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SERVICE INFORMATION FROM HEWLETT-PACKARD
2nd Quarter 1989
Safety Precautions
Brett Frymire Network). In fact, the consumer elec- When Working
Hew lett-Packard tronics industry uses fiber optics to
isolate digital (TTL) from low-level Around
Your voltmeter no longer works. You analog circuits. For example, a n
open the cover and begin to trouble- Onkyo Compact Disc (CD) player Fiber Optics
shoot. You trace the fault to some uses fiber optics in this application.
components with a plastic cable. What
is this? What do you do?
CD players and the new DAT (digital
audio tape) both use this technology [WARNING! 1)
for high noise immunity. Look for
You have found a fiber-optic link that modern stereo systems from Japan to Handle fiber-optic components with
has special parameters and requires interconnect the CD player or DAT care, keeping in mind the following
special test equipment. Follow along to any other component with plastic precautions.
as this article covers the parameters, fiber optics. A new ANSI standard is
equipment, and basics of fiber-optic being developed for FDDI (fiber dis- The output from fiber-optic links can
troubleshooting, w i t h t i p s a n d tributed data interface), which is a cause serious damage to the eye,
examples. 100 Mbit/sec optical LAN. and the glass in the cable can pierce
the skin.
Background As in consumer electronics, more and
more industry instrumentation uses Use caution when viewing fiber ends
Fiber-optic applications include iso- fiber optics (see Figure 1). A good or optical ports under magnification.
lation, video, audio or digital data example of a n instrument-related For further precautions and more
transmission, and LANs (Local Area fiber-optic application is t h e detail, see ANSI 2-136.1 1986.
(continued on page 3)
Use caution when viewing the optical
port without knowing the optical
power level and the wavelength.
Handle bare fiber with care. The
core end of the fiber is glass that
can pierce the skin and break off.
This is a hazard only when termi-
nating a fiber end with a connector
or a splice.
Potential eye problems result from
invisible wavelengths, collimated
light, and light intensity of unknown
sources. As a rule of thumb, it is
always safer and more accurate to
use a meter to measure light output.
Figure 1. Typical HP fiber-optic products.
Pub. NO. 5952-0134 @ Hewlett-Packard 1989
WWW.HPARCHIVE.COM
Fiber Optic Terms
The following list defines a few of Eye Pattern: A term describing the milliwatts). Usually seen when refer-
the terms used in fiber optics. The oscilloscope display when a data ring to the output power of the
book, Fiber Optics Handbook, pattern is triggered on the clock transmitter (Pt) or to the power
available from Hewlett-Packard (HP signal. See the photo below. presented to the receiver.
P/N 5952-9654), contains a more
detailed list of terms. Optical Power Budget: The calcu-
lated amount of optical power
BAUD: The symbol rate of the fiber- needed to sustain correct operation
optic link. Depending on the encod- of the receiver, incorporating the
ing format, the symbol rate can be losses throughout the link. For ex-
the same or twice the apparent ample, the optical power out of the
signal rate. transmitter minus the loss of the
cable minus any losses due to
BER: Bit Error Rate. The ratio of splices or bulkhead connectors. If
errors to total number of bits sent in the level of light falls below this
a data stream, which defines the power level, the receiver output is
quality of that data (e.g., BER no longer valid. (See Sensitivity.)
of 1x l E-7).
Overdrive: The tendency of the
Bulkhead or Splice: A special con- Fiber Optics (FO): The glass or receiver to produce erroneous or
nector that joins two cables. Gener- plastic medium used to transmit distorted data when too much light
ally, a bulkhead connector is used signals between two points, or light is presented to it.
when the cable has to pass through via an optical fiber. This term can
a wall or partition. A splice repairs a also refer to parts of the link (e.g., Pr: The optical power presented to
break in the cable. See the photo the cable, transmitter, receiver or the receiver.
below. the complete circuit link).
Pt: The optical power at the
FDDI: Fiber Distributed Data Inter- transmitter.
face. A proposed ANSI standard for
high speed fiber-optic LAN. Sensitivity: The minimum optical
power level where the receiver is
IF: The forward current through the guaranteed to have valid data. This
LED portion of the transmitter. is usually specified at a particular
BER.
LAN: Local Area Network. A group
of terminals/computers linked to- Underdrive: The tendency of the
gether in a fashion to permit ex- receiver to produce erroneous data
change of data with protocols. when not enough light is presented
to it.
Manchester Code: A coding
scheme that has two properties; VF: The forward voltage across the
one, the average duty cycle is 51
00
', LED portion of the transmitter.
and two, it is self-clocking.
