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ponding to (x 2 x 3 ) and x 5 are rewritten, by replacing * by 0 and
1 in the column labelled (x 2 x 3 ) (if necessary), thus yielding
*1
0
x4
x2
0
0
1
1
0
0
1
1
1
*
Note that this table describes the part of the function/realised
by multiplexer II^ For every 2-tuple generated by (x 2 x 3 ) the
corresponding data-input terminal of II t is assigned
(a) xs (or x 5 ) if the corresponding entry in column x 5 is 1 (or 0)
(b) 1 if the corresponding entry in column x 5 is •.
Thus assigning, we get the final design of Fig. \c.
In conclusion, the letter presents a simple algorithm to
design a multiplexer logic circuit for a given function. The
approach is simple and, although elaborated here for clarity, a
few of the steps of the algorithm can be eliminated after some
practice. The algorithm, however, may lead to several alternatives of the final design. If the function is given in the irreducible sum-of-products form, the algorithm rapidly yields the
final design even if the number of variables is large, as the
authors have found from experience.
A. B. EKTARE
D. P. MITAL
E & CE Department
University of Roorkee
Roorkee 247 672, India
22nd May 1980
about the halfwidth of the pulses, i.e. 100 ps. The light pulses
were detected with a high-speed p-i-n diode mounted in a lowimpedance microstripline circuit.
Experimental considerations: The word generator used in this
experiment is an improved version of one published earlier.1 It
consists of a step recovery diode which delivers 100 ps wide
pulses with a 10 ns pulse period. This pulse train is divided into
four signal lines of different lengths and thus different propagation times. The delay difference is accomplished by combining
a transmission line of variable length, 10 to 30 cm, with fixed
transmission lines. The propagation time in each of the four
signal paths is then continuously variable between 0-5-10 ns,
and the difference can be set to form a digital word with a bit
rate of up to 10 Gbit/s. Each line employs a p-i-n diode switch
with a rise time of less than 4 ns. The four signal paths are later
combined with an OR gate built from Schottky diodes having
switching times in the 10 ps range. A block diagram of the
pulse generator is shown in Fig. 1.
p-i-n
switch
variable
delay
line
p-i-n
switch
variable
delay
1 ine
/
\
sr.d.
p-i-n
switch
r
variable
delay
line
References
1 MILLER, R. E.: 'Switching theory-vol. I' (Wiley, 1966), chap. 3
2 KOHAVI, z.: 'Switching and finite automata theory' (McGraw-Hill,
1970)
3 WHITEHEAD, D. c : 'Algorithm for logic circuit synthesis by using
multiplexers', Electron. Lett., 1977, 13, pp. 355-56
4 ALMAINI and WOODWARD: 'An approach to control variable selection problem of ULM', Digital Processes, 1977, 3, 3
5 LLOYD, A. M.: 'Design of multiplexer ULM networks using spectral
techniques', IEE Proc. Pt. E, Comput. & Digital Tech-, 1980,127, (1)
0013-5194/80/130495-03$! SO/0
p-i-n
switch
variable
delay
line
|200/i]
Fig. 1 Block diagram of word generator
The widest modulation bandwidth was obtained with the
t.j.s. Mitsubishi ML-2307 79-1022 laser.2 This laser was found
to operate in a single longitudinal mode during pulse modulation if the bias current was selected to be above threshold. The
dynamic spectrum was measured at three different time values
during a pulse, and the single-mode operation is shown in Fig.
2. The beam divergence of this particular laser was 5° x 35°
and the threshold current 35 mA. The laser was mounted in a
microstrip circuit with a bandwidth of more than 10 Gbit/s.
8 Gbit/s OPTICAL TRANSMISSION
WITH T.J.S. GaAIAs LASER AND
p-i-n DETECTION
Indexing terms: Lasers, Modulation, Optical transmission,
Photoelectric devices
Optical transmission at 8 Gbit/s has been performed with a
t.J3. laser and a low-impedance microstripline-mounted p-i-n
diode. The laser was modulated with 100 ps wide pulses and
no intersymbol interference was observed.
Introduction: Wideband laser diodes as well as detectors are of
great interest for future optical fibre transmission systems. An
attempt has therefore been made to find the upper p.c.m. bit
rate limit of currently commercially available laser diodes and
detectors. Since laser diode bandwidth increases with higher
bias current, the lasers were biased 5-30% above threshold,
which requires high power efficiency and good heat sink
design. The experimental method made use of a specially
designed modulating signal generator delivering four pulses
with continuously variable spacing.
