Development of a Radiofrequency Phase Shifter for use in
Phased and Smart Antenna Arrays
Octavian – Modest MANU
Ștefan cel Mare University
Suceava, Romania
octavianm@eed.usv.ro
Abstract: This paper presents the development of an ultra-high frequency phase shifter for use in
phased arrays or smart antenna arrays. Several methods used in obtaining phase shifts of radio frequency
signals are described along with a summary of advantages and disadvantages of each method. The solution
adopted in the end is based on the use of a vector modulator in order to obtain a phase shifter capable of
modifying both phase and amplitude of the signal. In the end the practical implementation of an advance phase
shifter based on an Analog Devices 1GHz vector modulator is presented.
Keywords: antenna arrays, phase shifter, ultra high frequency (UHF), vector modulator, vector network
analyzer (VNA).
I. Introduction
The efforts of developing phase shifter
systems have increased over the last years due
to their multiple uses in instrumentation and
metrology applications, amplifier linearization,
power combining, etc. One of the most
extensive uses of radio frequency phase shifters
is for carrying out the beam-forming in phased
antenna array applications. These smart
antennas are used in telecommunication industry
or military applications [1-4]. The non-inertial
electronic beam-forming and beam scanning of
the phased antenna arrays used for localization
and monitoring applications, allows a very fast
beam switching and target identification coupled
with the ability to monitor multiple targets at the
same time. These advantages are impossible to
achieve using traditional inertial systems of
rotating antennas.
In the field of communication applications the
higher antenna gain of the antenna arrays
increases the signal to noise ratio, allowing the
decreasing of the bit error rate over the
communication channel. Other optimizations
brought by smart antennas in the field of
communication systems include the increasing of
the spectral efficiency, the reducing of the
transmitted
power,
of
the
co-channel
interference, and of the multipath interferences
[5-6]. Smart antennas can also monitor and
track mobile users, allowing an improvement of
both signal range and the quality of service.
Figure 1 . Cross section through radiation pattern
(H plane) of 8 element linear phased antenna array
steered at 0°, 45° and -45° scanning angle.
Figure 1 depicts a cross section of radiation
pattern in horizontal plane of an eight elements
antenna array, having an inter-elements distance
d of half wavelength and scanned at 3 different
angles of -45°, 0° and 45° to the array normal.
In order to steer the main beams of a linear
antenna array to the desired angle Φ it is
necessary to apply a progressive phase shift Ψi
between antenna array elements:
i  k0 (K  i)d sin() for i  1,2,...K
where k0
wavenumber.
represents
the
free
(1)
space
II. Phase shifter designs
In order to implement a scanning antenna
array a phase shifter capable to variable phase
shift the signal applied to the antenna elements
must be developed. The simplest design uses
two delay lines with different length and
switches to select through which line the signal
is passing. The delay Φ obtained between the
two paths is given by:
  2 l
p
(2)
where l represent the length difference between
the two paths and the vp represent the phase
velocity of the transmission line. The lines can
be made of wave guides, radio frequency cables,
coplanar waveguides, or microstrip lines. The
main advantages of the delay line phase shifter
are relative simple construction and the fact that
the delays introduced are not influenced by the
signal frequency. Among disadvantages must be
noted that in order to obtain several different
phase shift values a number of switched delay
lines must be interconnected. Thus, the signal
attenuation is not constant over the entire phase
shift range. This type of phase shifter does not
allow a continuous variation of phase shift
values, providing only discrete phase shifts and
in the case of a large number of phase shift
values, the design complexity for this type of
phase shifter increases very much.
The directional properties of the half power
hybrid coupler are used in the development of
the analog reflection type phase shifter. This,
being an analog variable phase shifter, allows an
infinite number of phase steps over the entire
phase shifter range. The device consists of a 3dB
hybrid coupler and two variable reflective loads
connected between coupler and ground. The
signal is feed to Port 1 of the hybrid (input port)
and it is split equally between Port 2 and Port 3
with 90° phase difference between them. From
here signals are reflected back by the loads
connected to the ground.
Figure 2 . Analog reflection type phase shifter.
The variable reflective loads induce a phase shift
Φ on the reflected signal. At the Port 1 the half
power signal having a phase shift of Φ reflected
from Port 2 (coupled port) is added with the half
power signal reflected from Port 3 (output port),
which has a phase shift of 180° + Φ, thus
nulling each other. At the Port 4 (isolated port)
the half power signal reflected from Port 2
having a phase shift of 90° + Φ is added with
the half power signal reflected from Port 3 also
having a phase shift of 90°+Φ. Thus, at the
Port 4 a signal having full power and a phase
shift of 90°+Φ is obtained. If the variable
reflective loads can vary the impedance between
Zmin and Zmax, the phase shift ΔΦ induced by the
devices is given by the formula:

