TE4201-Communication Electronics 1 2. Noise and Frequency Spectrum  AM communications system AM communications systemAM communications system  Noise Noise  Signal to Noise ratio Signal to Noise ratioSignal to Noise ratio  Bandwidth Bandwidth

TE4201-Communication Electronics 2 Example in AM communications system Information = low freq. sine Carrier = high freq. sine AM modulator Transmitter Propagate in the air medium = propagation power loss High power transmitted Low power received Amplified AM wave Information detected from AM wave Amplified output of information signal

TE4201-Communication Electronics 3 Noise effect in receivers Noise Both P s and P n amplified + amplifier noise

TE4201-Communication Electronics 4 Noise power, noise voltage, short noise P n = Thermal Noise power(W) k= Boltzmann ’ s constant(1.38x J/K) T=Resistance temperature (degree Kelvin)=300 at room temperature of 27°Celsius  f=Bandwidth of the system(Hz) Determine the noise voltage produced by a 1 M  resistor at room temperature of 27  C over a 1 MHz bandwidth

TE4201-Communication Electronics 5 Noise power, noise voltage, short noise (cont ’ d) (a) Determine the short noise current of a diode with a forward bias of 1mA over a 100 kHz bandwidth (b) Determine the diode ’ s equivalent noise voltage (c) If the diode is in circuit with 500  series resistance, calculate the total output noise voltage at 27  C Transistor Noise: the noise discussed before does not include transistor noise called “ short noise ”.Transistor Noise: the noise discussed before does not include transistor noise called “ short noise ”. It is due to the discrete-particle nature of the current carriers in all forms of semiconductors. These current carriers even in dc conditions, are not moving in an exactly continuous flow since the distance they travel varies due to random paths of motion.It is due to the discrete-particle nature of the current carriers in all forms of semiconductors. These current carriers even in dc conditions, are not moving in an exactly continuous flow since the distance they travel varies due to random paths of motion. The short noise sounds like a shower of lead shot falling on a metal surface. It adds to the thermal noise.The short noise sounds like a shower of lead shot falling on a metal surface. It adds to the thermal noise. Equation of the short noise in a diode is:Equation of the short noise in a diode is:

TE4201-Communication Electronics 6 Signal to Noise ratio (S/N) Noise Ratio (NR) & Noise Figure (NF) NR = (S/N) i /(S/N) o = 1000/631=1.58 NF = 10log log631 = 30dB-28dB = 2dB S i / N i = (S/N) I = 1000 (S/N) I = 10log1000=30dB S o / N o = (S/N) o = 631 (S/N) o = 10log631=28dB NR = ? NF = ?

TE4201-Communication Electronics 7 Noise figure of BJT used at different frequencies NF = 2dB if 2N4957 is used at frequencies between 0.1MHz to 100MHz Otherwise BJT will be very noisy and (S/N) o will be very low if used at other frequencies Reactance Noise Effects

TE4201-Communication Electronics 8 Noise in amplifiers in cascade NR = ? NF = ? NR 1 P GN P G1 P G2 NR 2 NR N NR = overall Noise Ratio, P G = Power Gain

TE4201-Communication Electronics 9 A three-stage amplifier system has a 3 dB bandwidth of 200kHz determined by LC circuit at it ’ s input, operating at 22  C. The first stage has a power gain of14dB and a NF = 3dB. The second and third stages each has a power gain of20dB and NF=8dB. Input noise of the first stage is generated by 10k  resistor. The output is connected to a 300  load. Calculate (a) Noise voltage and the power at the input and output of this system if the amplifiers are noiseless. (b) Overall NF for the system (c) The actual output noise voltage and noise power P G =54dB =log -1 (54/10)=10 54/10 =log -1 (54/10)=10 54/10 = 2.51x10 5 = 2.51x10 5 P G1 =14dB P G2 =20dB P G3 =20dB (a ) Noiseless amplifiers NF = 0dB NR = 1 (S/N) in /(S/N) out = 1

TE4201-Communication Electronics 10 NF 1 = 3dB NR 1 = log -1 (3/10)= 2 NF 2 = 8dB NR 2 = log -1 (8/10)= 6.31 NF 3 = 8dB NR 3 = log -1 (8/10)= 6.31 P G1 =14dB =log -1 (14/10)=25.1 P G2 = P G3 =20dB =log -1 (20/10)=100 (c) Calculation including Amplifier Noise (b)

TE4201-Communication Electronics 11 Bandwidth Frequency spectrum of information signals Frequency spectrum of a 1kHz square wave signal (digital signals) 1kHz square wave signal (digital signals) Bandwidth is the amount of important harmonics (whose amplitude is at least one tenth of the fundamental amplitude) in the frequency spectrum of information signals

TE4201-Communication Electronics 12 Harmonic content & BW of different signal waveforms

TE4201-Communication Electronics 13 Harmonic content and Bandwidth of sawtooth signal

TE4201-Communication Electronics 14 Effect of filter BW for a sawtooth information signal