; Experiment 10 Performance and Go-NoGo tests on an Ellmax
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Experiment 10 Performance and Go-NoGo tests on an Ellmax


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									                   Ellmax Transmitter-Receiver at 650nm


The simple electronic circuitry required to interface a digital fiber optic link is
one of the factors assuring the high reliability of optical signal transmission
systems; paradoxically, the electronics is also the frequency-limiting factor.

In this experiment, the conversion of an electrical signal to light and then, the
conversion back into an electrical signal are demonstrated using the Ellmax
Training kit. This system operates as both, an analog, or digital, optical fiber link.
The transmitter uses red light around 650 nm wavelength.

The PIN photodiode on the receiver permits:
1. Analog transmission from 20 Hz to 25 kHz; and
2. Digital transmission from DC to 20 kBits/s for the standard RS232 output.

Figure 1 shows a simplified diagram of the transmitter and receiver. It shows the
simplicity of how the data is converted, transmitted, received, and finally
converted back to an electrical signal.
                                           Connectors                       Receiver

                                                Fiber       Photodetector

       Figure 1: simplified diagram of the transmitter and receiver system.

Equipment and Material

Ellmax fiber optic trainer
Square or Sine wave source
Fiber optic power meter (Ellmax)
Vernier Caliper
Rods of different diameters
Assorted cables

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Experiment Objectives
   1. To operate, analyze, and make typical measurements on the Ellmax
      trainer such as: signal delay, rise time, and frequency response;
   2. To perform Go-NoGo tests at various points during circuit assembly; and
   3. To investigate bending radius on attenuation and time delay

Before proceeding with the experiment, two terms that you should be familiar
with are:

Go-NoGo test
 This test provides a qualitative proof on whether or not a device works in a
given setting. It is not a performance test. To perform the test on an optical fiber,
normal room brightness is sufficient for a length up to 100m. For longer lengths,
a high intensity light such as a laser is used.

Transmission Loss (TL)
Transmission Loss is defined as losses caused by several intrinsic and extrinsic
factors. Intrinsic factors include fiber design, material absorption, material
scattering, guide scattering and leaky modes. Extrinsic factors include
connectors, mechanical deformation (macro- and micro-bending), and high
energy radiation. Recall that most fibers have a much higher attenuation for red
light than for infrared (IR).

Transmission Loss (TL) is given by: TL = attenuation (dB) of a fiber-link

Activities and Instructions

Exercise 1: Digital Operational Mode

Connect a 1:1 probe to the oscilloscope, and a 1:1 probe to the square/sine wave
signal generator.

Prepare four 30" long leads (if they are not already prepared) to be used with the
9V DC supplies. With reference to figures 2 and 3, slide the digital/analog switch
on both the transmitter and receiver to digital. Connect the prepared wires to the
+V and Ground connectors on the transmitter and receiver boards, and to the
voltage supplies, but do not turn on the supplies at this time. There are two
grounds – one for the supply and one for the signals – double check that you
are making the right connection – if in doubt ask the instructor.

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To prevent RF-ringing, connect four shorts (6") leads to the digital and ground
inputs on the transmitter and receiver (two wires on each board).

Connect a T-connector on the generator’s Sync output (+5V/0V). Connect the T
to the oscilloscope’s channel A (or 1), and the digital input of the transmitter.
Connect the receiver's output to channel B (or 2) of the scope. Trigger the scope
on channel B (or 2) if necessary.

Figures 2 and 3 below show the two units with their respective inputs, outputs,
and analog and digital switches.

      0V    +V                                                            +V    0V
                  Transmitter                              Receiver

                     switch                                      switch

                                                                          Gnd     Signal
Analog source:
radio/cassette/                                                           Amp/speaker
generator                                                                 or scope

        Figures 2 and 3: Transmitter and Receiver units showing their

Perform the optical fiber Go-NoGo test

Allow light (for example, from the fluorescence lights in the classroom) to enter
one end of the fiber. If light appears at the other end, the fiber is functional.
(Caution: if a laser is used as the source of light, shine the beam of light exiting
the fiber on a sheet of paper. Also, do not shine the laser into the receiver’s
detector. It may damage it.)

Perform the transmitter Go-NoGo test.

Turn on and adjust the DC power supplies to +9V. This supplies power to the
transmitter and receiver units. Turn on power to the signal generator and set the
frequency at about 60Hz . If red light appears at the TX-LED (transmitter LED),
the transmitter is working. This is the transmitter Go-NoGo test. If light does not

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appear, you will have to troubleshoot the system. Check wiring, correct applied
voltages, etc.

Perform the transmitter/fiber Go-NoGo test.

Connect the fiber cable to the transmitter output connector. Again, check the red
LED for confirmation of transmission of signal.

Set the signal frequency to 1 KHz and measure the optical power at the end
(output) side of the fiber (side that connectors to the receiver) using the power
meter (Ellmax or another). If the connectors do not match, simply point the fiber
into the receiver plug. If it works, you will have the receiver's input signal.