Z

Z

  2 tan1  L,max   tan1  L,min 
 Z0 
 Z0 






(3)

where Z0 is the reference impedance of the
hybrid being used. Since -90° < tan-1 (x) < 90°,
the maximum value of the phase shift obtained
with this type of phase shifter is 180°. The
reflective loads will always have a resistive
component, thus the phase shifter will exhibit
loss. Usually the variable reflective loads are
made with varactors which, due to their low
quality factor, exhibit large loss.
An improved method of developing a phase
shifter consists of using a phase modulator as
the active element. This splits the input signal
into two equal amplitude components phased 90
degree apart, which are then independently
amplified with variable gain amplifiers (VGA) and
then summed, forming the phase shifter output
signal. If the gain of the in-phase (I) and
quadrature (Q) VGAs is GI and GQ respectively,
the desired gain and phase setpoints of the
phase shifter are given by:
2
G



arctan 






180  arctan 




 


180  arctan 







 arctan 




GI  GQ
2
(4)
2
GQ 

GI 

GQ 

GI 

GQ 

GI 


GQ

GI 

if GI  0, GQ  0
if GI  0, GQ  0
(5)
if GI  0, GQ  0
if GI  0, GQ  0
The vector modulator phase shifter provides
several advantages over other designs,
described shortly. The variable gain amplifiers
can be developed using transistors and lumped
elements, resulting in a very compact size and, if
the gain of these amplifiers is greater than unity,
the phase shifter itself can have gain. The vector
modulator phase shifter can create any desired
phase shift between 0° and 360°. Another great
advantage is the possibility of adjusting both
amplitude and phase errors of the phase shifter
in site, only by varying the gain of the VGAs,
thus allowing easy tuning of a phased antenna
array comprised of several elements, equipped
with vector modulator based phase shifters.
The output of the VGAs are summed and
feed to the output amplifier which delivers a
differential output signal into the external load.
This amplifier also has an output disable
function.
The block diagram of the phase shifter circuit
is presented in Fig. 4, schematics in Fig. 5 and
the PCB layout in Fig. 6. The differential I and Q
control signals of the AD 8340 are provided by
two Linear Technology LTC 2602 dual 16 bit rail–
to–rail digital to analog converters (DAC)[8]. The
voltage reference for the DACs is supplied by the
National Semiconductor LM4140 high precision
low noise low dropout voltage reference [9].
III. Hardware implementation
The above mentioned benefits of the vector
modulators lead us to the development of a
phase shifter based on the Analog Devices
AD8340 vector modulator, with possible use in
the construction of a phased antenna array
operating in the 868 MHz industrial, scientific
and medical (ISM) band.
The first stage of the AD8340 vector
modulator, as seeing in the Fig. 3, consist of an
in-phase – quadrature splitter, which splits the
input signal into two components phased shifted
90° apart and having equal amplitude. The input
signal can be driven either differential or single
ended. This quadrature generator is made with a
multistage resistor – condenser polyphase
network tuned over the entire operating
frequency range of 700 MHz to 1000 MHz [7].
Since the passive network is perfectly linear, the
RF input signal amplitude and phase information
are transmitted faithfully to both channels. The I
and Q signals are feed to the corresponding
differential variable gain attenuators having
differential input control signals.
Figure 3 . AD8340
diagram.
Vector
modulator
Figure 4 . Phase shifter block diagram.
Figure 5 . Phase shifter schematics
block
Figure 6 . Phase shifter printed circuit board.
The DACs are controlled via a serial
peripheral interface (SPI) by ATMEGA 16, an 8
bit reduced instruction set computing (RISC)
microcontroller (MCU) manufactured by Atmel
[10]. The MCU receives the phase and the
attenuation values via an USB connection from a
computer. The USB to USART signal conversion
is made with FTDI 232BM integrated circuit. The
phase shifter and the control board prototypes
were manufactured on a dual layer FR4 copper
laminate with a permittivity of approx. 4.5 and a
loss tangent of 15·10-3 at a frequency of 1 GHz.
