RF Modelling and Antennas - Dec 2012

Electronics & Telecomm. (Semester 5)

TOTAL MARKS: 80
TOTAL TIME: 3 HOURS (1) Question 1 is compulsory.
(2) Attempt any three from the remaining questions.
(3) Assume data if required.
(4) Figures to the right indicate full marks. 1 (a) A lossless co-axial cable has wavelength of electric and magnetic field of l=20cm of 960MHz. Find the relative dielectric of insulation.(5 marks) 1 (b) A typical PCB substrate consists of Al₂O₃ with a relative dielectric constant of 10 and a loss tangent of 0.0004 at 10 GHz. Find the conductivity of the substrate(5 marks) 1 (c) For a parallel copper plate transmission line operated at 1GHz, the following parameters are given:
W=6mm, d=1mm, σ_dielectric=0.125mS/m, ε_r=2.25.
. Find the line parameters R, L, G and C per unit length.(5 marks) 1 (d) A lossless 50 Ω microstrip line is terminated into a load with admittance of 0.05mS. What additional impedance has to be placed in parallel with load to assure impedance of 50Ω . (5 marks) 2 (a) A transmission line of characteristic impedance of Z₀=50A transmission line of characteristic impedance of Z₀=50Ω and length d=0.15? is terminated into load impedance of Z_L =(25-j30)Ω.
Find τ₀,Z_in(d) and SWR. (10 marks) 2 (b) A lossless transmission line with Z₀=50Ω is 10cms long f=800MHz, v_p=0.77c. If the input impedance is Z_in=j60Ω, find C_L. What length of a short circuited transmission line would be needed to replace Z_L. (10 marks) 3 (a) To suppress noise in a digital communication system a bandpass RF filter is required with a passband from 1.9GHz to 2GHz. The minimum attenuation of filter at 2.1GHz and 1.8GHz should be 30dB. Assuming that a 0.5dB ripple in passband can be tolerated, design a filter that would be using minimum number of components(10 marks) 3 (b) Derive expression for internal, external and loaded quality factors for standard series and parallel resonant circuit. (10 marks) 4 (a) The intrinsic carrier concentration is typically recorded at room temperature.
For GaAs we find at T=300^oK the effective densities of state
N_c=4.7x10¹⁷ cm^-3,
N_v=7.0x10¹⁸ cm^-3. .
Assuming that the bandgap energy of 1.42eV remains constant,
(a) Find the intrinsic carrier concentration at room temperature.
(b)Compute ni at T=400°K.
(10 marks) 4 (b) Explain in brief the principle of operation of HEMT and RF FET along with their construction.(10 marks) 5 (a) Design a prototype low pass Butterworth filter that will provide at least 20dB attenuation at f=2f_3dB. Compute and plot the amplitude response for 0 to 5GHz.(10 marks) 5 (b) Explain Ebers-Moll model of a large signal BJT. Also give details of transport representation. (10 marks) 6 (a) Explain in brief the determination of AC parameters of bipolar transistors.(10 marks) 6 (b) Show that in the fig d12.1, the feedback capacitance C_cb can be expressed as
C_M1=C_cb(1-v_ce/v_be) and C_M2=C_cb(1-v_be/v_ce), on the input and output port
Assume that the input and output voltages are approximately constant, and keep in mind that v_ce is negative under CE configuration. (10 marks)

Write short notes on:

7 (a) DC characterization of BJT.(5 marks) 7 (b) Realization of RF filters using Kuroda s identities. (5 marks) 7 (c) Terminations used in microstrip line. (5 marks) 7 (d) Equivalent circuits of resistors, inductors and capacitors.(5 marks)