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softening which would allow the photodiode to move during
bonding. A finished assembly is shown in Fig. 2.
It is possible at this stage to functionally test the photodiode
and to actively check the accuracy of alignment between fibre
and photodiode before final assembly into the receiver circuit.
Receiver assembly: The photodiode block assembly has been
incorporated into a PIN/FET receiver module of the type
previously used with conventional top illuminated diodes4
(Fig. 3). This consists of a thick film hybrid circuit plus the
photodiode mounted in a solid-sidewall metal dual-in-line
package. The fibre exits horizontally from the package and so
the mounting block is attached to the gold plating with the
metallised face (C) down, using either an epoxy adhesive or a
low melting point solder. Wire bond connections are finally
made from the block to the hybrid circuit. As the photodiode is
now in the vertical plane, this final bonding is done to the gold
on the face B' of the step perpendicular to the area to which the
wires from the diode were attached.
over the main part of the passband, the first step is to determine a nonminimum-phase lowpass prototype network from
which various microwave structures, such as, for example,
direct-coupled cavity waveguide filters, can be obtained. The
prototype network is a folded ladder of admittance inverters
and shunt capacitors with additional crosscoupling by further
inverters used to realise the finite transmission zeros, see Fig. 1.
Fig. 1 Lowpass prototype filter
Conclusion: A quartz block containing an optical fibre has
been used as a mount for a substrate illuminated photodiode.
The method of construction of the block has been described,
and also the method by which the diode is aligned and connected to metallised areas on the block, to produce a testable
diode-to-fibre assembly. Photodiodes mounted on such blocks
have been successfully incorporated into PIN/FET receivers
for 1-3" jun optical communications systems.
Acknowledgments: Acknowledgment is made to the Director of
Research, British Telecom, for permission to publish this letter
and to J. Ancell, B. Clark and R. Taylor for their assistance in
preparing the blocks.
B. M. MACDONALD
11th August 1981
A. G. SAUNDERS
British Telecom Research Laboratories
Martlesham Heath, Ipswich, Suffolk IP5 7RE, England
References
1
SMITH, D . R., CHATTERJEE, A. K., REJMAN, M. A. Z., WAKE, D . , a n d
WHITE, B. R.: 'p-i-n/FET hybrid optical receiver for 11-1-6 //m optical communication systems', Electron. Lett., 1980, 16, (19), pp.
750-751
2
LEE, T. p., BURRUS, c. A., DENTAI, A. c , and OGAWA, K.: 'Small area
InGaAs/InP p-i-n photodiodes: Fabrication, characteristics and
performance of devices in 274 Mb/s and 45 Mb/s lightware
receivers at 1-31 /mi wavelength1, ibid., 1980, 16, (4), pp. 155-156
3
LEE, T. p., BURRUS, c. A., and DENTAI, A. G.: 'InGaAs/InP p-i-n
photodiodes for lightwave communications at the 0-95-1-65 /an
wavelength', IEEE J. Quantum Electron., 1981, QE-17, pp. 232-238
4
Several methods for the construction of the nonminimumphase lowpass functions are available from which the element
values of the prototype networks can be determined, but they
are not always easy to apply and require considerable computation. Even more important, however, is the fact that filters
with several crosscouplings tend to be difficult to tune.
Recently,1 a class of lowpass prototype functions with equiripple passband magnitude response has been introduced
having only one pair of finite transmission zeros, which are
located on the real axis of the complex frequency plane symmetrically about the origin at the distance ±ay. It has been
shown that for the values of ox close to unity a very flat delay
characteristic over the central part of the passband can be
obtained, while the skirt attenuation of the resulting filter is
almost identical to that for the all-pole Chebyshev filter of
order n — 1. Since only one extra crosscoupling is required to
realise one pair of real-axis zeros, the practical tuning of the
resulting network is greatly facilitated. The magnitude-squared
function of these prototypes has the form
SMITH, D . R., HOOPER, R. C , AHMAD, K., JENKINS, D., MABBITT, A. W.,
and NICKLIN, R.: 'p-/-n/FET hybrid optical receiver for longer wavelength optical communication systems', Electron. Lett., 1980, 16,
(2), pp. 69-71
0013-5194/81/220832-02$!.50/0
LEAST-SQUARES MAGNITUDE FILTERS
WITH SELF-EQUALISED DELAY
CHARACTERISTIC
A2(co) =
l)]2a>rn-3(co) - 2o\ Tn-2(co)
(2)
2{o\ + co2)
Uco) =
In the above expression, Tn(a>) is the Chebyshev polynomial of
the first kind and e2 is a constant which determines the inband
loss.
The aim of this letter is to demonstrate that further increase
in the bandwidth coverage of delay approximation can be obtained if, instead of providing an equiripple passband magnitude response, the prototype function is determined by
minimising the ratio of the reflected power to the transmitted
power over the normalised passband in terms of a weighted
least-mean-squares norm. Since in all cases of practical interest
the reflected power must be small, this corresponds very nearly
to the minimum of return loss. The weight function associated
with the Chebyshev polynomial of the first kind Tn(co) is used,
i.e. w(to) = (1 — co2)~' 2, and denoting the characteristic function by fn((o) = Pn(co)/(o)2 + a2), the error integral to be minimised takes the form
E=
P2{o>) dto
(3)
(1 - to2)- ' V + a\f
This minimisation problem can be solved explicitly. It has been
proved in a recent paper2 that the error integral reaches its
minimum value if, and only if, the polynomial Pn(co) is orthogonal on the interval [0, 1] with respect to the weight function
In the design of narrow bandpassfiltersat microwave frequencies with stringent magnitude and group delay specifications
ELECTRONICS LETTERS 29th October 1981
(i)
where the characteristic function fn(co) is defined by the rational Chebyshev function
Indexing terms: Filters, Microwave filters, Transfer functions
A new class of nonminimum-phase transfer functions with
one pair of real-axis transmission zeros is described. These
functions are suitable for determining the lowpass prototype
networks in the design of narrow bandpass filters at microwave frequencies with small passband loss and very flat delay
response over the central part of the passband.
