Automated Testing of C-Band Rotary Field Ferrite Phase Shifter

Dushyant Bansal Int. Journal of Engineering Research and Applications www.ijera.com
ISSN : 2248-9622, Vol. 4, Issue 12( Part 6), December 2014, pp.204-208
www.ijera.com 204|P a g e
Automated Testing of C-Band Rotary Field Ferrite Phase Shifter
Dushyant Bansal, Assistant Professor
B.S.A. College of Engineering & Technology, Mathura (U.P.) INDIA
Abstract
This paper presents the methodology for an automatic test procedure based on LabVIEW for RF
characterisation of C- band rotary field phase shifter. Using this software, the characteristics of the phase shifter
intended for phased array application viz. differential phase shift, insertion loss, return loss, rms phase error,
insertion phase are measured systematically and displayed graphically in user friendly format.
Keywords – Rotary field phase shifter, phase error, GPIB, LabVIEW, VNA
I. INTRODUCTION
LabVIEW (National Instruments, USA) is a very
popular and widely accepted graphical programming
environment using graphical programming language
instead of the conventional text based language and is
used extensively for developing automated
measurement, test, and control systems. This paper
describes an automated test procedure based on Lab
VIEW software for the R.F. characterisation of the
indigenously designed and developed C band Rotary
Field Phase Shifter(RFPS) using Vector Network
Analyser(VNA). This software has been developed
keeping in view the applicability of phase shifter in
phase array radar system.
The phase shifters are the key elements of the
multifunction phased array radar. A number of phase
shifters with radiating elements are arranged in an
array forming the steerable antenna. By giving a
suitable phase command to individual elements, the
beam can be steered in any desired direction in space.
Rotary field phase shifters have performance
advantages over the other class of ferrite phase
shifters such as dual mode ferrite phase shifters and is
well reported in the literature (ref. Please). The basic
configuration of RFPS is shown in fig.1. It consists of
a metallised ferrite rod coupled at each ends to
reciprocal polarisers and matching transformers[1].
The polarisers convert linear polarisation incident at
either ends to circularly polarised TE11 mode waves.
The matching transformers couple the phase shifter to
standard rectangular waveguide. The ferrite rod is
fitted with a 8-pole ferrite yoke which contains a set
of windings to produce a transverse rotatable four
pole magnetic field. This magnetic field provides a
bias to the ferrite rod for creating a birefringence of
180 differential phase (ie a half wave plate) [2]. This
field can be rotated electronically by proper setting of
the currents in the two orthogonal windings known as
„sine‟ and „cos‟ windings. The differential phase shift
equals to twice the relative rotation of the magnetic
field. The driver circuit does the interfacing between
the control system and the phase shifter.
The typical data required for complete
characterization of the phase shifter intended for
phase array radar application are :
 Phase Shift with Command states
 Average Insertion Loss : This is computed as
the arithmetic mean of the transmission
coefficients at all phase states at any given
frequency, expressed in dB.
 Fluctuation in Average Insertion Loss : This is
computed as the square root of the mean of the
squares of the difference between the average
transmission coefficient and each individual
transmission coefficients.
 Average Return Loss : This is computed as the
arithmetic mean of the reflection coefficients
at all phase states at any given frequency,
expressed in dB.
 RMS Phase Error : This is the RMS of the
deviation of RF phase shift values from the
commanded values.
 Insertion Phase : It is the phase angle of the
receive transmission coefficient for phase
command state defined as providing zero
degree differential phase shift.
The above parameters are needed over the
specified frequency band and over the operating
temperature range for complete RF characterisation
of the phase shifter.
II. DESIGN METHODOLOGY
Testing of the phase shifters is done using a
standard test jig and terminating the radiator end of
the phase shifter in a free space simulator. For a
triangular grid configuration of the phase shifters in
an array, the free space simulator is a square
waveguide[3]. Hence, the VNA test bench is
calibrated using a square waveguide cal kit. This is
critical for obtaining accurate measurements. A TRL
calibration routine has been chosen after creating the
TRL calibration kit. The block diagram of the test set
RESEARCH ARTICLE OPEN ACCESS

