Frequency-independent active phase shifters for UWB applications
ABSTRACT This paper describes an active phase shifter, which exhibits a frequency-independent phase shift over the ultra-wideband (UWB) frequency range. The design of this pure phase shifter relies on the use of a negative group delay (NGD) circuit. The topology of the active NGD cell is based on a Field Effect Transistor in cascade with a series resonant network. A 90°±5° phase shifter with a single NGD stage was designed, at first, and implemented in hybrid microstrip technology; its measurements showed a 75% relative bandwidth and validated the proposed approach. Then, a compact pure phase shifter with a multi-stage NGD cell was designed. It was aimed at meeting specifications in the UWB frequency range, i.e. 3.1-10.6 GHz (110% relative band). Over this band, the corresponding simulation results showed a constant transmission phase of -45°±3°, an insertion gain from 2 to 5.5 dB and insertion losses better than 9 dB.
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ABSTRACT: With a conventional lens sharpness of the image is always limited by the wavelength of light. An unconventional alternative to a lens, a slab of negative refractive index material, has the power to focus all Fourier components of a 2D image, even those that do not propagate in a radiative manner. Such "superlenses" can be realized in the microwave band with current technology. Our simulations show that a version of the lens operating at the frequency of visible light can be realized in the form of a thin slab of silver. This optical version resolves objects only a few nanometers across.Physical Review Letters 11/2000; 85(18):3966-9. · 7.94 Impact Factor
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ABSTRACT: We present experimental scattering data at microwave frequencies on a structured metamaterial that exhibits a frequency band where the effective index of refraction (n) is negative. The material consists of a two-dimensional array of repeated unit cells of copper strips and split ring resonators on interlocking strips of standard circuit board material. By measuring the scattering angle of the transmitted beam through a prism fabricated from this material, we determine the effective n, appropriate to Snell's law. These experiments directly confirm the predictions of Maxwell's equations that n is given by the negative square root of epsilon.mu for the frequencies where both the permittivity (epsilon) and the permeability (mu) are negative. Configurations of geometrical optical designs are now possible that could not be realized by positive index materials.Science 05/2001; 292(5514):77-9. · 31.03 Impact Factor
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ABSTRACT: Recently, three-dimensional composite periodic media comprising split-ring resonators (SRR) and thin wires have been shown to exhibit a negative refractive index in the frequency range around the SRR resonance. In this letter, we propose transmission line models for studying and interpreting the electromagnetic propagation behavior of such materials. Based on these equivalent transmission line models, we show that by periodically loading a network of transmission lines with series capacitors and shunt inductors, a negative refractive index medium can be synthesized without excess resonators, thus leading to wideband behavior. These proposed media have tailorable properties over a broad frequency range. Moreover, they are completely planar, frequency scalable, more compact, and easier to implement for RF/microwave circuit applications than their SRR/wire counterparts.IEEE Microwave and Wireless Components Letters 03/2003; · 1.78 Impact Factor
Frequency-Independent Active Phase Shifters
for UWB applications
Blaise Ravelo1, Marc Le Roy2 and André Pérennec2
ESIGELEC, Technopôle du Madrillet, avenue Galilée, BP 10024, 76801, Saint-Etienne du Rouvray Cedex
2Lab-STICC, UMR CNRS 3192
UEB, Université de Brest (UBO), 6 avenue Le Gorgeu, CS93837, 29238 Brest cedex 3
Abstract— This paper describes an active phase shifter, which
exhibits a frequency-independent phase shift over the ultra-
wideband (UWB) frequency range. The design of this pure phase
shifter relies on the use of a negative group delay (NGD) circuit.
The topology of the active NGD cell is based on a Field Effect
Transistor in cascade with a series resonant network. A 90°±5°
phase shifter with a single NGD stage was designed, at first, and
implemented in hybrid microstrip technology; its measurements
showed a 75% relative bandwidth and validated the proposed
approach. Then, a compact pure phase shifter with a multi-stage
NGD cell was designed. It was aimed at meeting specifications in
the UWB frequency range, i.e. 3.1-10.6 GHz (110% relative
band). Over this band, the corresponding simulation results
showed a constant transmission phase of -45°±3°, an insertion
gain from 2 to 5.5 dB and insertion losses better than 9 dB.
Further to the study by Veselago  and those by Smith
and Pendry -, many authors have investigated,
theoretically and by experiments, the properties provided by
artificial materials, known as metamaterials. For example,
some resonant artificial materials and their equivalent 1-D
planar circuits - are able to generate a negative group
velocity over a limited frequency band where anomalous
dispersion occurs. Generally, these frequency bands are rather
narrow and associated with high attenuation values in
transmission. Electronic active circuits generating negative
group delay (NGD) were proposed in -, at first as
demonstrators to study this intriguing property, and finally to
compensate for losses and to widen the relative operating
frequency band. In these electronic circuits, the notion of
group delay is preferred to that of group velocity because the
circuit length may be undefined in the case of lumped, or
integrated, components. Nevertheless, as these circuits use
classical electronic components and particularly operational
amplifiers, their operating frequency bands are restricted
below a few hundreds of MHz -. The idea of a negative
group delay synthesizer was validated by Lucyszyn et al. ,
but as the device was operating in reflection, the range of
possible applications was restricted by the transmission losses
as observed for resonant planar metamaterial circuits.
