High Voltage Practical Robotic Two Port S Parameter Passive Circuit Using Spice

RF-controlled robot design requires S parameters to model and simulate various electronic passive resistors, inductors, and capacitor circuits represented as two-port networks. Examples demonstrating the usefulness of the technique and models in simulation are provided. The procedure and technique for simulation of two ports using Z, H parameters are extended by similar steps using analogue behavioural modelling, as is done for S parameters in a two-port with original SPICE2G for robotic circuits, using computer-aided simulation tools like SPICE/ Pspice at high frequencies.

Introduction

In the present age of modern electronics, almost all engineers and scientists are familiar with robots as they are extensively used in human life. The RF-controlled Robot utilises a transmitting device containing an RF transmitter and an RF Encoder, sending commands to the robots for specified tasks such as moving back and forth, reversing, turning right/left, and stopping. To design RF transmitters in RF and microwave systems, S-parameters [1-6] are common, and these parameters can be measured with the help of network analysers.

SPICE [7-16] is employed to simulate various electronic circuits comprising different types of devices and linear circuits. The elements used in typical CAD device environments using SPICE include resistors, capacitors, inductors (including mutual), voltage/current DC and AC sources, dependent linear and non-linear current/voltage sources, transmission lines, and electronic devices such as diodes, BJT, JFET, MOSFET. Two-port circuit models for microwave passive networks, both in classroom theory and industrial practice, are important for students and researchers and are useful in synthesising T-PI models [17-24]. There are two methods to simulate [18-24] active or passive circuits: the first is by using active or passive models for each component or group of components, and the second is by representing input and output voltages and currents by suitable port parameter equations when that portion of the circuit does not contain independent voltage and current sources. Models represented by non-linear analogue behavioural options of Pspice are added to these equations, and the system is solved for voltages and currents, either manually or using sophisticated simulation software. To model the two-port of an RF network for robotic use using existing software programs like Spice2g and Pspice, where there are no models to represent them, we must use existing model descriptions like behavioural models and adapt them suitably. Attempts have been made to achieve this using only the Spice2G program. The present method can be used in conjunction with original passive circuits to simulate networks. The analogue behavioural modelling option of PSPICE includes additional descriptions of DC and AC voltages and currents using expressions, tables, Laplacian operators, and complex voltage and current multiplications with complex numbers.

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Passive circuit modelling method

The power waves incident and reflected from the two ports of a linear network are defined as a1, a2, b1, b2, respectively. These are related to the scattering matrix that characterises the circuit.

b₁=S₁₁a₁+S₁₂a₂

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b₂=S₂₁a₁+S₂₂a₂ (1)

where,

Here, both ports are assumed to have the same characteristic impedance Z0. Substituting for a1, a2, b1, b2 in equation (1),

Similarly,

V₂=S₂₂(V₂+I₂Z₀)+S₂₁(V₁+I₁Z₀)+I₂Z₀ …(4)

putting S₂₂=Real{S₂₂}+ j Imag{S₂₂}

and S₂₁=Real{S₂₁}+ j Imag{S₂₁} …(5)

Now, equation (2) and (4) describing the two port voltages and currents (Fig.1a) could be represented by Fig. 1b. Now,

V₁ i.e., equation(3) can be written as V₁=I₁Z₀+E₁+E₂

V₂ i.e, equation (5) can be written as V₂=I₂Z₀+E₇+E₈

where E₇=Real{S₂₂}(V₂+I₂Z₀)+ j Imag{S₂₂}(V₂+I₂Z₀)

=E₉+j E₁₀

E₈=Real{S₂₁}(V₁+I₁Z₀)+ j Imag{S₂₁}(V₁+I₁Z₀)

=E₁₁+j E₁₂

If the S parameters are defined in rectangular(Cartesian) co-ordinates, i.e., S_ij=Real{S_ij}+jImag{S_ij}, i,j (1,2) the sub-circuit in Table I should be used for simulating two-port network between terminals 1 and 2.

If the S parameters are defined in polar coordinates, the Table II sub circuits should be used for defining two port networks. If SPICE2G version is used, a lossless transmission line of length NL chosen according to Polar angles of the S parameters is used to realise the e ^{jPhase Angle} term. If phase angle is negative NL(Spice transmission line parameter) is given by

=ANG1

And if Phase Angle is positive then NL is given by

=ANG2

Results

The overall S parameters of the entire circuit (Figure 2) which is used generally in radio transmitters to control robots, are computed by PSpice files.

A two-port model for the measured S parameters for radio frequency transmitter component (FIGURE 2) with S11, S21, S12, and S22 as inputs is obtained USING BOTH SPICE2G and Pspice circuit analyses and simulation software. Analog behavioural modelling technique is used with Pspice. Table I and Table II show the SPICE2G subroutines to obtain any two port equivalent usind scattering matrix for radio frequency transmitter component for robo control.

Table I. SPICE2G file model for Simulating Two Port with Cartesian S Parameters

Table II. SPICE2G file model to simulate Two ports with Polar S parameters

Tables III & IV show the circuit netlist to obtain S parameters for Figure 2. Table V shows the equivalent two-port circuit for Figure 2. with Pspice software. Figures 3a & 3b show the output voltage of the transmitter component with 100Ω source and load.

Table III. Netlist for S parameter determination S11, S21

Table IV Netlist for S parameter determination S12, S22

The results by running the file(Table V) show that the two port representations are correct and valid in the 1Ghz, and 2GHz and verified by Tables III and IV.

Table V Two-port representation of the Transmitter Control Circuit

For operating temperature analysis, parameters sensitive to temperature have to be modified. They can be increased/decreased according to the difference between measured and operating temperature. Simulation packages like Spice specify at which the analysis is being done and give results at this temperature. For various temperatures, various linear component values, and linearised small signal equivalent model parameters are calculated prior to simulation. The variation of small signal S, Z, and H parameters with respect to temperature can be stored in the form of look-up tables and polynomial or spline functions at a frequency and bias. To evaluate the circuit at a temperature we must load those values of two port parameters stored in the computer either in rectangular or polar coordinates and the rest of the simulation follows with no changes in the procedure for all three representations.

Methods and procedures to predict ac/rf responses of integrated circuits employed in robot design applications in which parts of the circuit are described by two ports (both active and passive) whose S parameters are known are described and explained. An example with capacitive loading showing the usefulness of the approach is provided. It is shown from the definition of two-port parameters we obtain two equations in V1, V2, I1 and I2. The S, Z, and H parameters can be used to represent two ports, with input and output voltages and currents. The complex terms in the description of each of these circuit equations contain a linear combination of two-port Z, Y, and S parameters with voltage/current at different input/output ports. As these are circuit/network equations, no change is needed in the method to obtain two-port models as explained in the case of S parameters for the other two port representations (combination of V1, V2, I1, I2) for Z, H parameters. Also, similar simulation procedures and techniques using other sophisticated simulation RF packages could be employed for two ports described by Z transmission and inverse transmission parameters.

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K. Bharath Kumar obtained a B. Tech degree in E & CE with highest honours from JNTUniversity, Anantapur in 1981and M. Tech degree from Indian Institute of Technology, Kharagpur in the area of Microwave and Optical Communication in the year 1983. Later joined Indian Telephone Industries, Bangalore and worked in the area of Fiber optics, and in the year 1990 obtained M. S. degree from the Illinois Institute of Technology, Chicago,USA and joined the Oki Electric, Japan as a researcher. He has over thirty-four publications retired, and his current address is 10-365,Sarojini Road, Anantapur,AP 515 001 and can be reached at email:[email protected]