20-10-2012, 01:49 PM
Line-Reflect-Reflect Technique
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INTRODUCTION
LRR- LINE REFLECT REFLECT is a new self-calibration
procedure for the calibration of vector network analyzers (VNA). VNA
measure the complex transmission and reflection characteristics of
microwave devices. The analyzers have to be calibrated in order to
eliminate systematic errors from the measurement results.
The LRR calibration circuits consist of partly unknown standards,
where L symbolizes a line element and R represents a symmetrical
reflection standard. The calibration circuits are all of equal mechanical
length. The obstacle, a symmetrical-reciprocal network is placed at three
consecutive positions. The network consists of reflections, which might
show a transmission. The calibration structures can be realized very easily
as etched structures in microstrip technology.
During the calibration [G], [H], which represents the systematic
errors of the VNA is eliminated in order to determine the unknown line and
obstacle parameters.
MICROWAVE DEVICES
Microwave devices are devices operating with a signal frequency
range of 1-300GHz. A microwave circuit ordinarily consists of several
microwave devices connected in some way to achieve the desired
transmission of a microwave signal.
The various microwave solid state devices are,
* Tunnel diodes
These are also known as Esaki diodes. It is a specially made PN
junction device which exhibits negative resistance over part of the forward
bias characteristic. Both the P and the N regions are heavily doped. The
tunneling effect is a majority carrier effect and is very fast. It is useful for
oscillation and amplification purposes. Because of the thin junction and
shot transit time, it is useful for microwave applications in fast switching
circuits.
* Transferred electron devices
These are all two terminal negative resistance solid state devices
which has no PN junction. Gunn diode is one of the transferred electron
devices and which works with the principle that there will be periodic
fluctuations in the current passing through an n-type GaAs substrate when
the applied voltage increases a critical value i.e. 2-4Kv/cm.
VECTOR NETWORK ANALYZERS
It measures the complex transmission and reflection characteristics
of microwave devices. This is achieved by comparing the signal input to the
device, with the signal either transmitted through or reflected back from the
device. They incorporate very narrow bandwidth receivers tuned to the
signal source frequency and give a direct readout of the four coefficients
contained in a scattering matrix-the scattering or ‘s’ parameters, each of
which has magnitude and phase elements. The network analyzer was
controlled by an external personal computer. The raw measurement data
have been read out and processed on a computer.
LRR METHOD WITH A WEAK TRANSMISSION
This algorithm is based on the representation of the obstacle
networks with pseudo transmission matrices. As the obstacles might also be
realized as pure reflections, the networks cannot be described with
transmission matrices, because in this representation a factor Δmi might
become zero.
MICROSTRIP LINES
Prior to 1965 nearly all microwave equipment utilized coaxial,
waveguide, or parallel strip-line circuits. In recent years microstrip lines
have been used extensively because they provide one free and accessible
surface on which solid state devices can be placed. It is an unsymmetrical
stripline, that is a parallel plate transmission line having dielectric substrate,
one face of which is metalised ground and the other(top) face has a thin
conducting strip of certain width ‘w’ and thickness ‘t’. Sometimes a
coverplate is used for shielding purposes but it is kept much farther away
than the ground plane so as not to affect the microstrip field lines. It is also
called an open strip line. Modes on microstrip are only quasi transverse
electromagnetic (TEM). Thus the theory of Tem- coupled lines applies only
approximately.
Microstrip transmission lines consisting of a conductive ribbon
attached to a dielectric sheet with conductive backing are widely used in
microwave technology. Because such lines are easily fabricated by printedcircuit
manufacturing techniques, they have a technical merit. Most
microstrip lines are made from boards of copper with a thickness of 1.4 or
2.8 mils. Line width of less than 0.1 is uncommon.
CONCLUSION
LRR technique is a new self-calibration procedure. The calibration
circuits are all of equal mechanical length. This is advantageous for the
calibration of vector network analyzers, such as, for instance, for
applications where the connectors of the analyzers measurement ports
cannot be displaced.
The robust functionality is confirmed by measurements, and for that
the calibration circuits can be realized in microstrip technology. In the LRR
method two solutions are there and depending on the realized calibration
structures, the appropriate way should be chosen in order to improve the
accuracy.