Degradation: The tendency of the
light emitting diode (LED) to produce Wavelength: The optical spectrum
Optical Port: The portion of the of the light emitted at the transmitter.
less light with constant current (IF) transmitter housing with which the
after an extended period of time. fiber-optic cable mates.
Encoding: The process of translat- Optical Power (Pt or Pr): The
ing data into a controlled average brightness, regardless of the wave-
duty factor range for use in an ac length of the light, measured in
coupled circuit. For example, NRZ either watts or dB (sometimes dis-
(Non-Return to Zero) data can be played as dBm, which is relative to
sent as Manchester coded.
2 BENCH BRIEFS WWW.HPARCHIVE.COM 2ND QUARTER 1989
(continued from page 1)
HP 3458A Multimeter, where fiber Test Equipment Basic Troubleshooting
optics provide a data link for high Procedure
voltage isolation between floating Troubleshooting fiber-optic circuits
measurement hardware and the requires specialized test equipment. Note: Each failure shown in the fol-
HP-IB section. Traditionally, pulse This equipment will allow you to lowing figures is not an actual failure
transformers have been used in this verify the light output of the trans- but is a forced condition for example
area. Other applications also include mitter, view the waveshape of the only.
RS-232C (serial interface connection) optical output, accurately vary the
modules, such as the DOClO2P and light amplitude (useful in trouble- One important resource for trouble-
DOClOlP, which can be used for shooting the receiver), and substitute shooting is the databook from the
higher noise rejection over long dis- your own optical signal. manufacturer on the specific parts
tances. Note that a standard hardwire used. Databooks are inexpensive (free
RS-232C connection is limited to ap- Hewlett-Packard manufactures sev- to approximately $15.00) and provide
proximately 50 feet according to EIA eral products that can be used for a wealth of data that are useful for
(Electronic Industries Association) fiber-optic troubleshooting (see Figure troubleshooting. Note that for trou-
specifications. The DOClO2P and 2), the first of which (and one of the bleshooting purposes you only need
DOClOlP are good to approximately most important) is the HP 8152A to look at typical values of the follow-
500 meters. Hewlett-Packard's new Optical Power Meter. This meter ing elements.
Precision Architecture computer uses measures light output (brightness)in
an optional fiber-optic interface for dBm so that optical power (Pt or Pr) VF (forward voltage measured
the connection between the disk drive can be verified. The ability to check across the LED).
and the CPU. The HP 37204A Mul- correct optical modulation makes the
tipoint HP-IB Extender also uses an HP 81519A Optical Receiver very tr, tf (for both parts).
optional fiber-optic interface. Fiber important. Other equipment that may
optics play a big part in isolating the prove helpful include the HP 8158B Pt for specific current.
effects of TTL noise from small-signal Optical Attenuator, the HP 8154B
analog electronics. Possessing both Optical Source, and of course, a good IF (usually in graphical form).
fiber-optic basic knowledge and dual-trace oscilloscope and digital
troubleshooting skills are valuable voltmeter (the HP 1980B Oscilloscope Sensitivity of the receiver.
aids that will help you repair instru- and HP 3435A Digital Multimeter
ments now and in the future. are good choices). Attenuation of the fiber-optic cable.
Definition of a
Fiber-optic Link
Before we discuss fiber-optic trouble-
shooting and equipment, you need to
understand the basic components of
a fiber-optic link along with related
terms and acronyms that you will
encounter i n t h e course of
troubleshooting.
Fiber-optic links, as they are referred
to, are made up of three functional
blocks. The transmitter is the first
block, containing an LED (light emit-
ting diode) and any needed support
circuitry to convert electrical signals
to light. The second block is the cable
and correct connectors for the link,
which carries the light signal from
the transmitter to the receiver, which
is the third block. The receiver con-
sists of a photodiodelphototransistor
and support circuitry to convert the
Figure 2. HP fiber-optic test equipment.
light back to an electrical signal.
2ND QUARTER 1989 WWW.HPARCHIVE.COM BENCH BRIEFS 3
Troubleshooting fiber-optic circuits If the LED is driven by a TTL gate Check the Optical Power
requires a logical, organized ap- (see Figure 3A) or other voltage
proach, just like troubleshooting other source, the ac excursion will range Remembering the safety precautions
circuits or instruments. (But be care- between ground and VF (1.5 volts), or for the transmitter, separate it from
ful. Even though the approach is between Vcc and VF, depending on the rest of the circuit and check it for
similar to many other circuits, don't the drive circuit used. For example, correct operation as follows. Remove
ignore the special safety precautions a pre-bias circuit (see Figure 3B) can the fiber-optic cable from the trans-
associated with fiber optics. Remem- affect the voltage swings in a different mitter receptacle. You should use a
ber that the fact that a fiber-optic manner. Pay attention to the dc volt- meter (HP 8152A or equivalent) that
circuit is used may indicate the pres- age across the LED as the diode must is calibrated for the wavelength of
ence of a high voltage.) The following have a high enough VF to forward the LED to check the light output.
