Name: ______________________ IFT-372 Mid-Term Practice Exam
Spring 2016
1. A continuous-spectrum baseband signal has a spectrum extending from 3 kHz to 8 kHz. It is applied as one input to an ideal balanced modulator. The
carrier input is a sinusoid having a frequency of 100 kHz. (a) Determine the range of frequencies contained in the DSB output spectrum. (b) Determine the
required transmission bandwidth for an AM modulated system.
The spectrum of the ranges from 5 to 8 kHz and the carrier frequency is 100 kHz. Therefore, f c=100 kHz and W=5Hz. The lowest frequency is fc – W =
100 kHz – 5 kHz = 95 kHz.
The highest frequency is fc + W = 100 kHz + 5 kHz = 105 kHz.
The range of frequencies is thus 95 kHz to 105 kHz.
(b) W = 5 kHz. Therefore, the transmission bandwidth is BT = 2W = 2 *5 kHz = 10 kHz
2. What are the (a) noise figure, (b) noise factor, and (c) effective temperature (Te) for a system with a 26 dB input SNR and a 21 dB output
SNR?
(NF) noise figure:
26dB - 21dB = 5dB noise figure
(F) noise factor:
10^(noise figure/10)
10^(5/10) = 3.16
(Te) effective temp:
noise factor = 1 + (Te/290)
3.16 = 1 + (Te/290)
2.16 = (Te/290)
Te = 290 * 2.16 = 626.4
1
, Name: ______________________ IFT-372 Mid-Term Practice Exam
Spring 2016
3. wha are
4. A baseband signal has frequency components from 5 kHz to 15 kHz. Determine (a) the theoretical minimum sampling rate and (b) the maximum time
interval between successive samples.
F = 5kHz - 15kHz
Nyquist = 2 * fmax
2(15) = 30
F = 1/T or T = 1/F
T = 1/15 = .067 sec.
5. A digital communications system is being designed with the goal of transmitting 32 kbits/s over a channel having a bandwidth of 8 kHz. Determine (a) the
number of encoding levels required and (b) the minimum signal-to-noise ratio required in the channel.
a) Capacity = 2Blogbase2(M)
32/10 = (2 * 8logbas2(M)) / 10
2(logbase2(M))
M = 4 levels
b) C = Blogbase2(1+SNR)
32 = 8logbase2(1+SNR)
4 = logbase2(1+SNR)
2^4 = 1+SNR
SNR = 15
2
Spring 2016
1. A continuous-spectrum baseband signal has a spectrum extending from 3 kHz to 8 kHz. It is applied as one input to an ideal balanced modulator. The
carrier input is a sinusoid having a frequency of 100 kHz. (a) Determine the range of frequencies contained in the DSB output spectrum. (b) Determine the
required transmission bandwidth for an AM modulated system.
The spectrum of the ranges from 5 to 8 kHz and the carrier frequency is 100 kHz. Therefore, f c=100 kHz and W=5Hz. The lowest frequency is fc – W =
100 kHz – 5 kHz = 95 kHz.
The highest frequency is fc + W = 100 kHz + 5 kHz = 105 kHz.
The range of frequencies is thus 95 kHz to 105 kHz.
(b) W = 5 kHz. Therefore, the transmission bandwidth is BT = 2W = 2 *5 kHz = 10 kHz
2. What are the (a) noise figure, (b) noise factor, and (c) effective temperature (Te) for a system with a 26 dB input SNR and a 21 dB output
SNR?
(NF) noise figure:
26dB - 21dB = 5dB noise figure
(F) noise factor:
10^(noise figure/10)
10^(5/10) = 3.16
(Te) effective temp:
noise factor = 1 + (Te/290)
3.16 = 1 + (Te/290)
2.16 = (Te/290)
Te = 290 * 2.16 = 626.4
1
, Name: ______________________ IFT-372 Mid-Term Practice Exam
Spring 2016
3. wha are
4. A baseband signal has frequency components from 5 kHz to 15 kHz. Determine (a) the theoretical minimum sampling rate and (b) the maximum time
interval between successive samples.
F = 5kHz - 15kHz
Nyquist = 2 * fmax
2(15) = 30
F = 1/T or T = 1/F
T = 1/15 = .067 sec.
5. A digital communications system is being designed with the goal of transmitting 32 kbits/s over a channel having a bandwidth of 8 kHz. Determine (a) the
number of encoding levels required and (b) the minimum signal-to-noise ratio required in the channel.
a) Capacity = 2Blogbase2(M)
32/10 = (2 * 8logbas2(M)) / 10
2(logbase2(M))
M = 4 levels
b) C = Blogbase2(1+SNR)
32 = 8logbase2(1+SNR)
4 = logbase2(1+SNR)
2^4 = 1+SNR
SNR = 15
2
