EC312 - ExerciselS AM,Gain,Noise,Filter Problems

An AM transmitter is tested using a 50 Ohm resistive ("dummy") load in place of theantenna. The voltage waveform across this load is shown below. Using this waveform,determine:

in Volts

/ ,/ L>

* S 6Time in/fn^m Sec

I/ rv>

50

a. The frequency of the carrier.

b. The frequency of the modulat ing signal.

c. The voltage of the carrier alone.

d. The modulat ion index.

fm - ~J~r* ~

Vc = 100

m =

,,,Mitl

e. Write an expression for t h i s voltage waveform (\\4iichshowsallthefrequencies).

- /O .:-)5.n (ztr 2^1 -25'

f. Determine the channel bandwidth required to transmit this AM waveform.

BWAM"

EC312 - ExerciselS AM,Gain,Noise,Filter Problemsj^"027i9/2oT4'~~~

g. (2 pts) Determine the average power in the carrier.

PC :zk "h. (2 pts) Determine the total power.

PT !•)

2. A 5-kHz carrier is amplitude modulated by a single 200-Hz tone. Determine thefrequencies generated.

5COO , 5"2OQ

3. A 740-kHz carrier is modulated by a voice signal that has frequency components from100 Hz to 3 kHz. Sketch the spectrum of the resulting signal and indicate the range offrequencies generated for the upper and lower sidebands.

100 Hz to 3 kHz

sin 2n 740,000

Ifo,l

fuss:

4. What is the bandwidth of the AM signal generated in the previous example? ' ^'( " " "

5. Overmodulation and sideband splatter can occur when the modulation index exceeds /OQ %.

6. WWL, AM 870 in New Orleans broadcasts with a total power of 50,000 W. If the stationuses a modulation factor of 95%, how much power is in the carrier?

(1,^51 2 Av J

EC312 - ExerciselS AM,Gain,Noise,Filter ProblemsRev 02/19/2014

7. For the circuit below, describe the attenuation) A(jco)|= jVout(jco)/Vs(jco)| ( Magnitude notphase) in terms of the impedances and their variables ( Think Voltage divider)

CL

Vs RLOAD

+

VOUT

Remembering the impedances: Zc= 1/jcoC, ZL = jcoL, R=R/_0.

_J&^^_ -jAGco)|= Z(Z, -<- Z L+ ^ a

__Ti t , i l ' /R + j u) L — j >UJ<L

What happens to the attenuation when ZC=ZL? A = J

Does this describe a filter function? H i f S ~ IVr ••"/.• LcL-CK

•J. ••:.':'} CYll *~1 &Jkzx* --1 A-

8. Given: The signal input to the RF amplifier in an AM radio has a signal amplitude of 3.0 uV.A noise voltage of 0.2 uV was added to the signal by the pre-selector

Reqd: What is the SNR as a ratio and dB value at the input to the RF amp?

Soln: *</\ 'J

a. A d

9. Given: The input to the RF amplifier has a SNR of 23.5 dB.The output of the RF amplifier is a signal of 75 uV, and a noise of 6.3 uV.

Reqd:What is the SNR at the output of the RF amp'?

Soln: ,- y

i**

EC312 - ExerciselS AM,Gain,Noise,Filter Problems______ ....,, ,,,, ., _ _ _

10. Given: The gain of the three stages of the IF filter/amplifier are 17.0 dB, -0.5 dB, and 20 dB.N F , » 2 0 4 d B l iF s «30dB NF5-9.29«iB

signal input

i, IF 1 IF1 Filter 1 "(Am output

4 = 11D dB .4, = .Q y dB A> = 20,0 dB

Reqd: What is the total gainjorthis circuit?

Soln:

Ar

Ja\. Given: The bandwidth of an AM receiver with a 75-H input resistance is 10 kHz. The

temperature of the receiver is 35°C. The signal input power is 25 pW.

Reqd: Find the open circuit noise voltageF4a^-the~mnrse-power

^e-the--SM^Tpttt-^f^>*ft€!r^B^ .

Soln:

' "75- Jt 10

v*;- i I 2.

1 2. What is the noise voltage produced by a 300-Q resistor at 20°C over a(o MHz Bandwidth?

73 )

13. For each of the types of internal noise, indicate where on the diagram they predominate.

n ' ' Devicenoise

shot noise

(j) flicker (1/f) noise

_ thermal noise

3 \ transit-time noise

F(Hz)