Conclusions:
The enhancements
to
the digital complex sam-
pling system presented eliminate the detrimental effects of DC
content in the A/D input and drastically reduce the distortion
caused by the sampling offset between the
I
and
Q
channels.
The techniques discussed are simple to implement. The over-
sampling of the input signal makes it possible to use the
simple interpolation circuit to provide
a
significant improve-
ment.
T.
E.
THlEL
General Elecrric Company
Corporate Research and Development
Schenectady,
NY
12301,
USA
G.
J.
SAULNIER
Electrical, Computer, and Systems Engineering
Rensselaer Polytechnic Institute
Troy,
NY
12181,
USA
Refer
ewes
5th February
1990
CONSIDINE, v.: 'Digital complex sampling',
Electron. Lett.,
1983,
19,
pp.
608-609
RICE,
U.
w., and wu,
K.
H.:
'Quadrature sampling with high
dynamic range',
IEEE Trans.,
1982,
AES-18,
(4),
pp.
736739
RAOER,
c.
M.:
'A
simple method for sampling in-phase and quadra-
ture components',
IEEE Trans.,
1984,
AESZO,
(6),
pp.
821-824
RAFFEXTY,
w., and
SAULMER,
G.
I.:
'Variable-delay, sinelcosine non-
coherent detector',
Electron. Lett.,
1985,21,
pp.
586587
SAULNIER,
G.
J.,
and
RAFFERTY,
w.: 'DSP-based non-coherent dual
detector demodulator
for
land mobile radio channels'.
Conf.
Record
of
ICC'86,
Toronto, pp.
1008-1012
CMOS
VHF
TRANSCONDUCTANCE-C
LOWPASS
FILTER
Indexing terms: Filters, Circuit theory and design
Experimental results of
a
VHF
CMOS
transconductance-C
lowpass filter are described. The filter
is
built with trans-
conductors as published earlier. The cutoff frequency can
be
tuned from
22
to
98
MHz
and the measured filter response
is
very close to the ideal response.
Introduction:
Several continuous-time high frequency inte-
grated filters have been reported in the literature.'-' Most
filters were built with transconductance elements and capa-
citors, to take advantage
of
these structures at high fre-
quencies. The maximal frequencies, however, were limited to
the lower megahertz range. Krummenacher and Joehl' report-
ed
a
4MHz lowpass filter and Kim and Geiger' reported
a
bandpass filter, programmable up to 16MHz.
Tsividis' has proposed another approach: minimal
transistor-only VHF filters. With this technique, filters at very
high frequencies (10-100MHz) can be made, but these filters
seem to be restricted to low quality factors and accuracy.
In
this paper an accurate transconductance-C lowpass filter
is presented with
a
cutoff frequency up to 98 MHz.
Transconductor:
Recently, a transconductance element for
VHF filters has been pre~ented.~ This circuit is given in Fig.
I.
The circuit is based
on
CMOS inverters and has good linear-
ity. The circuit has no internal nodes and an output resistance
tunable to infinity. The result
of
this is that an integrator built
with this transconductor has
a
high DC-gain and parasitic
poles located in the gigahertz region. This is
a
good starting
point for VHF filters.
The transconductance can
be
tuned with the supply voltage
v,:4
Y,
=
(Kd
-
K"
-
I
Kp
I
)JCS"
.
B,)
(1)
ELECTRONICS LETERS
29th March 1990
Vol.
26
No.
7
.
-.
The output resistance of the transconductor can be tuned with
Vdd.
The DC-gain
of
the transconductance-C integrator is
Vdd
101
%d
Ydd
Vdd
"dd
"0
1
"0
2
'nv3Inv4
["vi
'""6
(""2
Fig.
1
VHF linear transconductance element4
only limited by mismatch: It can be shown that the measured
0.5%
transconductance mismatch results in
a
DC-gain of
minimally
200,
which is high enough for most applications.
Filter:
A third order elliptic filter has been built with the
transconductor of Fig.
1.
The normalised passive prototype
filter is given in Fig.
2. The active implementation is shown in
0
2016
1203T
I
203T
Fig.
2
Normalised passive prototype
of
third order ellipricfilter
Fig.
3.
The filter is a direct implementation of the ladder filter
using
a
gyrator
(G,-G,)
to simulate the inductor.
The filter operates mainly
on
parasitic capacitances. The
parasitic capacitances are all at nodes where a capacitance
is
desired in the filter. The parasitic capacitances comprise
roughly
70%
of gate oxide capacitance
(Cox)
and are conse-
quently quite linear. C, is fully determined by parasitic capac-
itances. The other capacitances C, to C, are designed by
adding small extra capacitors. These extra capacitors are poly-
silicon n-well capacitors; also with gate oxide dielectric. The
time constants of the filter can be written
as
T
=
C/g,,
with
C
a
capacitance in Fig.
3
and
g.
the transconductance of the
transconductor. Both
C
and
g,
are approximately proportion-
al to
Cox,
The result of this is that the spread in
T
due to
spread in
CO,
is small. This results in quite accurate time
constants, even if the filter operates mainly on its own para-
sitics. Another advantage of operation
on
parasitics is that the
series resistance in the capacitances is very small, which is
important at very high frequencies.
The balanced input voltage of the filter is generated from a
single ended signal by means of an off-chip transformer
(T').
The output voltages
of
the filter are converted to output cur-
rents by means
of
G,.
