Attenuation measuring system

A fiber optic attenuation measuring system includesa transmitter for generating and transmitting a reference signal from a first end of fiber optic cable, and a receiver positioned to receive the signal from a second end of the cable. The receiver compares the transmitted signal with a reference signal to determine attenuation through the cable under test. In the preferred embodiment of the invention, the transmitter and receiver are independent units, which may be positioned separately with respect to one another, and the receiver is constructed to enable it to reproduce the reference signal through the use of a timing pulse transmitted with the test signal from the transmitter. In the alternative, a reference cable may link the transmitter and receiver for comparison with the test transmission.

BACKGROUND OF THE INVENTION 
This invention relates to a system for testing the attenuation of fiber 
optic cables. While the invention is described with particular reference 
to its use with aircraft systems, whose skilled in the art will recognize 
the wider applicability of the inventive principles disclosed hereinafter. 
Fiber optic cables are finding increased application in a variety of 
products, including aircraft control systems. They are useful in aircraft, 
for example, because the cables are light weight, because the cables do 
not require special shielding in that cross coupling between adjacent 
cables or between other electromagnectic sources and a particular cable 
does not affect data transmission, and because the cables still retain the 
flexibility and pliability of more conventional electrical conductors. For 
all their above-described advantages, however, fiber optic cables are 
difficult to test reliably to ensure that the cable's integrity is intact 
or that its performance has not been subject to degradation in use. This 
difficulty is particularly true with respect to aircraft flight systems, 
where access to the cables often is restricted physically. 
The invention disclosed hereinafter provides a relatively simple and 
effective device for testing the attenuation of a fiber optic conductor. 
In the preferred form of the invention, a separate transmitter and 
receiver are provided which are operatively connected at opposite ends of 
the cable under test. The transmitter is adapted to provide a test signal 
to the fiber optic conductor. A timing pulse also is generated and 
combined with the test signal for transmission. The amplitude of the 
timing pulse is higher than the test signal so that any cable whose 
attenuation is within the test range is automatically within the range of 
timing pulse recovery. The receiver separates the timing pulse from the 
test pulse and utilizes the timing pulse to demodulate the test signal to 
provide a DC level voltage proportional to the amplitude of the signal 
wave transmitted through the cable. This DC voltage is compared with a 
reference signal and any differences are displayed as attenuation to the 
receiver operator. Because the reference signals are generated in the 
respective transmitter and receiver of the system, no physical connection, 
except the cable under test, is required between the transmitter and the 
receiver. Alternatively, the transmitted signal may be passed both through 
the cable under test and a reference cable, and then compared in the 
receiver to establish the attenuation in the fiber optic cable under test. 
One of the objects of this invention is to provide a system for testing the 
transmission capabilities of fiber optic conductors. 
Another object of this invention is to provide a fiber optic conductor test 
system employing a transmitter and a receiver, which may be separate units 
interconnected only by the fiber optic conductor under test. 
Another object of this invention is to provide a system for testing fiber 
optic cables in which a test pulse and a timing pulse are transmitted 
simultaneously through the cable under test. 
Another object of this invention is to provide a fiber optic cable testing 
system in which a receiver is designed to obtain a test signal and a 
timing pulse through a cable under test, the timing pulse being used to 
provide a demodulation signal for the test signal. 
Other objects of this invention will be apparent to those skilled in the 
art in light of the following description and accompanying drawings. 
SUMMARY OF THE INVENTION 
In accordance with this invention, generally stated, a system for testing 
fiber optic conductor attenuation is provided which includes a transmitter 
for transmitting a test signal through the fiber optic conductor. A 
receiver attached to the second end of the conductor compares the 
transmitted signal through the conductor under test with a reference 
signal to obtain an error signal representation of attenuation through the 
conductor under test. In the preferred embodiment of the invention, the 
test signal includes a timing pulse having an amplitude greater than the 
test signal. The timing pulse is separated from the test signal in the 
receiver. The timing pulse is utilized in the receiver to generate a 
demodulation control signal which inverts a portion of the transmitted 
signal to provide a DC voltage representation of the transmitted signal 
for comparison with a reference voltage. Any difference between the 
reference voltage and the voltage representation of the transmitted signal 
is displayed as the attenuation through the conductor.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring now to FIGS. 1A and 1B, reference numeral 1 indicates one 
illustrative embodiment of attenuation measuring system of this invention. 
The system 1 includes a transmitter 2, shown in FIG. 1A, and a receiver 3, 
shown in FIG. 1B. In applicational use, the transmitter 2 and receiver 3 
are independently self-contained units which are operatively connected 
between first and second ends of a fiber optic conductor 4. 
The transmitter 2 includes an oscillator means 5 having an output 6 forming 
an input to a square wave generator amplifier means 7. The oscillator 5 
also has an output 8 forming an input to a demodulator 9, later described 
in greater detail. 
The square wave generator 7 has an output 10 connected to a first input 11 
of an amplifier 12. In the embodiment illustrated, the signal at the 
output 10 of the generator 7 is a square wave having a 0.2 millisecond 
pulse width. A differentiator 13 is operatively connected to the output 10 
of the square wave generator 7. The differentiator 13 has an output 14 
which passes through a diode 75 to provide an input signal of a single 
polarity to a one microsecond monostable multivibrator 15. The signal out 
of the multivibrator 15 at an output 16 is a pulse of one microsecond 
duration at 0.2 millisecond intervals. Output 16 is coupled to the input 
11 of the amplifier 12 through a resistor 17. A second input 60 of the 
amplifier 12 is connected to ground. 
