Abstract :
The leakage cancellation circuit is analysed using different line sections such as the transmission line, microstrip line and strip line. And also the performance of LCC using transmission line, microstrip line and strip line provides the leakage cancellation of about -12dB, -9dB and -14dB respectively. The design of leakage cancellation circuit is done by using Advanced Design System (ADS) software from Agilent Technologies.
Keywords: Leakage cancellation circuit(LCC), Advanced design system(ADS), strip line(SLIN), transmission line(TLIN), microstrip line(MLIN).
I. INRODUCTION :
In order to improve the T/R isolation, a Leakage Cancellation Circuit (LCC), such as the one shown in Figure 1, can be used. The arrows show the major signal flow and multiple reflections are ignored. The power splitters illustrate equal power split, however the coupling ratio can be adjusted so that only the minimum amount of power necessary for cancellation is tapped off [7].
Figure 1: Operation of the LCC
The cancellation signal level (C) is set to cancel the leakage from the circulator (L). In the ideal case, the cancellation voltage is equal in amplitude but opposite in phase to that of the leakage voltage.
Hence, the leakage signal can be eliminated by the cancellation signal. In terms of complex voltages
----(1)
where R is the residue. The other signal components, are the antenna mismatch (M), and desired signal (S). The cancellation improvement (CI, simply referred
to as cancellation) in dB is
----(2)
The minus sign in Equation (1) implies that the cancellation signal should be 180° out of phase with L+M. Generally, this is only true at a single frequency, where the LCC is set to null the leakage and mismatch. A typical LCC is comprised of a phase shifter as well as an attenuator. It is usually difficult to match both the phase and amplitude of the
leakage perfectly to achieve R = 0 . The balance of amplitude and phase between the terms L+M and C is very important. Suppose there are signals from two channels being added. The total voltage is
-----(3)
where channel 1 is used as a reference (V1=1) and α and φ are the relative amplitude and phase of channel 2 to the reference. Thus, the amplitude imbalance is simply α and the phase imbalance φ. Let V1 represent L+M and V2 the cancellation voltage C.
An analog leakage cancellation circuit can be added to suppress leakage [6]. This technique involves continuous tracking of the amplitude and the phase of leakage signal. In order to suppress the leakage signal, a cancellation signal that has equal amplitude but opposite phase is added. Digital leakage cancellation circuits that exploit the similarity or correlation between the transmitted and leakage signals are proposed in [7], [8]. Such digital cancellation techniques usually require low distortion in quantization for controlling component, digital linear correction and minimal delay between digital processing and analog-to-digital (ADC) conversion,
as suggested in [6]. The goal of the cancellation branch is to coherently cancel the leakage L and mismatch M, so that only the desired signal S remains.
Figure 2: General representation of LCC
As shown in Figure 2, the LCC can be represented by an attenuator and phase shifter. The source signal is divided by a power splitter. Some of the source power is circulated through a circulator to a 50 Ω load which represents the antenna. The other part of the source power is fed into the LCC. As illustrated in Figure 2, the output of the LCC is the cancellation signal C designed to cancel leakage signal L from the circulator. Ideally, the load antenna is perfectly matched and mismatch M from the load antenna is zero. For the case of complete cancellation, only the received signal S arrives at power detector, which is shown as a load in Figure 2.
A. Circulator:
A ferrite circulator is a three or four port device that can, in principle, offer isolation of the transmitter and receiver. The symbol used to describe a three-port circulator is illustrated in Figure3.The arrow indicates the direction of "signal circulation" from port to port. Ideally, the input signal to port 1, 2 and 3 should only emerge from port 2, 3 and 1, respectively.
Figure 3: Symbol of 3-port circulator
In a three-port circulator, as shown in Figure3, the transmitter may be connected to port 1. It radiates out of the antenna connected to port 2. The received echo signal from the antenna is directed to port 3, which connects to the receiver. In this paper SecII deals with different transmission line sections such as TLIN, MLIN and SLIN where as Sec III deals with results and discussions.
II. TRANSMISSION LINES:
In communications and electronic engineering, a transmission line is a specialized cable designed to carry alternating current of radio frequency, that is, currents with a frequency high enough that their wave nature must be taken into account. Transmission lines are used for purposes such as connecting radio transmitters and receivers with their antennas, distributing cable television signals, and computer network connections.
