1. The ability to change operating frequency rapidly without a lot of retuning is called:
a. agility c. VFO
b. expansion d. spread-spectrum
ANS: A
2. The difference between the DC power into a transmitter and the RF power coming out:
a. is a measure of efficiency c. may require water cooling b. heats the transmitter d. all of the above
ANS: D
3. Baseband compression produces:
a. a smaller range of frequencies from low to high b. a smaller range of amplitude from soft to loud c. a smaller number of signals
d. none of the above ANS: B
4. ALC stands for:
a. Amplitude Level Control c. Accurate Level Control b. Automatic Level Control d. none of the above ANS: B
5. In an AM transmitter, ALC is used to:
a. keep the modulation close to 100% c. maximize transmitted power b. keep the modulation below 100% d. all of the above
ANS: D
6. With high-level AM:
a. all RF amplifiers can be nonlinear c. minimum RF power is required b. minimum modulation power is required d. all of the above
ANS: A
7. With high-level AM:
a. the RF amplifiers are typically Class A c. the RF amplifiers are typically Class C b. the RF amplifiers are typically Class B d. the RF amplifiers are typically Class AB ANS: C
8. With low-level AM:
a. the RF amplifiers must be Class A c. the RF amplifiers must be linear b. the RF amplifiers must be Class B d. the RF amplifiers must be low-power
ANS: C
9. Power amplifiers must be linear for any signal that:
a. is complex c. has variable frequency
b. has variable amplitude d. all of the above ANS: B
10. In high-level AM, "high-level" refers to:
a. the power level of the carrier c. the power level of the final RF amplifier b. the power level of the modulation d. none of the above
ANS: D
11. In high-level AM, the power in the sidebands comes from:
a. the modulating amplifier c. the driver stage
b. the RF amplifier d. the carrier
ANS: A
12. In an AM transmitter with 100% modulation, the voltage of the final RF stage will be:
a. approximately half the DC supply voltage b. approximately twice the DC supply voltage c. approximately four times the DC supply voltage d. none of the above
ANS: C
13. Practical transmitters are usually designed to drive a load impedance of:
a. 50 ohms resistive c. 300 ohms resistive b. 75 ohms resistive d. 600 ohms resistive ANS: A
14. Which of the following can be used for impedance matching?
a. pi network c. both a and b
b. T network d. a bridge circuit
ANS: C
15. When a transmitter is connected to a resistor instead of an antenna, the resistor is called:
a. a heavy load c. a temporary load
b. a dummy load d. a test load
ANS: B
16. When a transmitter is connected to a resistor instead of an antenna, the resistor must be:
a. wire-wound c. 1% tolerance or better
b. noninductive d. all of the above
ANS: B
17. A Class D amplifier is:
a. very efficient c. essentially pulse-duration modulation
b. essentially pulse-width modulation d. all of the above ANS: D
18. To generate a SSB signal:
a. start with full-carrier AM c. start with a quadrature signal b. start with DSBSC d. all of the above
ANS: B
19. The carrier is suppressed in:
a. a balanced modulator c. a frequency multiplier
b. a mixer d. none of the above
ANS: A
20. To remove one AM sideband and leave the other you could use:
a. a mechanical filter c. both a and b
b. a crystal filter d. none of the above
ANS: C
21. A direct FM modulator:
a. varies the frequency of the carrier oscillator b. integrates the modulating signal
c. both a and b d. none of the above ANS: A
22. An indirect FM modulator:
a. requires a varactor in the carrier oscillator b. varies the phase of the carrier oscillator c. both a and b
d. none of the above ANS: B
23. AFC stands for:
a. Amplitude to Frequency Conversion c. Automatic Frequency Control b. Automatic Frequency Centering d. Audio Frequency Control ANS: C
24. Frequency multipliers are:
a. essentially balanced modulators c. essentially mixers b. essentially Class C amplifiers d. none of the above ANS: B