These currents are converted to volt-
ages by means
of
two off-chip
l00R
resistors. The differential
output voltage is converted to a single ended voltage
at
500
by means
of
a
transformer (T'). An on-chip reference path,
also buffered with a transconductor
(G9), is used to compen-
sate for all parasitic elements outside the filter during mea-
surements.
The chip was processed in a 3pm BICMOS process using
only the CMOS part of it.
Experimental results:
The measured filter responses are given
in Fig.
4 for three values
of
bd:
Vdd
=
2.5V,
=
5V
and
421
V,,
=
IOV. From this figure it can be seen that the measured
responses fit very closely the ideal response of the passive
ates mainly on parasitic capacitances, hut the accuracy is not
affected.
0
bond
pad
A
0
Volt5
analogue earth
reference
path
I
II
G9
LI7
filter
Fig.
3
Acme implementation
offilter
and
test
circuit
10
0
-I
0
-2
0
%
5
-30
n
-CO
-
50
-
60
10
100
4CG
-
'141q
frequency
MHz
Fig.
4
Filter responses
a
V,
=
2
5
V,
b
V,,
=
5
V,
c
V,
=
1OV
~ measured,
-
- - -
ideal
prototype filter of Fig
2 The cutoff frequency is varied from
22 MHz
(bd
=
2 5 V) to 98 MHz
(bd
=
10
V) The 98 MHz
filter curve is close
to
the ideal curve up to 350MHz This
implies that the transconductor has only parasitic poles in the
gigahertz region.
The experimental results are summarised in Table
1
The
lower limit for the dynamic range was chosen
as
the total
passband noise and the upper limit was the 1% total tnter-
modulation distortion (TIMD) input voltage level The TIMD
was measured with
a
two-tone input signal with frequencies
of
around half the cutoff frequency
of
the filter
Table
1
EXPERIMENTAL RESULTS
The results obtained with this technique indicate that accu-
rate integrated CMOS filters at very high frequencies are pos-
sible. Applications can he found in the field of TV IF filtering
and other VHF filters.
Acknowledgments:
I
thank
E.
Seevinck, W.
J.
A. de Heij, E.
Klumperink and
K.
Hoen for fruitful discussions and A. Cense
and
J.
P.
M. van Lammeren of Philips, Nijmegen, for making
processing
of
the chip possible.
B.
NAUTA
10th
November
1989
Faculty
of
Electrical Engineering
University
of
Twente
PO
Box
217,
7500
AE Enxhede,
The
Netherlands
References
1
KRUMMENACHER,
F.,
and
JOEHL,
N.'
'A
4-MHz
CMOS
COIllillUOUS-
time filter with on-chip automatic tuning',
IEEE
J.,
1988,
SC-23,
pp. 75C758
2
KIM, T. G., and GEIGER,
R. L.:
'Monolithic programmable
RF
filter',
Electron. Lett..
1988,
24,
pp. 1569-1571
3
TsiviDts,
Y.
P.:
'Minimal transistor-only micropower integrated
VHF
active filter',
ibid.,
1987,
23,
pp. 777-778
4
NAUTA,
U,,
and SEEVINCK, E.: 'Linear
CMOS
transconductance
element for
VHF
filters',
ibid.,
1989,
25,
pp. 448450
Parameter
bd
=
2.5V
bd
=
5V
bd
=
1OV
Cutoff frequency
22
MHz 63 MHz 98 MHz
Passband ripple
(0.28 dB)
0.3
dB 0.4 dB 0.8 dB
Dynamic range*
?
68 dB 72 dB
CMRR-passband 40dB 40 dB
40
dB
Transconductance 0.35 mA/V 1.06 mA/V 1.38 mA/V
Power dissipation 4mW 77mW 670mW
'id
2.50
V
4.76 V 8.10V
*
See text
The adjustment of
Vdd
(frequency tuning) and
Vid
(Q-tuning)
was done manually. However, circuitry for automatically
tuning
Vdd
and
V,,,
which is necessary to compensate for
process and temperature variations, is presently being devel-
oped.
Conclusions:
Experimental results for a VHF CMOS
transconductance-C lowpass filter are presented. The filter is
built with transconductors
as
published earlier." The maximal
cutoff frequency is 98MHz and the filter response fits very
well with the ideal response, up to 350 MHz. The filter oper-
422
PERFORMANCE CHARACTERISTICS
OF
HIGH POWER CW,
lcm
WIDE MONOLITHIC
AlGaAs LASER DIODE ARRAYS WITH A
2mm
TOTAL APERTURE WIDTH
Indexina terms: Lasers and laser aoolications. Measurement
Icm wide monolithic laser diode arrays emitting around
810nm with a 2mm total aperture width have
been
charac-
terised under CW conditions. CW operation up to
16
W has
been achieved at a heatsink temperature of 70°C. Several
arrays have been lifetested at
10
W CW
at a
20°C
heatsink
temperature for a few thousand hours and have projected
lifetimes of between 5000 and 17000 hours. The temperature
dependence of the degradation rate was characterised, from
which data an activation energy of 0.2eV was obtained.
Continuous-wave (CW) AlGaAs monolithic laser diode arrays
are reliable, have high power, high efficiency and are narrow
bandwidth sources of optical energy.',' CW output power as
high as 76 W has been demonstrated using
1
cm wide mono-
lithic arrays with a 3mm total active aperture width
(30%
packing density)
at
0°C.' A projected lifetime in excess
of
5000
ELECTRONICS LETTERS
29rh
March
1990
Vol.
26
No.
7
.
..