Electronic Communications: A Systems Approach

Jeffrey S. Beasley

Chapter 1 FUNDAMENTAL COMMUNICATIONS CONCEPTS - all with Video Answers

Educators

Chapter Questions

Problem 2

What is carrier frequency?

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00:35

Problem 3

Describe the two reasons that modulation is used for communications transmissions.

Gopal Sharma

Numerade Educator

Problem 4

List the three parameters of a high-frequency carrier that may be varied by a low-frequency intelligence signal.

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01:42

What are the frequency ranges included in the following frequency subdivisions: MF (medium frequency), HF (high frequency), VHF (very high frequency), UHF (ultra high frequency), and SHF (super high frequency)?

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Numerade Educator

Problem 6

A signal level of $0.4 \mu \mathrm{V}$ is measured on the input to a satellite receiver. Express this voltage in terms of $\mathrm{dB} \mu \mathrm{V}$. Assume a $50-\Omega$ system. $\left(-7.95 \mathrm{~dB}_\mu \mathrm{V}\right)$

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Problem 7

A microwave transmitter typically requires a $+8-\mathrm{dBm}$ audio level to drive the input fully. If a $+10-\mathrm{dBm}$ level is measured, what is the actual voltage level measured? Assume a $600-\Omega$ system. $(2.45 \mathrm{~V})$

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00:33

Problem 8

If an impedance matched amplifier has a power gain $\left(P_{\mathrm{our}} / P_{\mathrm{in}}\right)$ of 15 , what is the value for the voltage gain $\left(V_{\text {out }} / V_{\text {in }}\right)$ ? (3.87)

Ashley Volpe

Numerade Educator

03:42

Problem 9

Convert the following powers to their $\mathrm{dBm}$ equivalents:
(a) $p=1 \mathrm{~W}(30 \mathrm{dBm})$
(b) $p=0.001 \mathrm{~W}(0 \mathrm{dBm})$
(c) $p=0.0001 \mathrm{~W}(-10 \mathrm{dBm})$
(d) $p=25 \mu \mathrm{W}(-16 \mathrm{dBm})$

Willis James

Numerade Educator

03:34

Problem 10

The output power for an audio amplifier is specified to be $38 \mathrm{dBm}$. Convert this value to (a) watts and (b) dBW. (6.3 W, $8 \mathrm{dBW}$ )

Satpal Satpal

Numerade Educator

02:25

Problem 11

A $600-\Omega$ microphone outputs $a-70-\mathrm{dBm}$ level. Calculate the equivalent output voltage for the $-70-\mathrm{dBm}$ level. $(0.245 \mathrm{mV})$

James Kiss

Numerade Educator

Problem 12

Convert $50-\mu \mathrm{V}$ to $\mathrm{dB} \mu \mathrm{V}$ equivalent. $(34 \mathrm{~dB} \mu \mathrm{V})$

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Problem 13

A $2.15-\mathrm{V}$ rms signal is measured across a $600-\Omega$ load. Convert this measured value to its $\mathrm{dBm}$ equivalent. ( $8.86 \mathrm{dBm}(600)$ )

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Problem 14

A $2.15-\mathrm{V}$ rms signal is measured across a $50-\Omega$ load. Convert this measured value to its $\mathrm{dBm}(50)$ equivalent. (19.66 $\mathrm{dBm}(50))$

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02:43

Problem 15

Define information theory.

Sriram Soundarrajan

Numerade Educator

Problem 16

What is Hartley's law? Explain its significance.

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01:33

Problem 17

What is a harmonic?

Nathan Lee

Numerade Educator

01:48

Problem 18

What is the seventh harmonic of $360 \mathrm{kHz}$ ? $(2520 \mathrm{kHz})$

Shoukat Ali

Other Schools

Problem 19

Why does transmission of a $2-\mathrm{kHz}$ square wave require greater bandwidth than a $2-\mathrm{kHz}$ sine wave?

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Problem 20

Draw time- and frequency-domain sketches for a $2-\mathrm{kHz}$ square wave. The time-domain sketch is a standard oscilloscope display, while the frequency domain is provided by a spectrum analyzer.

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07:07

Problem 21

Explain the function of Fourier analysis.

Amit Srivastava

Numerade Educator

Problem 22

A $2-\mathrm{kHz}$ square wave is passed through a filter with a $0-$ to $10-\mathrm{kHz}$ frequency response. Sketch the resulting signal, and explain why the distortion occurs.

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01:02

Problem 23

A triangle wave of the type shown in Table 1-3(e) has a peak-to-peak amplitude of $2 \mathrm{~V}$ and $f=1 \mathrm{kHz}$. Write the expression $v(t)$, including the first five harmonics. Graphically add the harmonics to show the effects of passing the wave through a low-pass filter with cutoff frequency equal to $6 \mathrm{kHz}$.

Raj Bala

Numerade Educator

Problem 24

The FFT shown in Figure 1-16 was obtained from a DSO.
(a) What is the sample frequency?
(b) What frequency is shown by the FFT?
(FIGURE CAN'T COPY)
(FIGURE CAN'T COPY)

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Problem 25

Figure 1-17 was obtained from a DSO.
(a) What are the frequencies of the third and fifth harmonics?
(b) This FFT was created by inputting a $12.5-\mathrm{kHz}$ square wave into a DSO. Explain where $12.5 \mathrm{kHz}$ is located within the FFT spectrum.