The minimum spacing for resolving two adjacent pulses is
ELECTRONICS LETTERS
19th June 1980
Vol. 16 No. 13
Fig. 2 Dynamic spectrum of pulse modulated t.jj. laser in three cases
a 0-5 ns before pulse
b At top of pulse
c 0-5 ns after pulse
497
The receiver consisted of a Philco-Ford L4501 p-i-n detector
and a sampling oscilloscope. In order to improve the speed of
the detector, we loaded it with 11 O, which together with the
1-5 pF junction capacitance of the diode should give an RC
bandwidth of about 9 GHz. The p-i-n diode transit time bandwidth is 15 GHz. The diode/microstripline-mounting
combined bandwidth was measured to be 5-5 GHz, by observing the rise time of a reflected step voltage, as in the timedomain reflectometer technique. The bandwidth of the
sampling oscilloscope is 18 GHz. This gives an overall bit rate
capability of the receiver of approximately 11 Gbit/s with nonreturn-to-zero pulses (Fig. 3).
son
n
sampling
head
resistive II
load
single-mode laser and a low-impedance microstriplinemounted p-i-n diode. 10 Gbit/s may be a realistic limit of the
data rate for optical fibre transmission systems of the near
future.
R. TELL
S. T. ENG
Department of Electrical Measurements
Chalmers University of Technology
S-41296 Gbteborg, Sweden
20th May 1980
References
1
TELL, R., TORPHAMMAR, p , and ENG, s. T.: 'Multiplexer at 5 Gbit/s
2
for fibre-optical communication systems', Electron. Lett., 1977,13,
pp. 765-766
NAMIZAKL, H.: Transverse-junction stripe lasers with a GaAs p-n
home-junction', IEEE J. Quantum Electron., 1975, QE-11, pp.
427-431
0013-5194/80/130497-02$! .50/0
Fig. 3 Receiver circuit
In the laboratory experiment the laser beam passed through
two microscope objectives and 10 cm of free air. The laser was
biased at 41 mA and modulated with a word consisting of 20
mA pulses, as shown in the upper trace a of Fig. 4. The pattern
effect (intersymbol interference) was investigated by removing
one or two of the four 'ones'. No interference between adjacent
bits was observed. Six of the modulating words are shown in
Fig. 4, traces b-g.
SUBPICOSECOND MEASUREMENT OF
RESPONSE OF OPTICAL FIBRES
Indexing terms: Optical fibres, Wave propagation
Conclusions: Successful 8 Gbit/s p.c.m. modulation and wideband detection (11 Gbit/s) have been performed using a t.j.s.
DO1O1OOOOO11OOOO
A novel technique is described for measuring the impulse
response of short samples of monomode and multimode optical fibres, with a resolution better than 0-1 ps. The times of
arrival of the modes of a multimode fibre are resolved.
We describe here a novel technique for the measurement of the
impulse response of opticalfibres,only short samples of fibres
being required (01 m to 1 m). The temporal resolution is better
than 01 ps. The method consists of placing thefibreunder test
in one arm of a Mach-Zehnder interferometer, excited by a
broadband optical source. A similar technique without the
reference beam was reported in Reference 1 and related general
concepts can be found in References 2 and 3. In the actual
experiment we used a mechanically tuned dye laser, but short
(ps) pulses could be used as well. Only the width of the spectral
curve of the source matters here. The output of the MachZehnder interferometer is input to a spectrometer. The spectrogram obtained is then illuminated with a HeNe source, and the
impulse response of the fibre under test is directly observed in
the far-field pattern. More precisely, if h(t) denotes the field
impulse response of the opticalfibreunder test, and if we define
the Fourier transform of h(t) by
/J(v)= J h{t) exp (2nivt) dt
(1)
— oo
and the analytic response by
co
ha(t)= j fi{v) exp (-2nivt) dv
o
0
1
|20OK |
2
time . ns
Fig. 4
a Output electrical pulses from word generator
b-g Output pulses from detector with different 'ones' removed
498
(2)
we observe in the far field I(x) = |/jfl(f)|2, to within constant
factors. This simple result holds only if the spectral width of the
source exceeds the wavelength range over which the fibre
transmission is appreciable. Otherwise, a slightly degraded impulse response is measured, the temporal resolution being the
reciprocal of the source spectral width. Note that ha(t) and h(t)
have the same total duration, and thus useful information is
obtained from hjt). It is possible however, to observe the impulse response h(t) itself at the reconstruction stage by introducing a reference wave. Schematically, it would suffice to
perforate a small aperture in the spectrogram. In the previous
ELECTRONICS LETTERS
19th June 1980 Vol. 16 No. 13
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