J
Vol. 17 No. 22
= {\ -o)2
w(a>)
(I-co2)12
(co2 + a2)2
(4)
833
It has also been proved that the minimising polynomial Pn{a>)
can be obtained in the explicit form
\
I
,
4 jj I
l
i\
yi
/J
/c\
V /
where Un(co) is the Chebyshev polynomial of the second kind 3
-IT
-TT-.
(6)
-rrr: UW
and
(7)
In this way the prototype characteristic function of the leastsquares magnitude filters is completely determined as
-2z?[/ n - 2 (o;) + zt[/n(co)
Jn(CO) = K
,2
CO
a\
(8)
where K is a normalising constant such that/ n (l) = 1. It can
easily be verified that substituting Tk(co) for Uk(co) in eqn. 8,
and using the recurrence relation Tn(co) = 2a>Tn-^CD)—
Tn-2(co), the characteristic function with equiripple passband
magnitude response (eqn. 2) is recovered.
The magnitude and passband group delay responses of the
least-squares lowpass prototype filter and the equiripple solution (eqn. 2) for n = 10 are presented in Fig. 2, while in Table 1
the element values for both filters are given together with some
other relevant parameters. For comparison, included in Fig. 2
are also the magnitude and group delay characteristics of the
standard all-pole Chebyshev filter for n = 9. The least-squares
filter of the present design with one pair of real-axis zeros at
a = ± 0-7372 and normalised so as to produce the bandedge
attenuation of 0-8 dB at a> = 1 yields a stopband rejection
almost equal to that of the equiripple passband filter designed
for 0044 dB maximum passband attenuation with one pair of
real-axis zeros at c = + 0-8538, but it has considerably smaller
inband loss over the largest part of the passband. The bandwidth coverage of delay approximation of the least-squares
filter is also considerably improved. The delay bandwidth for
the least-squares filter with equiripple delay error of 0-6% is
od = 0-51, which is to be compared with the delay bandwidth
cod = 0-36 for the filter with equiripple passband magnitude
response corresponding to the same percentage delay error.
D. D. RAKOVICH
21st September 1981
Faculty of Electrical Engineering
University of Belgrade, POB 816
11001 Belgrade, Yugoslavia
M. Dj. RADMANOVICH
Department of Electronic Engineering
University of Nish, Yugoslavia
References
1 LEVY, R.: 'Filters with single transmission zeros at real or imaginary
frequencies', IEEE Trans., 1976, MTT-24, pp. 172-181
2
RAKOVICH, B. D., and POPOVICH, M. v.: 'Characteristic function of
least-mean-square passband filters with finite attenuation poles',
ibid., 1980, CAS-27, pp. 1225-1233
3
ABRAMOVITZ, M., and STEGUN, I. A.: 'Handbook of mathematical
functions' (Dover Publications, New York, 1965)
0013-5194/81/220833-02$!.50/0
SELF-ALIGNED NORMALLY-OFF GaAs
MESFET USING SN-DOPED SiO2 GLASS
0
05
10
20 30
normalised frequency
Fig. 2 Comparison of magnitude and group delay characteristics: (LS)
least squares with equalised delay for n = 10; {ER) equal ripple with
equalised delay for n = 10; (Ch) Chebyshev for n = 9
Table 1 ELEMENT VALUES FOR LOWPASS
PROTOTYPE (FIG. 1)
c2
c3
c4
c5
^Z'
Least squares
ffi = ± 0-7371567
10281706
0-8896558
1-4639700
1-5440387
1-9900473
1-9113172
An improved normally-off GaAs MESFET was fabricated by
employing Sn-doped SiO 2 glass, which was used as an Sndiflusant for making the N+ layer. An N+ layer with Rs = 100
fi/D and Ns = 3x 10 13 c m " 2 was successfully obtained
under 800°C, 20 min diffusion conditions. A new selfalignment technique, using doped-SiO 2 film, provided a high
performance normally-off GaAs FET.
Series resistance reduction is an effective way to improve performance of normally-off GaAs MESFETs used as active elements in direct coupled logic circuits.1 A typical ion
gate
x
1-6806248
1-8158227
21918833
21587845
0
0
0-2085087
0-2566048
drain
active layer
SI Ga As substrate
f
f
Knt2
Delay
error, %
Delay
bandwidth
834
Equal ripples
ff, = ± 0-8538461
Indexing terms: Semiconductor devices and materials, Fieldeffect transistors, Glass
0-8987582
0-6
0-363
0-8241118
doped Si O2
n_
1
1
diffused
1
1
—r
1
1
region
0-6
0-513
Fig. 1 FET cross-sections
a Conventional FET
b Presented FET employing doped SiO 2
ELECTRONICS LETTERS 29th October 1981
Vol.17
No. 22
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