|The function of a Phase Shifter is to control
the phase characteristics in the processing of
a microwave signal. This device will convert a
signal to a desired phase location via a digital
The theory of operation is to divide the input
signal into two equal signals 90 degrees apart,
I (in-phase) and Q (Quadrature). This allows
the magnitude of each signal to be re-located
along its vectors' axis. The two signals are
then combined. Using The Pythagorean Theorem,
the sum of the vectors produces the resultant
The circuit for this digital phase shifter
consists of a 3 dB, 90 degree Quadrature Hybrid,
two Variable Attenuators capable of a 180 degree
phase shift, an In-Phase Power Combiner and
drive circuits to control the variable attenuators
(Click Here for Schematic).
The key component is a 3 dB, 90 degree, Quadrature
Hybrid. The hybrid is used nine times. The input
signal is processed by the first hybrid. It
equally divides the amplitude with a 90 degree
phase shift to the Quadrature path and the two
signals are isolated. This places the I &
Q Vectors on their respective axis. The balances
of the hybrids are divided in the I & Q
channels for the Variable Attenuators. Each
attenuator controls the magnitude with 180 phase
shift allowing four quadrant operations. The
final stage is the In-Phase Power Combiner which
combines the signals in vector addition to the
In the RF design of a digital phase shifter
the preferred media is microstrip. Traditional
hybrids are designed in stripline. Whether incorporating
the hybrid as part of the circuit or as a discrete
component like hybrids for microstrip configurations,
the result is, the microwave fields of propagation
are excited. This creates a discontinuity within
the transmission line of the device. Techniques
can be employed to minimize the discontinuity;
but with nine 4 port hybrids, this event occurs
directly or in-directly 36 times. To this end,
changing media alters fringing fields, thus
creating adverse effects and degrading performance.
In a very natural progression, G. T. Microwave
developed hybrids in microstrip to eliminate
the 36 discontinuities; this complements the
design. In a stripline hybrid, tuning is performed
between measurements because the hybrid's circuitry
is not accessible. Microstrip hybrids allow
access to the circuitry while being measured.
This provides for a more precise tuning capability.
The technique yielded an overall improvement
in both the optimization and performance. The
hybrid exhibits a 9:1 Bandwidth with an Amplitude
Balance of +/- 0.6 dB, a Phase Balance of +/-
4.0 degrees, an Isolation of 18 dB and a V.S.W.R.
of 1.5:1 max.
With the transmission problems eliminated
by the entire microwave circuit topology being
in a true microstrip configuration, there were
other design considerations that were incorporated.
The choice of a diode is an important design
consideration. Using chip diodes with a ribbon
lead in a shunt configuration has its drawbacks.
In a high frequency broadband application, the
inductance of ribbon lead adversely effects
the attenuation and phase. A Beam Lead Diode
installed using a proprietary technique reduces
the series inductance and has a significant
improvement in performance.
To minimize adverse effects from biasing,
the networks are buried as far as possible from
the direct signal paths. The line length between
adjacent hybrids is zero in the I & Q channels
for optimal broadband performance.
Even with the improvement in the microwave
performance, it is the control circuitry that
provides the absolute accuracy for the overall
The digitally controlled section of the Phase Shifter is designed as a single input divided
into two independent drivers, hence I &
Q controls. Each driver has a resolution capable
of 64K to control and compensate the device.
For any desired resolution, the optimal performance
values are stored in the driver's EPROM’s
ready to be commanded from the external Digital
This is accomplished by using a computer with
I/O and IEEE controller cards, G. T. Microwave's
proprietary program and a Vector Network Analyzer.
The computer's I/O port sets the external control
input for the desired phase shift then ramps
each driver's 64K of resolution along the I
or Q axis using another I/O port to an internal
control input. While ramping the drivers the
Vector Network Analyzer measures each step and
sends the data to the computer via the IEEE
Bus. When the optimal location is determined,
the computer programs the driver's EPROM’s
for the external control input count.
The digital phase shifter being demonstrated
is optimized over a 9:1 bandwidth, 2.0-18.0
GHz with 360 degrees of phase shift and 0.088
degree resolution, 12 BITs. The unit's envelope
is 4.25 x 3.38 x 0.75 Inch. Using independent
test laboratories to verify results, the following
test data illustrates the typical performance
that is achieved using the techniques described
herein (Click Here for Plot Responses).
This technology hosts a variety of products
which include but is not limited to: BPSK, QPSK
& Vector Modulators, Phase Shifter and
Phase Free Attenuators. The models are offered
with options that include: digital control with
up to 64K of resolution, Linearized or any desired
control input slope characteristic, narrowband
optimized performance, temperature compensation,
video filtering and sub-assembly integration.
With the new millennium upon us, modulation
techniques will require technology to demand
a new generation of components. They will need
an improved performance at a lower cost. Now,
industry can welcome the arrival of ultra-broadband
digital phase shifter that will provide tomorrow's
capability today at G. T. Microwave, Inc., the
leading edge in performance.