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Problem 26

Define electrical noise, and explain why it is so troublesome to a communications receiver.

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Problem 27

Explain the difference between external and internal noise.

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Problem 28

List and briefly explain the various types of external noise.

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Problem 29

Provide two other names for Johnson noise and calculate the noise voltage output of a 1-M $\Omega$ resistor at $27^{\circ} \mathrm{C}$ over a $1-\mathrm{MHz}$ frequency range. $(128.7 \mu \mathrm{V})$

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Problem 30

The noise produced by a resistor is to be amplified by a noiseless amplifier having a voltage gain of 75 and a bandwidth of $100 \mathrm{kHz}$. A sensitive meter at the output reads $240 \mu \mathrm{V}$ rms. Assuming operation at $37^{\circ} \mathrm{C}$, calculate the resistor's resistance. If the bandwidth were cut to $25 \mathrm{kHz}$, determine the expected output meter reading. $(5.985 \mathrm{k} \Omega, 120 \mu \mathrm{V})$

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01:32

Problem 31

Explain the term low-noise resistor.

Manik Pulyani

Numerade Educator

Problem 32

Determine the noise current for the resistor in Problem 29. What happens to this noise current when the temperature increases? (129 pA)

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Problem 33

The noise spectral density is given by $e_n^2 / \Delta f=4 k T R$. Determine the bandwidth $\Delta f$ of a system in which the noise voltage generated by a $20-\mathrm{k} \Omega$ resistor is $20 \mu \mathrm{V}$ rms at room temperature. (1.25 MHz)

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Problem 34

Calculate the $S / N$ ratio for a receiver output of $4 \mathrm{~V}$ signal and $0.48 \mathrm{~V}$ noise both as a ratio and in decibel form. $(69.44,18.42 \mathrm{~dB})$

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Problem 35

The receiver in Problem 34 has an $S / N$ ratio of 110 at its input. Calculate the receiver's noise figure (NF) and noise ratio (NR). (1.998 dB, 1.584)

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Problem 36

An amplifier with $\mathrm{NF}=6 \mathrm{~dB}$ has $S_i / N_i$ of $25 \mathrm{~dB}$. Calculate the $S_d / N_o$ in $\mathrm{dB}$ and as a ratio. (19 dB, 79.4)

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Problem 37

A three-stage amplifier has an input stage with noise ratio $(\mathrm{NR})=5$ and power gain $\left(P_{\mathrm{G}}\right)=50$. Stages 2 and 3 have $\mathrm{NR}=10$ and $P_{\mathrm{G}}=1000$. Calculate the $\mathrm{NF}$ for the overall system. $(7.143 \mathrm{~dB})$

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Problem 38

A two-stage amplifier has a 3-dB bandwidth of $150 \mathrm{kHz}$ determined by an $L C$ circuit at its input and operates at $27^{\circ} \mathrm{C}$. The first stage has $P_{\mathrm{G}}=8 \mathrm{~dB}$ and $\mathrm{NF}=2.4 \mathrm{~dB}$. The second stage has $P_{\mathrm{G}}=40 \mathrm{~dB}$ and $\mathrm{NF}=6.5 \mathrm{~dB}$. The output is driving a load of $300 \Omega$. In testing this system, the noise of a $100-\mathrm{k} \Omega$ resistor is applied to its input. Calculate the input and output noise voltage and power and the system noise figure. $\left(19.8 \mu \mathrm{V}, 0.206 \mathrm{mV}, 9.75 \times 10^{-16} \mathrm{~W}, 1.4 \times 10^{-10} \mathrm{~W}\right.$, $3.6 \mathrm{~dB})$

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Problem 39

A microwave antenna $\left(T_{\text {eq }}=25 \mathrm{~K}\right)$ is coupled through a network $\left(T_{\text {eq }}=30 \mathrm{~K}\right)$ to a microwave receiver with $T_{\text {eq }}=60 \mathrm{~K}$ referred to its output. Calculate the noise power at its input for a 2-MHz bandwidth. Determine the receiver's NF. $\left(3.17 \times 10^{-15} \mathrm{~W}, 0.817 \mathrm{~dB}\right)$

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Problem 40

List and briefly describe the four basic troubleshooting techniques.

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04:13

Problem 41

Describe the disadvantages of using substitution at the early stages of the troubleshooting plan.

Kara Liskey

Numerade Educator

00:32

Problem 42

Explain why resistance measurements are done with power off.

Mrinal Rana

Numerade Educator

Problem 43

Describe the major types of circuit failures.

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Problem 44

Describe when it is more appropriate to use the signal injection method.

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Problem 45

You cannot guarantee perfect performance in a communications system. What two basic limitations explain this?

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Problem 46

You are working on a single-stage amplifier that has a $200-\mathrm{kHz}$ bandwidth and a voltage gain of 100 at room temperature. The external noise is negligible. A $1-\mathrm{mV}$ signal is applied to the amplifier's input. If the amplifier has a 5-dB NF and the input noise is generated by a 2-k $\Omega$ resistor, what output noise voltage would you predict? $(458 \mu \mathrm{V})$

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01:12

Problem 47

How does equivalent noise resistance relate to equivalent noise temperature? Explain similarities and/or differences.

Manik Pulyani

Numerade Educator

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