steps show the order of the procedure bias the LED, approximately 1.5 volts Do this by replacing the original
we will use to determine the failure. (a high VF or supply voltage at the fiber-optic cable with a short, known
anode with the cathode near ground good cable approximately one meter
1.First localize the problem. (Is it would indicate an open LED). At this in length. Make sure the test cable
located in the fiber-optic portion of point, any incorrect measurements has the correct connectors for the
the instrument?) other than those listed would indicate transmitter being tested. First, verify
the support circuitry. an output then measure and record
2. Gather information about the prob-
lem without disturbing the circuit
or instrument. This is sometimes
referred to as milking the front
panel. (This assumes that the prob-
lem has been traced to the fiber-
optic portion of the instrument.)
3. Using the test equipment men-
tioned earlier, isolate the problem
to one of the functional blocks of
the fiber-optic circuit (transmitter,
cable, or receiver).
4. Determine what is wrong with the
defective block.
Troubleshooting the
Transmitter Figure 3A. Typical shunt drive.
Troubleshooting the transmitter is
divided into three parts; checking for
signals to the transmitter, checking
the optical waveform, and checking
the optical power.
Check Signals to the Transmitter
Probe the anode of the LED with an
oscilloscope. Does a signal exist at
this point? Depending on the support
circuitry, a current source might be
driving the LED. In this case the
voltage excursion (if it is measurable)
will be very small, on the order of 50
millivolts ac, which will ride on a VF
of 1-2 volts dc. Figure 38. Pre-bias circuit example.
4 BENCH BRIEFS WWW.HPARCHIVE.COM 2ND QUARTER 1989
the output from the transmitter for Check the Optical Waveform If possible, use a dual-trace scope and
later use during the cable test. If trigger on the input signal. Compare
there is no output, the failure has Next, check the optical waveform the input signal to the output signal.
been isolated to this functional block. with a waveform analyzer. Use the The signal from the optical receiver
To verify the LED output, measure HP 81519A Optical Receiver o r should match the signal seen across
the voltage loop containing the LED equivalent and an oscilloscope (refer the LED. Figure 6 is an example of
(refer to Figure 3C), then calculate to Figure 5 ) . While attaching the a good waveform. It will not be an
the current through the LED (IF) optical receiver, verify that the oscil- exact duplicate because of slow re-
and check this value with the IF vs. loscope termination matches the ana- sponses in the LED. In particular,
Pt on the graph in Figure 4. lyzer requirements (usually 50 ohms). pay attention to the rise/fall times of
the transmitter, as a long fall time
(sometimes called long-tail effect) can
cause problems (see Figure 7). A
long-tail response can be recognized
by the characteristic changing slope
in the fall time. At approximately
the 20% point on the fall-time line,
I 1
the slope changes to a more gradual
sloped line. Usually the rise time on
a long-tail LED will also be slow, but
slow rises are usually fixed by the
I designer using a peaking circuit (see
Figure 3 0 . A peaking circuit can be
recognized where the output of the
Example O f P e a k i n g C I r c u I t drive circuit is connected in series
- with two resistors, one of the resistors
being bypassed with a capacitor.
When the first edge occurs there will
Figure 3C. Peaking circuit example. be a surge of current causing a fast
turn on of the LED. Unfortunately,
this does not help the falling edge.
Depending on the receiver, if the
long-tail response occurs near the
threshold of the amplifier/comparator,
noise (which rides on the signal) will
again pass through the threshold.
This will cause jitter o r multiple
switchings on the receiver output.
Examples of this problem will be
shown later.
The Cable
Testing the cable consists of compar-
ing the optical output from the trans-
mitter using a short and known good
cable to the output of the original
cable at the receiver. Using the one
meter known good cable, measure
the optical output at the transmitter
(as was done earlier when you
checked the optical power). Reconnect
the original cable to the transmitter
and then move to the receiver. Re-
move the cable end from the receiver
and measure the power. The power
Figure 4. Typical output power vs. drive current. from the cable at this point should
2ND QUARTER 1989 BENCH BRIEFS 5
WWW.HPARCHIVE.COM
Table 1. Receiver data sheet specifications. This is a partial example from the HFBR-0501 Series Technical Data Sheet.
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