design and development of onboard x-band conical

design and development of onboard x-band conical
Maria Siddiqua
RF/Microwave Communication System Laboratory
Abstract--This paper presents the design & simulation
of a QPSK Modulator (Quadrature Phase Shift Keying)
based on Direct Carrier Modulation technique with a very
simple circuitry. In this design, two double balanced
mixers, Branch-line coupler & Wilkinson power combiner
are used. The selection of a Double Balanced Mixer, being
the main component of the design, is a critical issue.
Specifications of mixer such as Conversion loss, Interport
isolation etc are discussed. Branch-line coupler is used to
produce in phase & quadrature signals & Wilkinson power
combiner is used to combine the two modulated orthogonal
signals at the output. These passive components are easy to
fabricate, occupy reasonable area and provide satisfactory
performance as compared to their counterparts. The
simulated spectrum and static constellation diagrams of the
modulator are presented. The circuit has been designed at
8.3 GHz X Band for 150Mbps bit rate.
monolithic applications [3]. With MMIC technology, a
tunable resistor can be realized using FET and pHEMT.
However, there were amplitude & phase variations in the
output signal dictated by the bias dependency of the
intrinsic resistances & parasitic capacitances of these
devices. These problems can be removed using passive
double balanced mixers. The double balanced mixer
having balanced diodes and transmission line
transformers remove the spurious response of RF & LO
frequencies. This eliminates the need of filters which
makes the hardware bulky and complex [1].
The design and simulation of modulator is carried out
using Harmonic Balance & Circuit Envelop Analysis.
High performance & affordable transmitters for digital
communication and radar systems are in demand. In
traditional microwave transmitter systems, first
modulation is carried out at an intermediate lower
frequency and then the modulated signal is upconverted
to the required transmitting frequency. To filter out the
unwanted sidebands at the carrier frequency, a complex
chain of filters and amplifiers are required [3]. The
performance of the circuit is very good but due to the
long chain of filters & amplifiers, hardware is bulky.
Direct Modulation has attained considerable importance
due to the minimal hardware complexity and cost
especially for microwave wireless applications such as in
satellites. Furthermore direct modulation eliminates the
need of extra filters to reject the sidebands. Direct
Modulators are widely used in cellular phones and
wireless LAN communications.
Research has been done on direct modulation
techniques based on Reflection Topology using
FET(Field Effect Transistor), pHEMT (pseudomorphic
High Electron Mobility Transistor) and PIN diodes as
switches. Traditionally, current-controlled PIN diodes
were used to realize a variable resistor. However, due to
more expensive foundry processing & the significant
control power required, PIN diodes are not often used in
Depending on the frequency of operation &
performance criteria, there are various ways to implement
the direct modulator. One of the microwave direct
modulators is Tandem modulator also known as I-Q
vector modulator which has the ability to control both
amplitude and phase of the transmitted signal
simultaneously [3]. Fig. 1 shows typical vector modulator
with a 90° splitter, two mixers and an in-phase combiner.
Fig. 1. Typical QPSK Modulator
I-Q modulator consists of a quadrature 3dB power divider
which creates two orthogonal channels. The two I (in-phase)
& Q (90° out of phase), NRZ (non return to zero) data streams
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