25. With mixing:
a. the carrier frequency can be raised b. the carrier frequency can be lowered
c. the carrier frequency can be changed to any required value
d. the deviation is altered ANS: C
COMPLETION
1. The accuracy and stability of a transmitter frequency is fixed by the ____________________ oscillator.
ANS: carrier
2. In the USA, the ____________________ sets requirements for accuracy and stability of a transmitter's frequency.
ANS: FCC
3. In Canada, _________________________ sets requirements for accuracy and stability of a transmitter's frequency.
ANS: Industry Canada
4. Frequency ____________________ is the ability of a transmitter to change frequency without a lot of retuning.
ANS: agility
5. Power output of SSB transmitters is rated by ____________________.
ANS: PEP
6. Reducing the dynamic range of a modulating signal is called ____________________.
ANS: compression
7. The opposite of compression is called ____________________.
ANS: expansion
8. ALC is a form of ____________________.
ANS: compression
9. High-level modulation allows the RF amplifiers to operate more ____________________.
ANS: efficiently
10. Low-level modulation requires the RF amplifiers to be ____________________.
ANS: linear
11. To isolate the oscillator from load changes, a ____________________ stage is used.
ANS: buffer
12. The peak collector voltage in a Class C RF amplifier is ____________________ than the DC supply voltage.
ANS: higher
13. Most practical transmitters are designed to operate into a ____________________-ohm load.
ANS: 50
14. Transmitters built with transistor RF amplifiers often use a ____________________ network for impedance matching.
ANS: T
15. Matching networks also act as filters to help reduce ____________________ levels.
ANS: harmonic
16. Severe impedance ____________________ can destroy a transmitter's output stage.
ANS: mismatch
17. Transceivers combine a transmitter and a ____________________ into one "box".
ANS: receiver
18. To allow a high modulation percentage, it is common to modulate the ____________________ as well as the power amplifier in transistor modulators.
ANS: driver
19. Pulse-width modulation is the same as pulse-____________________ modulation.
ANS: duration
20. Switching amplifiers are sometimes called Class ____________________ amplifiers.
ANS: D
21. Because the sideband filter in a SSB transmitter is fixed, ____________________ is used to operate at more than one frequency.
ANS: mixing
22. To generate a SSB signal, it is common to start with a ____________________ signal.
ANS: DSBSC
23. Indirect FM is derived from ____________________ modulation.
ANS: phase
24. Using a varactor to generate FM is an example of a ____________________ modulator.
ANS: reactance
25. The modern way to make a stable VFO is to make it part of a ____________________ loop.
ANS: phase-locked
SHORT ANSWER
1. If a 50-MHz oscillator is accurate to within 0.001%, what is the range of possible frequencies?
ANS:
50 MHz ± 500 hertz
2. What is the efficiency of a 100-watt mobile transmitter if it draws 11 amps from a 12-volt car battery?
ANS:
75.8%
3. The power amplifier of an AM transmitter draws 100 watts from the power supply with no modulation.
Assuming high-level modulation, how much power does the modulation amplifier deliver for 100%
modulation?
ANS:
50 watts
4. If the final RF amplifier of an AM transmitter is powered by 100 volts DC, what is the maximum collector voltage at 100% modulation?
ANS:
400 volts
5. Suppose the output of a balanced modulator has a center frequency of 10 MHz. The audio modulation frequency range is 1 kHz to 10 kHz. To pass the USB, what should be the center frequency of an ideal crystal filter?
ANS:
10.005 MHz
6. Suppose you have generated a USB SSB signal with a nominal carrier frequency of 10 MHz. What is the minimum frequency the SSB signal can be mixed with so that the output signal has a nominal carrier frequency of 50 MHz?
ANS:
40 MHz
7. Suppose you have an FM modulator that puts out 1 MHz carrier with a 100-hertz deviation. If frequency multiplication is used to increase the deviation to 400 hertz, what will be the new carrier frequency?
ANS:
4 MHz
8. Suppose you had an FM signal with a carrier of 10 MHz and a deviation of 10 kHz. Explain how you could use it to get an FM signal at 100 MHz with a deviation of 20 kHz.
ANS:
First, put the signal through a frequency doubler to get a 20-MHz carrier with a 20-kHz deviation. Then mix that signal with an 80-MHz carrier to generate a 100-MHz carrier with 20-kHz deviation.
Chapter 6: Receivers
MULTIPLE CHOICE
1. The two basic specifications for a receiver are:
a. the sensitivity and the selectivity
b. the number of converters and the number of IFs c. the spurious response and the tracking
d. the signal and the noise ANS: A
2. The superheterodyne receiver was invented by:
a. Foster c. Armstrong
b. Seeley d. Hertz
ANS: C
3. Trimmers and padders are:
a. two types of adjusting tools c. small adjustable inductors b. small adjustable resistors d. small adjustable capacitors ANS: D
4. "Skin effect" refers to:
a. the way radio signals travel across a flat surface b. the tissue-burning effect of a strong RF signal c. the increase of wire resistance with frequency d. none of the above
a. intermediate frequency c. indeterminate frequency b. intermodulation frequency d. image frequency ANS: A
7. AGC stands for:
a. Audio Gain Control c. Active Gain Control b. Automatic Gain Control d. Active Gain Conversion ANS: B
8. The frequency of the local oscillator:
a. is above the RF frequency
b. is below the RF frequency
c. can be either above of below the RF frequency d. is fixed, typically at 455 kHz.
ANS: C
9. The local oscillator and mixer are combined in one device because:
a. it gives a greater reduction of spurious responses b. it increases sensitivity
c. it increases selectivity d. it is cheaper
ANS: D
10. Basically, sensitivity measures:
a. the weakest signal that can be usefully received
b. the highest-frequency signal that can be usefully received c. the dynamic range of the audio amplifier
d. none of the above ANS: A
11. Basically, selectivity measures:
a. the range of frequencies that the receiver can select
b. with two signals close in frequency, the ability to receive one and reject the other c. how well adjacent frequencies are separated by the demodulator
d. how well the adjacent frequencies are separated in the mixer ANS: B
12. When comparing values for shape factor:
a. a value of 1.414 dB is ideal c. a value of 1.0 is ideal b. a value of 0.707 is ideal d. there is no ideal value ANS: C
13. When comparing values for shape factor:
a. a value of 2 is better than a value of 4 c. both values are basically equivalent b. a value of 4 is better than a value of 2 d. none of the above
ANS: A
14. Distortion in a receiver can occur in:
a. the mixer c. the IF amplifiers
b. the detector d. all of the above
ANS: D
15. Phase distortion is important in:
a. voice communications systems c. monochrome video receivers b. color video receivers d. all of the above
ANS: B
16. The response of a receiver to weak signals is usually limited by:
a. the AGC c. the dynamic range of the receiver
b. noise generated in the receiver d. the type of detector circuit being used ANS: B
17. Image frequencies occur when two signals:
a. are transmitted on the same frequency
b. enter the mixer, with one being a reflected signal equal to the IF frequency
c. enter the mixer, one below and one above the local oscillator by a difference equal to the d. enter the mixer, and the difference between the two signals is equal to twice the IF IF
ANS: C
18. An image must be rejected:
a. prior to mixing c. prior to detection
b. prior to IF amplification d. images cannot be rejected ANS: A
19. Image frequency problems would be reduced by:
a. having an IF amplifier with the proper shape factor b. having a wideband RF amplifier after the mixer c. having a narrowband RF amplifier before the mixer d. none of the above
22. Germanium diodes are used in AM detectors because:
a. they are faster than silicon diodes b. they are cheaper than silicon diodes c. they minimize distortion from nonlinearity d. all of the above
24. BFO stands for:
a. Beat Frequency Oscillator c. Bipolar Frequency Oscillator b. Barrier Frequency Oscillator d. Bistable Frequency Oscillator ANS: A
25. To demodulate both SSB and DSBSC, you need to:
a. use a Foster-Seeley discriminator b. reinject the carrier
c. use double conversion
d. use one diode for SSB and two diodes for DSBSC ANS: B
26. Which would be best for DSBSC:
a. carrier detection c. envelope detection b. coherent detection d. ratio detection ANS: B
27. An FM detector that is not sensitive to amplitude variations is:
a. Foster-Seeley detector c. a PLL detector b. a quadrature detector d. all of the above ANS: C
28. The function of a limiter is:
a. to remove amplitude variations c. to limit dynamic range b. to limit spurious responses d. to limit noise response ANS: A
29. Suppressing the audio when no signal is present is called:
a. AGC c. AFC
b. squelch d. limiting
ANS: B 30. LNA stands for:
a. Limited-Noise Amplifier c. Low-Noise Audio
b. Low-Noise Amplifier d. Logarithmic Noise Amplification ANS: B
31. AFC stands for:
a. Audio Frequency Compensator c. Automatic Frequency Control b. Autodyne Frequency Compensation d. Autonomous Frequency Control ANS: C
32. The function of AFC is:
a. maintain a constant IF frequency
b. match the local oscillator to the received signal
c. lock the discriminator to the IF frequency d. none of the above
ANS: B 33. SAW stands for:
a. Symmetrical Audio Wave c. Silicon-Activated Wafer b. Surface Acoustic Wave d. Software-Activated Wave ANS: B
34. The important property of a SAW is:
a. it stabilizes the audio in a receiver c. it is a stable bandpass filter b. it allows software radios to be built d. none of the above
ANS: C
35. The main function of the AGC is to:
a. keep the gain of the receiver constant b. keep the gain of the IF amplifiers constant
c. keep the input to the detector at a constant amplitude d. all of the above
ANS: C 36. DSP stands for:
a. Dynamic Signal Properties c. Distorted Signal Packet b. Direct Signal Phase d. Digital Signal Processor ANS: D
37. SINAD stands for:
a. Sinusoidal Amplitude Distortion b. Signal and Noise Amplitude Distortion c. Signal-plus-Noise-to-Noise Ratio
d. Signal-plus-Noise and Distortion-to-Noise and Distortion Ratio ANS: D
38. TRF stands for:
a. Tuned Radio Frequency c. Transmitted Radio Frequency b. Tracking Radio Frequency d. Tuned Receiver Function ANS: A
COMPLETION
1. Almost all modern receivers use the _________________________ principle.
ANS: superheterodyne
2. The first radio receiver of any kind was built in the year ____________________.
ANS: 1887
3. When two tuned circuits ____________________ each other, it means that when the frequency of one is adjusted, the other changes with it.
ANS: track
4. The ____________________ effect causes the resistance of wire to increase with frequency.
ANS: skin
5. The superhet was invented in the year ____________________.
ANS: 1918
6. In a receiver, the ____________________ refers to the input filter and RF stage.
ANS: front end
7. In a superhet, the output of the ____________________ goes to the IF amplifiers.
ANS: mixer
8. In a superhet, the ____________________ frequency is the difference between the local oscillator frequency and the received signal frequency.
ANS:
intermediate IF
9. The ____________________ circuit adjusts the gain of the IF amplifiers in response to signal strength.
ANS: AGC
10. An ____________________ converter uses the same transistor for both the local oscillator and the mixer.
ANS: autodyne
11. In low-side injection, the local oscillator is ____________________ than the received signal frequency.
ANS: lower
12. ____________________ is the ability of a receiver to separate two signals that are close to each other in frequency.
ANS: Selectivity
13. ____________________ is the ability of a receiver to receive and successfully demodulate a very weak signal.
ANS: Sensitivity
14. A receiver with two different IF frequencies is called a double-____________________ receiver.
ANS: conversion
15. A multiple-conversion receiver will have better rejection of ____________________ frequencies.
ANS: image
16. A demodulator is also called a ____________________.
ANS: detector
17. An ____________________ detector uses a diode to half-wave rectify an AM signal.
ANS: envelope
18. A ____________________ detector is used for SSB signals.
ANS: product
19. A BFO produces a locally generated ____________________.
ANS: carrier
20. A DSBSC signal requires a ____________________ detection circuit.
ANS: coherent
21. FM detectors have a characteristic ____________________-shaped curve.
ANS: S
22. While still commonly found, the Foster-Seeley and ratio detectors are ____________________.
ANS: obsolescent
23. Unlike the PLL detector, the quadrature detector is sensitive to changes in ____________________ of the input signal.
ANS: amplitude
24. A dual-____________________ MOSFET is useful for AGC.
ANS: gate
25. Diode mixers are too ____________________ to be practical in most applications.
ANS: noisy
26. The IF amplifiers in an AM receiver must be Class ____________________.
ANS: A
27. A double-tuned IF transformer is usually ____________________ coupled for the response to have a flat top and steep sides.
ANS: over
28. Multiple IF stages can be ____________________-tuned to increase the bandwidth.
ANS: stagger
29. Compared to tuned circuits, ceramic and crystal IF filters do not require ____________________.
ANS: adjustment
30. Up-conversion is when the output of the mixer is a ____________________ frequency than the incoming signal.
ANS: higher
31. In a block converter, the frequency of the first local oscillator is ____________________.
ANS:
fixed constant
32. Typically, AGC reduces the gain of the ____________________ amplifiers.
ANS: IF
33. An ____________________-meter is designed to indicate signal strength in many communications receivers.
ANS: S
34. The effectiveness of FM ____________________ is measured by a receiver’s quieting sensitivity.
ANS: limiting
35. A ____________________ refers to any kind of FM or PM detector.
ANS: discriminator
SHORT ANSWER
1. Suppose the bandwidth of a tuned circuit is 10 kHz at 1 MHz. Approximately what bandwidth would you expect it to have at 4 MHz?
ANS:
20 kHz
2. Using high-side injection for a 1-MHz IF, what is the frequency of the local oscillator when the receiver is tuned to 5 MHz?
ANS:
6 MHz
3. An IF filter has a –60 dB bandwidth of 25 kHz and a –6 dB bandwidth of 20 kHz. What is the shape factor value?
ANS:
1.25
4. Suppose a receiver uses a 5-MHz IF frequency. Assuming high-side injection, what would be the image frequency if the receiver was tuned to 50 MHz?
ANS:
60 MHz
5. Suppose a SSB receiver requires an injected frequency of 1.5 MHz. What would be the acceptable frequency range of the BFO if the maximum acceptable baseband shift is 100 hertz?
ANS:
1.5 MHz ± 100 hertz
6. The transformer of a double-tuned IF amplifier has a Q of 25 for both primary and secondary. What value of kc do you need to achieve optimal coupling?
ANS:
0.06
7. What value of transformer coupling would a double-tuned 10-MHz IF amplifier with optimal coupling need to get a bandwidth of 100 kHz?
ANS:
0.01