For digital operation the average optical power at the receiver's fiber input
connector is:
Power 650nm ______________________ μW
P peak ___________________________ μW

Connect the second end of the fiber cable to the receiver.

Measure the maximum usable frequency (that is, the frequency at which the
output pulse is not "distinct" anymore.) Is there any ringing in the output?

Measure the delay between the input signal slope and the output signal at
f=30 kHz. The oscilloscope should be triggered on the input signal, if necessary.

Maximum usable frequency (MUF) __________________________ kHz
Signal delay time measured at 30 kHz, t delay __________________ μS

Measure the rise time for the input and output pulse slopes (time required for a
slope voltage to change from 10% to 90%).

Input pulse rise time: tr _______________ μS
Output pulse rise time: tr ______________ μS

In your report discuss the results you obtained, and also, comment on the Go-
No-Go test procedures you investigated.

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Exercise 2: Analog Operation mode

Turn off the power supplies.

Disconnect the wires from the digital inputs to the transmitter and receiver and
connect them to their respective analog inputs. Connect the output of the
generator to its output port, not the sync output. Set and adjust the signal source
to about 200mV, 1 kHz sinewave and reconnect it to the transmitter's analog
input. In this part of the experiment, trigger the scope on channel A (or 1), if

Leave the fiber between the transmitter and receiver connected.

Slide the analog/digital switches on the transmitter and receiver circuit boards to
the analog position. Reconnect the scope to the receiver’s analog output. Turn on
the DC power to the transmitter and receiver. You should observe a sine wave


Disconnect the fiber at the receiver end and measure the optical power using the
available power meter. Again, since the connectors do not match, just point the
fiber end in the input of the power meter and take your reading.

For analog operation the average optical power at the receiver's input fiber
connector is:
P 650nm ___________________ μW

Reconnect the fiber cable to the receiver input. Set the sinewave generator at
1kHz and adjust the amplitude until you get a maximum undistorted receiver
output signal. Measure the peak-to-peak input and output voltages on the scope.

Voltage input at maximum undistorted output:
V input ____________________ V pp
V output ___________________ V pp

Measure the lower, fL ( low), and upper, fu (high), cutoff frequencies. Calculate the
rise times and time constant for the complete link.

For 3dB drop you find:
fL (low) _________________ Hz
fu (high) _________________ kHz
tr ______________________ μS

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tr ______________________ μS
τ (time constant) __________ μS

Exercise 3: Investigating the Bend Radius losses in fibers

Reconnect the apparatus for digital transmissions (as was done in exercise 1.)

Set the frequency of the signal generator to 60 kHz, and observe it on the

Observe the receiver output on the second channel of the oscilloscope.

Measure the amplitude of the signal, and note time of the trailing edge on the
oscilloscope. Input your data in the table. Use the vernier caliper to measure the
diameter of the largest rod (~3cm). Bend the cable around it and measure the
resulting amplitude of the output signal on the oscilloscope. Also measure the
time delay between the input signal and the resulting output singal. Record your
observations in the table below. Repeat for different bend radius for the
following diameters (~3cm, ~2.5cm, ~2.0cm, ~1.5cm, and ~1.0cm).

Table 1: Bend diameter (2xradius) vs Amplitude

  Diameter (cm)         Radius (cm)       Amplitude (Volts)       Delay Time (s)
   No bending

Plot a graph of Amplitude (y-axis) versus radius (x-axis), and from it determine
the average slope. Explain the meaning of the slope, and why the amplitude is

Explain why the delay between the original signal and the edge of the received
signal is increasing.

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What is required for this lab

Write a summary of what you did in the lab quoting results and your analyses.
In your summary you may want to suggest what you learned, how the
experiment can be improved, etc.

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Exercise 3: Impedance measurements (Optional Exercise)

Measure the transmitter input impedance with a series test-resistor, Rseries.
The circuit diagram is shown in figure 4. The input impedance is calculated as

      R source +R series
Z in =
               − −1
Vo: open terminal source voltage
VL: with Rsource + R series, the loaded input voltage
R series: 33k Ω or 22k Ω series resistance
R source: 50 Ω signal source impedance

                         RSource    R series            RSource    R series

                         V source                                                       Transmitter
                                         V0             V source        VL              Input

         Figure 4: Circuit arrangement to measure the input impedance

Measure the receiver output impedance with a parallel test-resistor, Rparallel
The circuit diagram to determine this measurement is shown in figure 5. The
input impedance is calculated as follows:

Output Impedance

Z out = R parallel (      − 1)
Vo : open source voltage

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VL: with R (parallel) loaded output
R parallel : resistance large enough to produce a measurable voltage drop from
            Vo to VL without distortion. Try 1k Ω , 22k Ω , 100 Ω , or similar.

            Z0                                    Z0

            V source   V0                         V source    VL                  RP

                       Z0                                      Z0

       Receiver                                              Receiver
       Output                                                Output
       (Unloaded)                                            (Loaded)

       Figure 5: Circuit arrangement to measure the output impedance.

Calculate and report your results for the impedances.

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