The dielectric thickness is 1.5mm and the
conductive layer height is 35 µm. The radio path
of the circuit was design using with the help of
freely available microwave numerical tools. An
experiment was conducted in order to verify the
correctness of the numerical tools. Several
microstrip lines with different widths were
etched on the same FR4 laminate. The lines
were terminated with SMD resistors and the
impedances were measured using Agilent Field
a.
b.
c.
d.
e.
f.
Figure 7 . Measured impedance of microstrip lines with width of 3 mm (a), 2,863 mm (b), 2,5 mm (c), 2 mm
(d), 1,7 mm (e), and 1,3 mm (f).
Figure 8 . Controller printed circuit board.
Figure 10 . S21 magnitude vs. phase.
Figure 9 . Experimental setup.
Fox N9912A Radiofrequency Analyzer between
600 MHz and 1 GHz. Smith Charts with the
results are presented in Fig. 7. The computed
width of a 50 ohm microstrip line for the above
mentioned laminate was 2.863mm. As seen in
Fig. 7 b) the 2.863 mm microstrip line has a
complex impedance of 49.5 –j0.2 ohms, which is
the closest value to the desired 50 ohm
impedance. The phase shifter module prototype
is compact having dimensions of 35 mm per 38
mm. Phase shifter uses SMA connectors for RF
path and a HDR1X7 connector for power supply
and serial communication with the controller
board. The controller board is made on the same
FR4 laminate, having the dimensions 64 mm by
41 mm. This circuit incorporates Atmega 16
microcontroller, FT 232 signal converter, an ISP
connector for in-circuit serial programming of
microcontroller and several LED used for
indicating serial traffic status and diagnosis.
The phase shifter characteristics were measured
using an HP 8753E Vector Network Analyzer.
The reflection coefficient was characterized
between 800 MHz and 1 GHz including the RFID
865 MHz to 868 MHz band. and it was found to
be between -42dB to -15dB over the entire
frequency range. The complex transmission
coefficient was measured at a given frequency of
866.5MHz for two attenuation values of 0% and
Figure 11 . S21 absolute phase error.
Figure 12 . Phase linearity.
25% over the entire phase range of 360° with a
phase step of 1. As shown in Fig. 10 the
variation of the S21 magnitude is lower than
0.85 dB resulting in a good stability of signal
gain over the phase variation. The absolute error
of the measured phase shift versus phase set
point over the entire 360° interval is plotted in
Fig. 11 and has a maximum value less than 4°.
Fig. 12 depicts the linearity of measured phase
over the entire phase setpoint interval.
IV.
Conclusions
This article presented an analysis of several
techniques for the development of radio
frequency phase shifters for use in phased array
antennas. The solution adopted for a practical
implementation of a phase shifter consisted in
using a vector modulator, due to the advantages
offered over the other types of phase shifters
such as: compact size; the ability to create any
desired phase shift between 0° and 360°; easy
in-site tuning of both phase and gain for each
phase shifter. The hardware implementation
uses the Analog Devices AD 8340 vector
modulator which has an operating frequency
between 700 MHz and 1 GHz. Controlling the
vector modulator was done via two LTC 2602
digital to analog converters. The prototype was
made on a dual layer FR4 copper laminate.
Beside the phase shifter a controller board was
also developed based on an 8 bit RISC
microcontroller which transmitted the values of
phase and gain set points via an SPI interface to
the DACs. Testing the prototype was done with
an HP 8753E Network Analyzer. The results
showed that the developed phase shifter has
good phase shift accuracy and stability in the
frequency region of interest.
V.
Acknowledgements
O.M. thanks CNCSIS-UEFISCSU PROJECT
NUMBER RU-107/2010 for the financial support.
VI.
[1]
[2]
[3]
[4]
[5]
[6]
[7]
[8]
[9]
[10]
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Octavian - Modest MANU
PhD student, Ştefan cel Mare University from
Suceava, Faculty of Computer Science and
Electrical Engineering, PhD thesis: Contributions to
the development of smart antennas and
applications, PhD supervisor: Prof. Eng. Adrian
GRAUR, PhD.