AM Radio and Optical Time-Domain Reflectometer

Dr. Fang Xu

5/4/2014

AM Radio and OTDR

• AM radio, invented in early 10th century, uses an audio frequency signal to modulate amplitude of a radio frequency signal in order to transmit the audio signal for long distance.

• OTDR, invented a half century later, uses an optical light impulse to shot into an optic fiber in order to use returning light from the optic fiber to identify connection characteristics of that fiber

• Why do we study them together?

AM Radio and OTDR, what’s in common?

• They both use a carrier:– In AM radio, it is the radio frequency

– In OTDR, it’s the light

• They both exploit carrier’s amplitude– In AM radio, this will recover the audio signal

– In OTDR, this will discover connection events, such as connectors or loss

• They all use filter to remove noise – the reason we study them together

A Little History

• When OTDR was invented, I was high school student and try to optimize my AM radio.

• Filter was best friend to reduce noise.

• But when filter was too efficient, it could transform a violin concerto to a drum ensemble.

• So filter of intermediate frequency stage is designed under the AGC control: when carrier signal is strong, IF filter should be dumped to reduce Q, which allows more high frequency contents to pass through.

Filter in OTDR

• Then I discovered that OTDR is also use filters.• But for each test setup, you can either have filter

on or off.• OTDR standard defines dynamic range as

backscatter at origin of fiber to average noise density (either rms. or 98% from peak) when backscatter disappeared in noise.

• This definition encourage manufactures of OTDR to put highly efficient filters.

• Sometime, such filter is capable of transforming a violin concerto to a drum ensemble.

Solution

• I created a filter which improved dynamic range by more than a dB without transforming a violin concerto to a drum ensemble.

• It works like filter in my old AM radio