Transmission lines use specialized construction such as precise conductor dimensions and spacing, and impedance matching, to carry electromagnetic signals with minimal reflections and power losses. Types of transmission line include ladder line, coaxial cable, dielectric slabs, stripline, optical fiber, and waveguides. The higher the frequency, the shorter are the waves in a transmission medium. Transmission lines must be used when the frequency is high enough that the wavelength of the waves begins to approach the length of the cable used. To conduct energy at frequencies above the radio range, such as millimeter waves, infrared, and light, the waves become much smaller than the dimensions of the structures used to guide them, so transmission line techniques become inadequate and the methods of optics are used.
Ordinary electrical cables suffice to carry low frequency AC, such as mains power, which reverses direction 100 to 120 times per second (cycling 50 to 60 times per second). However, they cannot be used to carry currents in the radio frequency range or higher, which reverse direction millions to billions of times per second, because the energy tends to radiate off the cable as radio waves, causing power losses. Radio frequency currents also tend to reflect from discontinuities in the cable such as connectors, and travel back down the cable toward the source. These reflections act as bottlenecks, preventing the power from reaching the destination.
A. SLIN:
A stripline circuit uses a flat strip of metal which is sandwiched between two parallel ground planes. The insulating material of the substrate forms a dielectric. The width of the strip, the thickness of the substrate and the relative permittivity of the substrate determine the characteristic impedance of the strip which is a transmission line.
It consists of a central thin conducting strip of width W>>thickness(t) placed inside a low-loss dielectric substrate of thickness (b) between two wide ground plates. Striplines have the same relative dielectric constant on both sides of the trace and have better grounding/reference . The stripline trace is also slower than a microstrip because the effective εr of microstrip is lesser than the strip line's.
Where,
W - Trace width
H - Diâ€electric thickness
T - Trace Thickness
Εr - Relative diâ€electric constant
Strip lines are excited by a coaxial line which interfaces the strip lines by means of special connector.It is suitable for the design of passive circuits but inconvenient for mounting active components. As shown in the diagram, the central conductor need not be equally spaced between the ground planes. In the general case, the dielectric material may be different above and below the central conductor.To prevent the propagation of unwanted modes, the two ground planes must be shorted together. This is commonly achieved by a row of vias running parallel to the strip on each side. http://www.microwaves101.com/encyclopedia/images/stripline1.jpg
http://www.microwaves101.com/encyclopedia/images/stripline2.jpg
http://www.microwaves101.com/encyclopedia/images/stripline3.jpg
Figure 4: General SLIN structure
For stripline and offset stripline, because all of the fields are constrained to the same dielectric, the effective dielectric constant is equal to the relative dielectric constant of the chosen dielectric material. For suspended stripline, if it is "mostly air", the effective dielectric constant will be close to 1.
B.Advantages of stripline:
Like coaxial cable, stripline is non-dispersive, and has no cutoff frequency. Good isolation between adjacent traces can be achieved more easily than with microstrip. Stripline provides for enhanced noise immunity against the propagation of radiated RF emissions, at the expense of slower propagation speeds.
Stripline filters and couplers always offer better bandwidth than their counterparts in microstrip, and the rolloff of stripline BPFs can be quite symmetric (unlike microstrip). Stripline has no lower cutoff frequency (like waveguide does).
Another advantage of stripline is that fantastic isolation between adjacent traces can be achieved (as opposed to microstrip). The best isolation results when a picket-fence of vias surrounds each transmission line, spaced at less than 1/4 wavelength. Stripline can be used to route RF signals across each other quite easily when offset stripline is used.
C. MLIN:
A microstrip line consists of a single ground plane and a thin strip conductor on a low loss dielectric substrate above the ground plate. It also have a radiation loss (which is proportional to the square of the frequency) due to thin open structure and any current discontinuities in the strip conductor.It also include the dielectric loss in the substrate and the ohmic loss in the strip conductor and the ground plane due to finite conductivity. A microstrip configuration where the relative dielectric constant
(εr) of typically around 4.5(for FR4) is on one side while a εr of 1(for air) is on the other side. the effective εr of microstrip is lesser than the strip line's. A microstrip circuit uses a thin flat conductor which is parallel to a ground plane.
Microstrip can be made by having a strip of copper on one side of a printed circuit board (PCB) or ceramic substrate while the other side is a continuous ground plane. The width of the strip, the thickness of the insulating layer (PCB or ceramic) and the dielectric constant of the insulating layer determine the characteristic impedance. Microstrip is an open structure whereas coaxial cable is a closed structure.