Output of the amplifier 12 is a square wave voltage pulse having the one 
microsecond timing pulse superimposed on its leading edge. The output of 
the amplifier 12 is passed through a current amplifier means 18 having an 
output 19 operatively connected to a light-emitting diode 20. 
The output of the light-emitting diode 20 is directed through a beam 
splitter 21 so that a portion of the output from the light-emitting diode 
20 passes through the conduit or conductor 4 under test, while a portion 
of the light from the light-emitting diode 20 is directed so that it 
impinges a large area photodetector 22. The output of the photodetector 22 
is an input to an amplifier 23 which provides an output voltage 
proportional to the current signal of the photodetector 22. The input 8 
from the oscillator 5 is used to demodulate the signal output of the 
amplifier 23 in the demodulator 9 to provide a first input 24 to a D.C. 
voltage amplifier 25. A second input 66 of the amplifier 25 is obtained 
from a variable voltage source 27. The voltage source 27 includes a 
potentiometer 28 and a zener diode 29 arranged in a conventional manner to 
provide a varaible but constant source of DC voltage to the amplifier 25. 
An output 30 of the amplifier 25 is fed to the square wave generator 7 and 
is utilized to provide amplitude control of the output 10 of the square 
wave generator 7. The operation of the components just described sets the 
power output of the light-emitting diode 20 by controlling square wave 
generator 7, the control being obtained by the optical feedback from the 
photodetector 22. This is an important feature of our invention in that 
the transmitter 1 is automatically compensated for anomalies occurring in 
the operation of the light-emitting diode 20 caused by temperature 
variations, or variations in a power supply 31, for example. 
The power supply 31 includes a suitable battery source, for example, nickle 
cadium rechargeable batteries 32, which supplies power to an input 33 of 
DC to DC converter 34. The converter 34 provides the desired operating 
voltages for the transmitter 2 at an output side 80. 
After passing through the conductor 4 under test, the output of the 
light-emitting diode 20 impinges a second beam splitter 35 which directs 
the output so that and a first portion impinges a high speed photodiode 36 
while a second portion of the output impinges a large area photodetector 
37. 
An output 38 of the photodiode 36 is an input to an amplifier 39 which 
filters or separates the test signal from the timing pulse. An output 81 
of the amplifier 39 is a first input to a comparator 40. 
An output 41 of the photodetector 37 is an input to an amplifier 42 which 
converts the current output of the photodetector 37 to a voltage 
proportional to the input at its output side 43. A low pass filter 
rectifier means 44 is connected between the output side 43 of the 
amplifier 42 and a second input 45 of the comparator 40. The signal output 
of the amplifier 42 passes through the rectifier and low pass filter means 
44 and provides an average DC reference level proportional to the test 
signal at the second input 45 of the comparator 40. The input 45 sets the 
level for the comparator 40 so that only actual timing pulses transmitted 
through the conductor 4 appear at an output side 46 of the comparator 40. 
The timing pulse thus recovered is the same one microsecond pulse at 0.2 
millisecond intervals generated in the transmitter 2. The pulse forms an 
input to a monostable multivibrator 47. The multivibrator 47 generates a 
square wave at its output side 48. The output 48 of the multivibrator 47 
is an input to a demodulator means 49. The demodulator means 49 also has 
an input 50 operatively connected to the output 43 of the amplifier 42. 
The output of the monostable multivibrator 47 is utilized to provide a 
demodulation signal which functions to enable the demodulator means 40 to 
invert the pulsed input from the amplifier 42 at proper times so that a DC 
voltage proportional to the amplitude of the test signal transmitted 
through the conductor 4 is available at an output side 51 of the 
demodulator 49. 
A constant voltage reference source 52 is a first input to a Logarithmic 
Amplifier module means 53, while the output 51 of the demodulator means 49 
is a second input thereto. The amplifier module means 53 includes suitable 
circuitry for comparing the reference signal from the source 52 and the 
received signal 51 logarithmically. It then generates an output signal 54 
that is proportional to the logarithm of the ratio of the two input 
voltages 52 and 51. The voltage at the input 52 is chosen to the 
equivalent to a zero (0) db attenution. The output signal 54 forms an 
input to a suitable display means 55. Display means 55 may be a liquid 
crystal display, for example. 
Again, a suitable power supply 56 is provided to operate the receiver 3 of 
the system 1. 
FIG. 2 is a block diagrammatic view of a second illustrative embodiment of 
system of this invention. Like numerals are utilized to represent like 
parts, where approximate. The primary difference in the systems shown in 
FIGS. 1A, 1B and 2 is that a reference cable 70 is connected between the 
amplifier 25 and an amplifier 71 of the receiver 3. The signal passed 
through the reference conductor 70 is compared with that transmitted 
through the cable 4 under test in the amplifier 71, any error signal again 
being displayed on a display means 55. While the system shown in FIG. 2 
works well for its intended purpose, the use of independent receiver and 
transmitter eliminates the need for the physical connection of the 
reference cable 70 and greatly enhances the mobility of the system 1. 
It thus is apparent that a system for measuring attenuation in fiber optic 
conductor is provided meeting all the ends and objects herein set forth 
above. 
Numerous variations, within the scope of the appended claims, will be 
apparent to those skilled in the art in light of the foregoing description 
and accompanying drawings. Thus, various details in circuit design may 
vary in other embodiments of this invention. The block diagrammatic views 
shown in the drawings will enable skilled practioners in the art to 
construct physical circuits to accomplish the descriptions set forth. 
Although various parameters in the form of pulse width and oscillator 
frequencies have been shown or described in conjunction with the operation 
of the circuit of this invention, these and other parameters may vary in 
other embodiments of the system 1. These variations are merely 
illustrative.