up is shown in fig 2. The system is composed of the
following :
 HP 8510C Network Analyzer
 C-band rotary field phase shifter with
integrated 6 bit digital Driver circuit (DUT)
 LabVIEW Version 8.2
 Computer with GPIB card
The computer is interfaced with the network
analyser through GPIB while the PC‟s parallel port is
used as I/O for the Command generator. Based on
the commanded phase state, the Command generator
generates the required input waveforms on the input
of the Driver which converts them to an equivalent
pulsed current and voltage waveforms to drive the
phase shifter coils (Sine and Cos windings) for
setting the desired phase state. The driver employs a
microcontroller and a microstepping device for this
purpose. One switching cycle is completed in
approximately 200 µsec.
The phase shifter is tested by commanding it to
different phase states from 0 to 63(for 6 bit control)
which correspond to differential phase shift from 0 to
354.4 degrees (max) and recording the data from the
VNA. The data set consists of all four S parameters
(Mag and Phase) at 64 phase setting and at the
specified number of frequency points (min 51 points,
max 801 points) within the given frequency band.
The automatic test procedure (ATP) program
developed inhouse using LabVIEW performs all the
tasks of instrument control, data acquisition,
computation and processing for display routines. The
computer plays the roles of talker (writes a control
string to the instrument with the address), listener
(receive data string from the instrument with the
address) and controller (manage GPIB and parallel
port). The program flowchart is shown in fig 3. It has
a user friendly input panel where the user can enter
the details like freq range, no of points, parameter etc
which are specified for initialisation and calibration
and the DUT number, Cal set, temperature, no. of
observation points etc as shown in fig 4. The first
phase state is set four times for precise setting of 1st
phase state. The data is recorded and averaged.
Thereafter, phase states of the phase shifter are varied
in steps from 0 to 63. For every phase setting, the
phase shifter RF response is measured and recorded.
A delay of typical 1.2 msec is kept in between phase
settings to take care of switching time of the driver
and the VNA response time. The data is processed
and the required RF parameters are calculated by the
computer for display in both tabular and graphical
formats. Simultaneously the results are saved in an
Excel sheet and the graphs are saved in jpeg format.
This ATP is a very productive tool in designing and
producing results in desired formats.
LabVIEW programs are called Virtual
Instruments (VIs). These are analogous to main
programs, subroutines and functions in text based
programming languages. In designing this
measurement software, a modular approach has been
chosen. The main VI, ATP.VI consists of various sub
VIs which are designed for specific tasks as shown in
fig.5. These sub VIs can be executed and debugged
independently. The functions of the sub VI are as
follows :
(i) INITIALIZE.VI: The system is initialised by
giving commands to HP8510C (VNA) and
command generator through the GPIB and
parallel ports respectively.
(ii) FREQ_LIST.VI: This VI gives the list of
frequency points at which data is presented
depending upon the no. of points chosen
within the given frequency band.
(iii) VNA_SUB.VI: This VI reads data at each
phase state from the VNA through the GPIB
port.
(iv) MAG_PHASE.VI: The data recorded from
the VNA is in string format. This VI is used
to calculate the magnitude and phase from
the string.
(v) COMPUTE.VI: This VI calculates the
desired parameters such as insertion loss,
phase shift, rms phase error, insertion phase
etc. from the recorded data.
(vi) REFERENCE.VI: A reference look up table
is generated for computation of RMS phase
error.
(vii) PLOT.VI and PRINT.VI: These VIs
presents the calculated data in the form of
graphs and provide a hard copy of the data
respectively.
(viii) SUM_2XCL.VI: The final results are saved
in EXCEL sheet.
III. TYPICAL RESULTS
Fig 6(a-d) shows the graphical plot of various RF
parameters of the phase shifter. Fig 6(a) shows the
phase shifter phase response with respect to the
commanded states. The measured results show that
linear phase characteristics are obtained. Fig 6(b)
shows the RMS phase error of the phase shifter at
various frequencies. As mentioned earlier this is the
RMS of the deviation of RF phase shift values from
the commanded value of the phase shift. A plot of
the differential phase shift vs. Command state has
been shown in fig6(a). As shown in fig. 6(b), it is
observed that r.m.s. phase error is within 3 degrees.
The average insertion loss and average return loss
characteristics are plotted in fig 6(c) and 6(d)
respectively.

IV. CONCLUSION
Automatic Test Procedure (ATP) software
based on LabVIEW for C- band rotary field phase
shifter has been successfully developed, resulting in
automated data acquisition, analysis and display of
measured parameters in appropriate formats. This
automatic test procedure has been implemented in
the product ionisation of the C-band rotary field
phase shifter.
REFERENCES
[1] C R Boyd Jr, “A latching ferrite rotary field
phase shifter”, IEEE Microwave Symposium
Digest, Vol-1, pp 103-106, May 1995
[2] A G Fox, S E Miller, M T Weiss, “Behavior
and application of ferrites in the microwave
region”, Bell Syst Tech J, vol 34, pp 5-103,
Jan 1955
[3] M A Balfour, "Active impedance of a phased
array antenna element simulated by a single
element in a waveguide," IEEE Trans.
Antennas Propagation, vol. AP-15, no. 2, pp
313-314, March 1967.
Fig 1 : C-band rotary field phase shifter
Fig 2 : Block diagram of the test bench set up

Fig 3. Flow chart of ATP software
Fig 4. Input Panel of user interface

Fig 5. Main VI and Sub VIs
Fig 6 (a). Plot of Diff phase shift Vs Command
states
Fig 6 (c). Plot of Average Insertion Loss Vs
Frequency
Fig 6 (b). Plot of RMS phase error Vs Frequency
Fig 6 (c). Plot of Average Return Loss Vs
Frequency
Fig 6 . Measured RF parameters of the C-band rotary field phase shifter

Automated Testing of C-Band Rotary Field Ferrite Phase Shifter

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