loop oscillator is activated to obtain a sample of the
loop frequency.
When the loop oscillator is activated the combination of the inductance of the road loop and the
added capacitance within the oscillator circuitry produces a resonant frequency. The oscillating
signal in the road loop sets up an alternating magnetic field surrounding the loop itself. When the
conductive metal in a vehicle enters the magnetic field it acts in a similar manner to a shorted
turn and the effective inductance of the loop is reduced thus increasing the resonant frequency.
Block Diagram of NT11/NT11-E Loop Sensor Unit
Loop
Oscillator
Frequency
Measuring
Circuit
Microcontroller
Indicators
Switches
Switches
Indicators
Page 7
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