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, σdielectric=0.125mS/m, εr=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 ZL =(25-j30)Ω.
Find τ0,Zin(d) and SWR. (10 marks) 2 (b) A lossless transmission line with Z0=50Ω is 10cms long f=800MHz, vp=0.77c. If the input impedance is Zin=j60Ω, find CL. What length of a short circuited transmission line would be needed to replace ZL. (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=300oK the effective densities of state
Nv=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=2f3dB. 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 Ccb can be expressed as
CM1=Ccb(1-vce/vbe) and CM2=Ccb(1-vbe/vce), on the input and output port
Assume that the input and output voltages are approximately constant, and keep in mind that vce 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)