What are in common between an AM radio and an OTDR? Technique used in old AM radio can boost modern OTDR performance.

1. AM Radio and Optical Time-
Domain Reflectometer
Dr. Fang Xu
5/4/2014

2. 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?

3. 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

4. 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.

5. 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.

6. 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