ENE 428 Microwave Engineering Lecture 9 Scattering parameters

ENE 428 Microwave Engineering Lecture 9 Scattering parameters and their properties. 1

Impedance and Admittance Matrices • Consider an arbitrary N-port network below, 2

The impedance [Z] matrix relates voltages and currents. So we can write [V] =[Z][I] V 1 = Z 11 I 1 + Z 12 I 2 V 2 = Z 21 I 1 + Z 22 I 2, etc. 3

The admittance [Y] matrix relates currents and voltages. So we can write [I] =[Y][V] I 1 = Y 11 V 1 + Y 12 V 2 I 2 = Y 21 V 1 + Y 22 V 2, etc. 4

Zij or Yij can be found by o/c or s/c at all other ports and • Zij can be found by driving port j with the current Ij, open -circuiting all other ports and measuring the open-circuit Voltage at port i. • Yij can be found by driving port j with the voltage Vj, short-circuiting all other ports and measuring the shortcircuit current at port i. 5

Reciprocal Network • Many practical networks are reciprocal (not containing any nonreciprocal media such as ferrites or plasmas, or active devices) • Impedance and admittance matrices are symmetric, that is and 6

Lossless Network • If the network is lossless, then the net real power delivered to the network must be zero. Thus, Re{Pav} = 0. • Then for a reciprocal lossless N-port junction we can show that the elements of the [Z] and [Y] matrices must be pure imaginary where m, n = port index. 7

Single- and Two-port Networks • The analysis can be done easily through simple input-output relations. • Input and output port parameters can be determined without the need to know inner structure of the system. • At low frequencies, the z, y, h, or ABCD parameters are basic network input-output parameter relations. • At high frequencies (in microwave range), scattering parameters (S parameters) are defined in terms of traveling waves and completely characterize the behavior of two-port networks. 8

Basic definitions • Assume the port-indexed current flows into the respective port and the associated voltage is recorded as indicated. 9

Ex of h and ABCD parameters for two-port network • H parameters • ABCD parameters These two-port representations (Z, Y, H, and ABCD) are very useful at low frequencies because the parameters are readily measured using short- and open- circuit tests at the terminals of the two-port network. 10

Two-port connected in series 11

Two-port connected in shunt 12

Two-port connected in cascade fashion 13

Disadvantages of using these parameters at RF or microwave frequency • Difficult to directly measure V and I • Difficult to achieve open circuit due to stray capacitance • Active circuits become unstable when terminated in short - and open- circuits. 14

Scattering Matrix (1) • The scattering matrix relates the voltage waves incident on the ports to those reflected from the ports • Scattering parameters can be calculated using network analysis techniques or measured directly with a network analyzer. 15

Scattering Matrix (2) • A specific element of the [S] matrix can be determined as • Sii is the reflection coefficient seen looking into port i when all other ports are terminated in matched loads. • Sij is the transmission coefficient from port j to port i when all other ports are terminated in matched loads. 16

Reciprocal networks and lossless networks • [S] matrix for a reciprocal network is symmetric, [S]=[S]t. • [S] matrix for a lossless network is unitary that means 17

Ex 1 A two-port network has this following scattering matrix Determine if the network is reciprocal, and lossless 18

Introduction of generalized scattering parameters (S parameters) 1. Measure power and phase 2. Use matched loads 3. Devices are usually stable with matched loads. S- parameters are power wave descriptors that permits us to define inputoutput relations of a network in terms of incident and reflected power waves 19

Introduction of the normalized notation (1) Let’s define we can write and 20

Introduction of the normalized notation (2) We can also show a(x) and b(x) in terms of V(x) and I(x) as and 21

Normalized wave generalization • For a two-port network, we can generalize the relationship between b(x) and a(x) in terms of scattering parameters. Let port 1 has the length of l 1 and port 2 has the length of l 2, we can show that or in a matrix form, Observe that a 1(l 1), a 2(l 2), b 1(l 1), and b 2(l 2) are the values of incident and reflected waves at the specific locations denoted as port 1 and port 2. 22

The measurement of S parameters (1) • The S parameters are seen to represent reflection and transmission coefficients, the S parameters measured at the specific locations shown as port 1 and port 2 are defined in the following page. 23

The measurement of S parameters (2) (input reflection coefficient with output properly terminated) (forward transmission coefficient with output properly terminated) (output reflection coefficient with input properly terminated) (reverse transmission coefficient with input properly terminated) 24

The advantages of using S parameters • They are measured using a matched termination. • Using matched resistive terminations to measure the S parameters of a transistor results in no oscillation. 25

The chain scattering parameters or scattering transfer parameters (T parameters) (1) • The T parameters are useful in the analysis of cascade connections of two-port networks. • The relationship between S and T parameters can be developed. Namely, 26

The chain scattering parameters or scattering transfer parameters (T parameters) (2) and We can also write 27

Review (2) • Normalized notation of the incident a(x) and reflected waves b(x) are defined as • The relationship between the incident and reflected waves and the scattering matrix of the two-port network, 28

Shifting reference planes • S parameters are measured using traveling waves, the positions where the measurements are made are needed to be specified. The positions are called reference planes. 29

Scattering matrix of the shifting planes • At the reference planes at port 1 and port 2, we write the scattering matrix as and at port 1’ and port 2’ as • We can show that 30