Lab 5:
Fiber Optic Communication
Physics 352L Section: 450
Friday, February 31st, 2014
Dr. Hugon
Harris Pharr
I have neither given nor received any unauthorized aid
on this assignment:
Abstract/Introduction:
The purpose of this lab was to introduce ourselves to the possibilities of
transferring data optically instead of electrically. This can be done using Fiber optic data
links which are controlled by VI’s written in LabView and sent and received by a
transmitter and a receiver. In section A and B we construct the transmitter and receiver
which we use in section C and D to transmit actual messages with a partner (Laura
Cusenza) from one VI, across the DAQ, circuit, into the other DAQ, and displayed on
your partners receiver front panel.
Section A: Transmitter.
The purpose of section A was to transmit an electrical signal by converting it into
an optical signal. The circuit functions mainly due to 3 main components: the buffer, the
driver, and the optical source. The “data in” depicted below in figure 1, can be produced
from either the clock on the blackbox (BB) or from the DAQ. The signal was fed into the
port of a 7400 NAND gate with a constant 5V feeding a HIGH signal into the other port.
In order for the NAND to output a LOW, the “data in” must be HIGH. If the output of the
first NAND is LOW, then the following NAND will have 2 LOW inputs which
ultimately produces a HIGH output. Conversely, if the output of the first NAND is
HIGH, the following NAND will output LOW. Each NAND chips are both supplied with
5V and grounded. This output controls the base of the NPN bipolar transistor seen below.
The transistor requires a HIGH signal to allow the current from the 5V source to flow
across the LED to ground via the emitter causing the LED to illuminate. Resistors R1 and
R2 serve to limit the flow of current; they also control the voltage potentials when the
“data in” is LOW. The test point (TP) shown in figure 1 below between the 2nd NAND
gate and the transistor is used to determine whether the base of the transistor is
conducting or not. This test point will mirror the “data in” and the LED also will emit
light when both TP’s and “data in” are held HIGH.
Figure 1: The transmitter constructed and analyzed in section A.
Section B: Receiver
In section B, working my brilliant lab partner Laura Cusenza, we utilized the
transmit constructed in section A of the circuit to be paired with a receiver we
constructed according to the circuit diagram in figure 2 below.
Figure 2: The Receiver constructed in section B of the Lab
This receiver displayed above consists of three main elements: a photo-detector, an
amplifier, and a digitizer. The receiver utilizes the Darlington configuring which consists
of a phototransistor and a NPN bipolar transistor shown on the left portion of figure 2.
This signal is then connected to the base of the photo transistor so that the transistor will
conduct when the input signal is HIGH. This will ultimately produce a HIGH signal at
the base of the transistor permitting current to flow into cascaded NAND gates in series.
The Darlington configuration amplifies the current gain of the system; with current
flowing, a HIGH signal is inputted into both inputs of the 1st NAND gate which produces
a LOW output, therefore a HIGH output of the 2nd NAND gate. The opposite of this
statement is also valid. The TP2 is LOW, the 1st NAND outputs HIGH and the 2nd NAND
outputs LOW. Just as in Section A, all the NAND gates are powered by a +5V source and
are grounded. TP1 was measured to be HIGH and read 4.8V with TP2 reading low at a
value of around 0V. While constructing this circuit, the test points 1 and 2 were very
useful when it came to trouble shooting. TP1 was used in order to determine whether the
current is flowing through the phototransistor from the 5-volt source to the base of the 2nd
transistor or not. This also inevitably determines whether the base of the phototransistor
is functioning or not. If both transistors are conducting causing current to flow, TP2 will
be HIGH. Originally our Phototransistor’s was broken and the lab was out so we had to
wait for another group to finish to borrow their phototransistor. When completed, the
voltage at TP2 was found to be 4.8V when HIGH. When Low, the voltage was 0. The
circuits in section A and section B transmit data via a fiber optic cable originating from
the transmitter and decoded and displayed on the receiver. The data out of the receiver
was then fed into the DAQ and LABVIEW.
Part C: Operating the Link.
In this section we made use of the transmitter with a square wave input at 1Hz. This
ultimately caused the LED to blink one time per second. When the receiver was tested by
covering and uncovering the input, no signal was displayed on the oscilloscope. The
transmitter was then connected to the receiver via a fiber optic cable and the signal was
sent through and observed on the oscilloscope. The frequency was then adjusted to
determine at what frequency distortion would occur when transmitting a square wave
through the link. Distortion occurred around 500Hz and the observed delay in Figure 3
was found to be 2.01us. I believe the distortions are cause by the maximum output from
the DAQ which in previous labs we found to be around 500Hz..
Figure 3: Observing the delay in the input and output of the receiver/transmiter
Section D: LabView-Driven Operation:
In section D, Laura and I used our Transmiter and Reciever VI’s on separate
computers to transmit messages from the Vi, to the DAQ, across the fiber optic cable,
into the other DAQ and decoded and displayed by the receiver VI on the other computer.
The first Fiber-T VI acted as the transmitter which sent the message to the transmitter
circuit which in turn transmits the code to the 2nd .VI, Fiber-R. This .VI reads the data
from the TTL line connected to the receiver. The Boolean signals and simply strings of
letters or words which are converted into an 8-bit Boolean array. The functionality of the
VI’s and the circuit that Laura and I constructed was shown to Robert our TA and he
signed off on it. The Pictures of our VI’s are displayed below in the appendix.
Conclusion:
This lab introduced me to the fundamentals of fiber optic data transmission along with
the LabView skills on not only how to transmit the signal but also to receive it. I was
introduced to the importance of timing in the signals and information can be transmitted
optically instead of electrically. Sections A and B introduced me to constructing the VI’s
and how to operate the transmitter and receiver where C and D put them to the test
forcing myself and my partner Laura to be able to send an receive actual messages via an
external fiber optic connection.
Appendix:
*Note these screenshots are on my mac so the DAQ components do not correlate to the
ones I used in the lab.
Fiber T:
Fiber R: