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Electronic Communications: A Systems Approach

Jeffrey S. Beasley

Chapter 6

RECEIVERS - all with Video Answers

Educators

Chapter Questions

Problem 1

Explain the following: sensitivity of a receiver; selectivity of a receiver. Why are these important characteristics? In what units are they usually expressed?

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Problem 2

Explain why a receiver can be overly selective.

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Problem 3

Draw a diagram of a tuned radio-frequency (TRF) radio receiver.

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02:14

Problem 4

A TRF receiver is to be tuned over the range 550 to $1550 \mathrm{kHz}$ with a $25-\mu \mathrm{H}$ inductor. Calculate the required capacitance range. Determine the tuned circuit's necessary $Q$ if a $10-\mathrm{kHz}$ bandwidth is desired at $1000 \mathrm{kHz}$. Calculate the receiver's selectivity at 550 and $1550 \mathrm{kHz}$. $(0.422$ to $3.35 \mathrm{nF}, 100,5.5 \mathrm{kHz}, 15.5 \mathrm{kHz})$

Averell Hause
Averell Hause
Carnegie Mellon University

Problem 5

Draw a block diagram of a superheterodyne AM receiver. Assume an incident signal, and explain briefly what occurs in each stage.

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Problem 6

What type of radio receiver contains intermediate-frequency transformers?

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Problem 7

The AM signal into a mixer is a $1.1-\mathrm{MHz}$ carrier that was modulated by a $2-\mathrm{kHz}$ sine wave. The local oscillator is at $1.555 \mathrm{MHz}$. List all mixer output components and indicate those accepted by the IF amplifier stage.

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Problem 8

Explain the purpose and operation of the first detector in a superhet receiver.

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01:12

Problem 9

Explain how the variable tuned circuits in a superheterodyne receiver are adjusted with a single control.

Manik Pulyani
Manik Pulyani
Numerade Educator

Problem 10

If a superheterodyne receiver is tuned to a desired signal at $1000 \mathrm{kHz}$ and its conversion (local) oscillator is operating at $1300 \mathrm{kHz}$, what would be the frequency of an incoming signal that would possibly cause image reception? $(1600 \mathrm{kHz})$

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01:19

Problem 11

A receiver tunes from 20 to $30 \mathrm{MHz}$ using a $10.7-\mathrm{MHz}$ IF. Calculate the required range of oscillator frequencies and the range of image frequencies.

Suzanne W.
Suzanne W.
Numerade Educator

Problem 12

Show why image frequency rejection is not a major problem for the standard AM broadcast band.

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Problem 13

What are the advantages to be obtained from adding a tuned radio-frequency amplifier stage ahead of the first detector (converter) stage of a superheterodyne receiver?

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Problem 14

If a transistor in the only radio-frequency stage of your receiver shorted out, how could temporary repairs or modifications be made?

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Problem 15

What advantages do dual-gate MOSFETs have over BJTs for use as RF amplifiers?

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Problem 16

What is the mixer in a superheterodyne receiver?

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Problem 17

Why is the bulk of a receiver's gain and selectivity obtained in the IF amplifier stages?

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Problem 18

A superhet receiver tuned to $1 \mathrm{MHz}$ has the following specifications:
$R F$ amplifier: $P_G=6.5 \mathrm{~dB}, R_{\mathrm{in}}=50 \Omega \quad$ Detector: $4-\mathrm{dB}$ attenuation
Mixer: $P_G=3 \mathrm{~dB}$ Audio amplifier: $P_G=13 \mathrm{~dB}$
3 IFs: $P_G=24 \mathrm{~dB}$ each at $455 \mathrm{kHz}$
The antenna delivers a $21-\mu$ V signal to the RF amplifier. Calculate the receiver's image frequency and input/output power in watts and $\mathrm{dBm}$. Draw a block diagram of the receiver and label $\mathrm{dBm}$ power throughout. (1.91 MHz, $8.82 \mathrm{pW},-80.5 \mathrm{dBm}, 10 \mathrm{~mW}, 10 \mathrm{dBm}$ )

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Problem 19

A receiver has a dynamic range of $81 \mathrm{~dB}$. It has $0.55 \mathrm{nW}$ sensitivity. Determine the maximum allowable input signal. $(0.0692 \mathrm{~W})$

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Problem 20

Define dynamic range.

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03:04

Problem 21

List the components of an AM signal at $1 \mathrm{MHz}$ when modulated by a 1-kHz sine wave. What is (are) the component(s) if it is (they are) converted to a usb transmission? If the carrier is redundant, why must it be "reinserted" at the receiver?

James Kiss
James Kiss
Numerade Educator

Problem 22

Explain why the BFO in an SSB demodulator has such stringent accuracy requirements.

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02:09

Problem 23

Suppose the modulated signal of an SSBSC transmitter is $5 \mathrm{kHz}$ and the carrier is $400 \mathrm{kHz}$. At what frequency must the BFO be set?

Amit Srivastava
Amit Srivastava
Numerade Educator

Problem 24

What is a product detector? Explain the need for a low-pass filter at the output of a balanced modulator used as a product detector.

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02:09

Problem 25

Calculate the frequency of a product detector that is fed an SSB signal modulated by $400 \mathrm{~Hz}$ and $2 \mathrm{kHz}$ sine waves. The $\mathrm{BFO}$ is $1 \mathrm{MHz}$.

Amit Srivastava
Amit Srivastava
Numerade Educator

Problem 26

What is the purpose of a discriminator in an FM broadcast receiver?

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00:56

Problem 27

Explain why the AFC function is usually not necessary in today's FM receivers.

Hast Aggarwal
Hast Aggarwal
Numerade Educator

Problem 28

Draw a block diagram of a superheterodyne receiver designed for reception of FM signals.

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03:01

Problem 29

The local FM stereo rock station is at $96.5 \mathrm{MHz}$. Calculate the local oscillator frequency and the image frequency for a 10.7-MHz. IF receiver. (107.2 MHz, 117.9 MHz)

Linda Winkler
Linda Winkler
Numerade Educator

Problem 30

Explain the desirability of an RF amplifier stage in FM receivers as compared to AM receivers. Why is this not generally true at frequencies over $1 \mathrm{GHz}$ ?

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Problem 31

Describe the meaning of local oscillator reradiation, and explain how an RF stage helps to prevent it.

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Problem 32

Why is a square-law device preferred over other devices as elements in an RF amplifier?

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Problem 33

Why are FET's preferred over other devices as the active elements for RF amplifiers?

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Problem 34

Explain the need for the RFC in the RF amplifier shown in Figure 6-15.

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Problem 35

What is the purpose of a limiter stage in an FM broadcast receiver?

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Problem 36

Draw a diagram of a limiter stage in an FM broadcast receiver.

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Problem 37

Explain fully the circuit operation of the limiter shown in Figure 6-16.

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Problem 38

What is the relationship among limiting, sensitivity, and quieting for an FM receiver?

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01:40

Problem 39

An FM receiver provides $100 \mathrm{~dB}$ of voltage gain prior to the limiter. Calculate the receiver's sensitivity if the limiter's quieting voltage is $300 \mathrm{mV} .(3 \mu \mathrm{V})$

Nicholas Majtenyi
Nicholas Majtenyi
Numerade Educator

Problem 40

Draw a block diagram for a double-conversion receiver when tuned to a $27-\mathrm{MHz}$ broadcast using a 10.7-MHz first IF and 1-MHz second IF. List all pertinent frequencies for each block. Explain the superior image frequency characteristics as compared to a single-conversion receiver with a $1-\mathrm{MHz}$ IF, and provide the image frequency in both cases.

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Problem 41

Draw block diagrams and label pertinent frequencies for a double-conversion and up-conversion system for receiving a 40-MHz signal. Discuss the economic merits of each system and the effectiveness of image frequency rejection.

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Problem 42

A receiver tunes the $\mathrm{HF}$ band ( 3 to $30 \mathrm{MHz}$ ), utilizes up-conversion with an intermediate frequency of $40.525 \mathrm{MHz}$, and uses high-side injection. Calculate the required range of local oscillator frequencies. (43.5 to $70.5 \mathrm{MHz}$ )

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Problem 43

An AM broadcast receiver's preselector has a total effective $Q$ of 90 to a received signal at $1180 \mathrm{kHz}$ and uses an IF of $455 \mathrm{kHz}$. Calculate the image frequency and its $\mathrm{dB}$ of suppression. $(2090 \mathrm{kHz}, 40.7 \mathrm{~dB})$

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00:00

Problem 44

Explain the operation of a diode detector.

Vinnu M
Vinnu M
Numerade Educator

Problem 45

Describe the advantages and disadvantages of a diode detector.

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Problem 46

Provide the advantages of a synchronous detector compared to a diode detector. Explain its principle of operation.

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Problem 47

Draw a schematic of an FM slope detector and explain its operation. Why is this method not often used in practice?

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Problem 48

Draw a schematic of a Foster-Seeley discriminator, and provide a step-by-step explanation of what happens when the input frequency is below the carrier frequency. Include a phase diagram in your explanation.

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Problem 49

Draw a diagram of an FM broadcast receiver detector circuit.

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Problem 50

Draw a diagram of a ratio detector and explain its operation.

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Problem 51

Explain the relative merits of the Foster-Seeley and ratio detector circuits.

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Problem 52

Draw a schematic diagram of each of the following stages of a superheterodyne FM receiver:
(a) Mixer with injected oscillator frequency.
(b) IF amplifier.
(c) Limiter.
(d) Discriminator.

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Problem 53

Describe the process of quadrature detection.

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Problem 54

Explain in detail how a PLL is used as an FM demodulator.

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01:23

Problem 55

Explain how separate left and right channels are obtained from the $(\mathrm{L}+\mathrm{R})$ and $(\mathrm{L}-\mathrm{R})$ signals.

Jeff Vermeire
Jeff Vermeire
Numerade Educator

Problem 56

What is SCA? What are some possible uses of SCA?