To overcome these limitations and give the opportunity to
apply this “non-commonly-found-in-nature” property, the
authors have proposed in previous studies a new NGD active
topology -. It consists in an active microwave circuit
that provides a significant NGD value and gain. The topology
is based on a Field Effect Transistor (FET) so as to permit
operation in different frequency bands or over broad
bandwidths according to the type of the chosen FET. As the
first realization completely fulfilled these objectives , two
major application domains were identified: i) the initial
microwave topology was converted into a baseband one
dedicated to high-speed signals in order to equalize or
compensate for interconnect or passive-circuit parasitic effects
,. ii) In the second domain of applications, i.e.
microwave devices, the negative group delay which
corresponds to a positive phase slope, was directly used as a
basic element to design a pure phase shifter -. A pure
phase shifter is a phase shifter with a constant (i.e. frequency-
independent) transmission phase; this notion was introduced
in 1920 for analogue modulations . More recently, several
pure phase shifters were proposed at higher frequency bands
-. But their transmission losses were non negligible,
or they operated in narrow frequency bands or their
implementation was difficult. In , an ultra-wideband
(UWB) phase shifter easy to implement was described but it
presented a constant phase shift between two output accesses
and not a constant transmission phase as proposed here.
Moreover, this passive distributed circuit proved to exhibit
In this paper, after a brief recall of the NGD active circuit
theory, the principle of the proposed frequency-independent
phase shifter is detailed in the case of a single NGD cell
circuit. The design and implementation of this 90° phase
shifter are reported, and the results from simulations and
measurements are compared and discussed. Then, to widen
the operating bandwidth, a multistage NGD pure phase shifter
was designed. The simulations showed a -45° constant
transmission phase over the 3.1-10.6 GHz band with a gain
within 2 and 5.5 dB and return losses better than 9dB. The
advantages and limitations of this technique are discussed and
some improvements are considered.
hal-00524304, version 1 - 7 Oct 2010
Author manuscript, published in "40th European Microwave Conference, Munich : Germany (2010)"
DOI : 10.978-2-87487-016-3 © 2010 EuMA
II. ACTIVE NGD CELL THEORY
The NGD unit cell, required by the design principle of the
proposed phase shifter, simply consists of an RLC series
network in cascade with a FET .
Fig. 1. Unit cell of active NGD circuit and its low frequency model.
In , by considering the low frequency model of the FET,
i.e. a voltage-controlled
transconductance, gm, in cascade with the drain-source resistor,
Rds, simple analytical expressions for the S-parameters of this
cell were obtained. Moreover, it was demonstrated
theoretically and experimentally that the group delay is always
negative at the resonance frequency,
where Z0 = 50 Ω is the reference impedance port. For a given
FET, the cell may also generate gain under certain conditions
of compromise between L and C:
Indeed, by inverting these equations, the network synthesis
equations are expressed as:
where S21 = |S21(ω0)| and the group delay
objectives. C is simply deduced from the resonance frequency:
C = 1/(Lω0
A series resistor is usually used to match the output access and
a parallel one for the input access.
current source with a
ωτ τ =
III. PRINCIPLE AND DESIGN OF FREQUENCY-INDEPENDENT
The proposed topology of the frequency-independent phase
shifter is depicted in Fig. 2. In fact, a positive group delay
(PGD) circuit, for example a transmission line, is cascaded
with a negative group delay circuit. To get a frequency-
independent transmission phase, the group delay values of the
PGD and the NGD devices must be alike, but of opposite sign.
Theoretically, the total group delay is then close to zero. It
also corresponds to cascading devices with identical phase
slope of opposite signs to achieve an overall constant phase
value over a specified frequency band.
Fig. 2. Bloc diagram of the proposed phase shifter.
A. Design and Validation of a One Stage 90° Pure Phase
As proof-of-concept circuit, a one stage phase shifter (PS)
 was designed by first using the synthesis relations. A
single NGD cell presents at its resonance a phase shift close to
180° (Fig. 3) due to the drain-source current direction. Then,
to achieve the objective of 90°, the line length is set in a value
such that its phase shift is around -90° at this particular
Fig. 3. Transmission phases of the transmission line and of the NGD cell.
1.25 1.501.75 2.00 2.25 1.002.50
Group Delay (ps)
S21 phase (deg)
Fig. 4. NGD cell-, transmission line- and both cascaded-group delays;
cascaded (PGD+NGD) transmission phase.