D. TLIN:
The generalized lumped-element model of a transmission line can be used to calculate characteristic impedance, phase velocity, and both parts of the propagation constant (phase and attenuation). The model uses an infinitesimally small section of a transmission line with four elements as shown below. Here the series resistance, series inductance, shunt conductance and shunt capacitance are all normalized per unit length (denoted by the "prime" notation).
http://www.microwaves101.com/encyclopedia/images/Transmission%20lines/general_Tline.jpg
Figure 5: General L-C TLIN
The characteristic impedance Z_0of a transmission line is the ratio of the amplitude of a single voltage wave to its current wave. Since most transmission lines also have a reflected wave, the characteristic impedance is generally not the impedance that is measured on the line. For a lossless transmission line, it can be shown that the impedance measured at a given position l from the load impedance Z_Lis
Z_\mathrm{in} (l)=Z_0 \frac{Z_L + jZ_0\tan(\beta l)}{Z_0 + jZ_L\tan(\beta l)}
where \beta=\frac{2\pi}{\lambda}is the wavenumber.
In calculating\beta, the wavelength is generally different inside the transmission line to what it would be in free-space and the velocity constant of the material the transmission line is made of needs to be taken into account when doing such a calculation.
III. RESULTS AND DISCUSSIONS:
Thus the leakage cancellation for different types of transmission sections are analysed from the following figures using the ADS software. Following Figure 7,9 and 11 shows the simulation result of LCC using TLIN,MLIN and SLIN respectively. And also the performance of LCC using SLIN is very good when compared with TLIN and MLIN. Figure 7,9 and 11 produces a leakage cancellation of about 8dB, 11dB and 20dB respectively. From this it is shown that SLIN plays a better performance when compared with other transmission line sections.
TLIN:
Figure 6: LCC using TLIN in ADS
Figure 7: Simulation result of figure 6
MLIN:
Figure 8: LCC using MLIN in ADS
Figure 9: Simulation result of figure 8
SLIN:
Figure 10: LCC using SLIN in ADS
Figure 11: Simulation result of figure 10
IV. CONCLUSION:
Thus it can be concluded that the LCC circuit with different transmission line sections are analysed such as TLIN, MLIN and SLIN. This can be done using Agilent ADS software. From this we came to know that SLIN plays better performance with 20dB leakage cancellation when compared with TLIN and MLIN.
V. REFERENCE:
[1]Wikimedia.http://upload.wikimedia.org/wikipedia/ commons/7/7a/Hawk-radarhatzerim-1-1.jpg, accessed on 26 Oct 2010.
[2] M. L. Skolnik, Introduction to Radar Systems, 2nd ed, McGraw-Hill, pp 83, 750-751, 1980.
[3]DS030_09_08HSCOUTMk2_LR[1].pdf www.thalesgroup.com, downloaded on 31 Oct 2010.
[4] P. D. L. Beasley, A. G. Stove, B. J.Reits, B-O. As, "Solving the Problems of a Single Antenna Frequency Modulated CW Radar," IEEE International RadarConference, 1990.
[5] "Microwave Devices & Radar," class notes for EC 4610, Department of Electrical and Computer Engineering, Naval Postgraduate School, pp. 115, Summer2010.
[6] G. P. Rodrique, "Circulators from 1 to 100 GHz," Microwave J. State of the Art Reference, vol.32, pp. 115-132, 1989.
[7] W. H. Cheng, "Cancellation Circuit for Transmit-Receive Isolation," Naval Postgraduate School Master's Thesis, Sept. pp xiv, 11-12, 27, 59, 2010.
[8] D. C. Jenn, Y. C. Tsai, J. H. Ryu, R. Broadston, "Adaptive Phase synchronization in Distributed Digital Array," NASA/ ESA Conference on Adaptive Hardware and Systems, Anaheim, CA, June 2010.
[9]AgilentTechnologies.http://edocs.soco.agilent.com/display/ads2009U1/Home, accessed on 6 Oct 2010.
[10]AgilentTechnologies.http://edocs.soco.agilent.com/display/ads2009/dbm%28%29+Measurement,accessed on 22 Nov 2010.
[11] P. Djerf and I. Tornazakis, "Development of a Distributed Digital Array Radar(DDAR)," Naval Postgraduate School Master's Thesis, Sept. pp. 39, 2008.