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Problem 57

Determine the maximum reproduced audio signal frequency in an SCA system. Why does SCA cause less FM carrier deviation, and why is it thus less noise resistant than standard FM? (Hint: Refer to Figure 6-33.) $(7.5 \mathrm{kHz}$ )

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Problem 58

We want to operate a receiver with $\mathrm{NF}=8 \mathrm{~dB}$ at $\mathrm{S} / N=15 \mathrm{~dB}$ over a $200-\mathrm{kHz}$ bandwidth at ambient temperature. Calculate the receiver's sensitivity. $(-98 \mathrm{dBm})$

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Problem 59

Explain the significance of a receiver's $1-\mathrm{dB}$ compression point. For the receiver represented in Figure 6-37, determine the 1-dB compression point. ( $\equiv 10 \mathrm{dBm}$ )

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Problem 60

Determine the third-order intercept for the receiver illustrated in Figure 6-37. ( $\cong+20 \mathrm{dBm})$

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Problem 61

The receiver described in Problem 58 has the input/output relationship shown in Figure 6-37. Calculate its dynamic range. $(78.7 \mathrm{~dB}$ )

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Problem 62

A receiver with a $10-\mathrm{MHz}$ bandwidth has an $S / N$ of $5 \mathrm{~dB}$ and a sensitivity of $-96 \mathrm{dBm}$. Find the required NF. $(3 \mathrm{~dB})$

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Problem 63

A high-quality FM receiver is to be tested for SINAD. When its output contains just the noise and distortion components, $0.015 \mathrm{~mW}$ is measured. When the desired signal and noise and distortion components are measured together, the output is $15.7 \mathrm{~mW}$. Calculate SINAD. (30.2 dB)

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00:22

Problem 64

Explain SINAD.

Joanna Quigley
Joanna Quigley
Numerade Educator

Problem 65

Discuss the advantages of delayed AGC over normal AGC and explain how it may be attained.

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Problem 66

Explain the function of auxiliary AGC and give a means of providing it.

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Problem 67

Explain the need for variable sensitivity and show with a schematic how it could be provided.

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Problem 68

Explain the need for variable selectivity. Describe how VBT is accomplished if the oscillator in Figure 6-45 is changed to $2650 \mathrm{~Hz}$.

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Problem 69

What is the need for a noise limiter circuit? Explain the circuit operation of the noise limiter shown in Figure 6-46.

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Problem 70

List some possible applications for metering on a communications transceiver.

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Problem 71

What is the purpose of a squelch circuit in a radio communications receiver?

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Problem 72

List two other names for a squelch circuit. Provide a schematic of a squelch circuit and explain its operation. List five different squelch methods.

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Problem 73

Describe the effects of EMI on a receiver.

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Problem 74

Describe the operation of an ANL.

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Problem 75

Which of the factors that determine a receiver's sensitivity is more important? Defend your judgment.

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Problem 76

Would passing an AM signal through a nonlinear device allow recovery of the low-frequency intelligence signal when the AM signal contains only high frequencies? Why or why not?

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Problem 77

Justify in detail the choice of a superheterodyne receiver in an application that requires constant selectivity for received frequencies.

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Problem 78

A superheterodyne receiver tunes the band of frequencies from 4 to $10 \mathrm{MHz}$ with an IF of 1.8 $\mathrm{MHz}$. The double-ganged capacitor used has a $325 \mathrm{pF}$ maximum capacitance per section. The tuning capacitors are at the maximum value $(325 \mathrm{pF}$ ) when the $\mathrm{RF}$ frequency is $4 \mathrm{MHz}$. Calculate the required RF and local oscillator coil inductance and the required tuning capacitor values when the receiver is tuned to receive $4 \mathrm{MHz}$ and $10 \mathrm{MHz}(4.87 \mu \mathrm{H}, 2.32 \mu \mathrm{H}, 52 \mathrm{pF}$, $78.5 \mathrm{pF}$ )

Eduard Sanchez
Eduard Sanchez
Numerade Educator
01:10

Problem 79

If you were concerned with the sensitivity rating of a communications system, would noise reduction capability be a major factor in your decision-making? Why or why not?

Yujie Wang
Yujie Wang
College of San Mateo

Problem 80

Explain why a limiter minimizes or eliminates the need for the AGC function.

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Problem 81

Draw a block diagram of an FM stereo demodulator. Explain the function of the AM demodulator and the matrix network so nontechnical users can understand. Add a circuit that energizes a light to indicate reception of a stereo station.

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Problem 82

Describe the process of up-conversion. Explain its advantages and disadvantages compared to double conversion.

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Problem 83

You have been asked to extend the dynamic range of a receiver. Can this be done? What factors determine the limits of dynamic range? Can they be changed? Explain.

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Problem 84

In evaluating a receiver, how important is its ability to handle intermodulation distortion? Explain the process you would use to analyze a receiver's ability to handle this distortion. Include the concept of third-order intercept point in your explanation.

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Problem 85

The receiver in Problem 61 has a 6-dB NF preamp (gain $=20 \mathrm{~dB}$ ) added to its input. Calculate the system's sensitivity and dynamic range. $(-99.94 \mathrm{dBm}, 66.96 \mathrm{~dB}$ )

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