TL (PGD) group delay
NGD cell group delay
(PGD+NGD) group delay
hal-00524304, version 1 - 7 Oct 2010
Then, the NGD positive phase slope is optimized to get the
opposite of the line one over the widest frequency band. Figs.
3 and 4 illustrate this principle: Fig. 3 shows the simulated
PGD and NGD transmission phases and Fig. 4 depicts the
group delays of the three circuits (PGD, NGD and PGD+NGD)
s well as the overall simulated transmission phase. Finally, a
final slight optimization under an EM software was performed
by taking into account the transistor and the passive
component S-parameters. This circuit was implemented in
planar microstrip technology and biased by a classical passive
Fig. 5. Detailed schematic of the 90° phase shifter: PHEMT EC2612 FET
(gm = 98.14 mS and Rds = 116.8 Ω, bias network in thin lines (Vgs = -0.1 V,
Vds = 3 V, Ids = 30 mA). R1 = 51 Ω, R2 = 22 Ω, R = 33 Ω, L = 4.7 nH, C = 1 pF,
Cb = 22 µF, Lb = 220 nH, Rb = 1 kΩ. ansmission lines, TL: w = 952 µm and
d = 6.2 mm.
Figs. 6 and 7 compared simulation (run under Momentum
software) and experimental results. They show that the
agreement is globally satisfying. The measured group delay
and constant phase show frequency characteristics widened by
comparison to simulation results. It is worth noting that the
measured gain is slightly lower than to the simulated one. The
measured gain is kept within -2 and 2 dB. The measured phase
is 90°±5° from 1.08 GHz to 2.42 GHz, which corresponds to a
76.5% relative band and the return losses are better than 10 dB
over this band.
Moreover, the total group delay is effectively close to zero
and the active phase shifter has a compact size of 2 cm by
Fig. 6. Transmission phase and group delay values obtained by simulations
Fig. 7. Simulated and measured S21-magnitude and return losses.
B. Design and Simulations of a -45° UWB Pure Phase Shifter
using Multistage of NGD Cells
Further to this proof-of-concept validation, a -45° phase
shifter was designed by using the same approach as the one
detailed in  to widen the operating frequency band. As put
forward in , it is advised to use an even number of FETs
to get phase values between 0 and -180°. Then, three different
resonant cells and two EC-2612 PHEMT FETs were used to
get -45° with gain for UWB applications, i.e. for a frequency
band extending from 3.1 to 10.6 GHz. Each cell was designed
according to both the synthesis equations and the previously
detailed process. Then, a final optimization was performed by
taking into account the FET S-parameters. Fig.8 shows the
ADS schematic. The group delay and the transmission phase
are both plotted in Fig. 9, whereas the S-parameter
magnitudes are presented in Fig. 10.
Fig. 8. ADS schematic of the UWB phase shifter with the component values
and the substrate characteristics.
One should note that, in the UWB, the gain lies within 2
and 5.5 dB, and the return losses are better than 9 dB, the S21-
phase is close to -45° with a variation from -41° to -47°. Once
again, it is worth underlining that the device is of compact size.
The whole group delay is close to zero, and even slightly
negative from 3.1 to 7.4 GHz. Due to the FET non-reciprocity,
the PS also presents a high isolation value, which may be an
advantage depending on the final application.
hal-00524304, version 1 - 7 Oct 2010
Fig. 9. Transmission phase and group delay of the -45° UWB phase shifter.
Fig. 10. Simulated S-parameters.
Using lumped components in simulations up to 10.6 GHz
may seem inconsistent with a hybrid implementation,
particularly for inductances. Nevertheless, chip thin film
inductors, operating up to these frequencies, are now available
and an MMIC integration is also possible.
Design examples of pure phase shifters were presented. To
get a frequency-independent transmission phase, two devices
were cascaded with identical absolute phase slopes but of
opposite signs. In theory, this means that the cascaded
elements have opposite constant group delay value; this
results in a total group delay close to zero. This kind of phase
shifter should be distinguished from most of the available
phase shifters: here, the transmission phase is constant but not
the phase shift between two branches as in classical phase
shifters. To provide a positive phase slope, a negative group
delay circuit is needed. The authors proposed an NGD
topology able to provide gain and NGD, and used this circuit
to implement a 90° frequency-independent phase shifter in
hybrid technology. The experimental results validated the
principle of design and the synthesis relations. Indeed, this
single-NGD-stage showed a 90°±5° transmission phase over a
75% relative bandwidth centred around 1.75 GHz, with a gain
between -2 and 2 dB. Then, multi-stage NGD cells were used
to design a -45° UWB pure phase shifter in order to widen the
operating bandwidth. The simulation results confirm
a -45°±3° transmission phase from 3.1 to 10.6 GHz with a
gain within 2 and 5.5 dB. EM simulations of this latter circuit
are in progress and its implementation is planned. Ongoing
researches are focused on the design of tunable pure PSs.
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