Deck 5: Frequency Modulation: Transmission

Question 1

The standard time constant used for pre-emphasis in a non-Dolby system is:

A) 75 us
B) 50 us
C) 25 us
D) 175 us

Answer 1

A

Question 2

An FM signal has a center frequency of 154.5 MHz but is swinging between 154.45 MHz and 154.55 MHz at a rate of 500 times per second. Its index of modulation is:

A) 100,000.
B) 500
C) 50,000
D) 100.

Answer 2

D

Question 3

The first working FM system was developed by:

A) Armstrong.
B) Marconi.
C) Hartley.
D) Sarnoff.

Answer 3

A

Question 4

In an FM modulator, the intelligence frequency determines the:

A) amount of carrier frequency deviation
B) phase angle of the carrier frequency component
C) rate of carrier frequency deviation
D) power level of the FM signal

Question 5

An FM signal has a center frequency of 154.5 MHz, but is swinging between 154.45 MHz and 154.55 MHz at a rate of 500 times per second. Its input intelligence frequency is:

A) 100 kHz
B) 500 Hz.
C) 154.5 MHz
D) 50 kHz.

Question 6

An FM signal has an intelligence frequency of 5 kHz and a maximum deviation of 30 kHz. Its index of modulation is:

A) 6
B) 35
C) 60
D) 150

Question 7

The two types of angle modulation are:

A) amplitude and frequency modulation
B) phase and amplitude modulation
C) phase and frequency modulation
D) pulse and frequency modulation

Question 8

Carson's rule is:

A) a quick approximation method to find the index of modulation of a PM signal
B) a quick approximation method to find the bandwidth of a PM signal.
C) a quick approximation method to find the bandwidth of an FM signal
D) a quick approximation method to find the index of modulation of an FM signal

Question 9

Noise is usually clipped by:

A) limiter circuits
B) phase detectors
C) ARC circuits
D) amplifiers.

Question 10

In an FM modulator, the amplitude of the intelligence signal determines the:

A) rate of the carrier frequency deviation
B) amount of carrier frequency deviation
C) power level of the FM signal
D) phase angle of the carrier frequency component

Question 11

An FM signal has a center frequency of 154.5 MHz but is swinging between 154.45 MHz and 154.55 MHz at a frequency of 500 times per second. Its input carrier frequency is:

A) 154.5 MHz
B) 100 kHz
C) 500 Hz
D) 50 kHz.

Question 12

Standard FM broadcast stations use a maximum intelligence frequency of:

A) 5 kHz
B) 150 kHz
C) 15 kHz
D) 75 kHz

Question 13

If the S/N of the input signal is 4 and the intelligence signal is 10 kHz, determine the deviation

A) 200 kHz
B) 100 kHz
C) 145 kHz
D) 160 kHz

Question 14

The amount of frequency deviation is proportional to the amplitude of the intelligence signal in:

A) a PM signal
B) an FM signal
C) both FM and PM signals
D) neither FM nor PM signals

Question 15

Which of the following is indirect FM generation?

A) Crosby modulator
B) varactor diode
C) Armstrong modulator
D) reactance modulator

Question 16

Standard FM broadcast stations use a maximum frequency deviation of:

A) 75 kHz
B) 5 kHz
C) 200 kHz
D) 150 kHz

Question 17

To solve for the frequency components of an FM signal, what high-level mathematical tool is used?

A) Fourier transforms
B) LaPlace transforms
C) Bessel functions
D) Calculus

Question 18

The modulation index is:

A) top envelope divided by center frequency
B) VCO voltage divided by center frequency
C) deviation divided by intelligence frequency
D) all of the above

Question 19

Viewing an FM signal on a spectrum analyzer, if the carrier frequency component is zero and there are four or five sidebands on either side of the carrier frequency, the index of modulation is:

A) 0
B) 5.5
C) 2.2
D) 8.65

Question 20

Phase modulation is an indirect form of:

A) frequency modulation
B) angle modulation
C) amplitude modulation
D) pulse modulation

Question 21

The advantage of indirect FM modulation over direct FM modulation is:

A) in their ability to work at higher intelligence frequencies
B) in their increased frequency deviation
C) in their ability to work with small signal amplitudes
D) in their improved frequency stability

Question 22

In this modulator, FM is created by having the intelligence signal cause a change in the transconductance of a JFET, which causes a change in the JFET amplifier's input capacitance, which shifts the resonant frequency of a tank circuit:

A) PLL modulator
B) VCO modulator
C) reactance modulator
D) varactor diode modulator

Question 23

Pre-emphasis is:

A) increasing the relative strength of low-frequency components before being fed into the modulator of an FM transmitter.
B) decreasing the relative strength of high-frequency components at the output signal of an FM detector in an FM receiver
C) decreasing the relative strength of low-frequency components of the output signal of an FM detector in an FM receiver.
D) increasing the relative strength of high-frequency components before being fed into the modulator of an FM transmitter.

Question 24

Narrowband FM stations:

A) are found in police, aircraft, taxicabs, weather service, and industrial applications
B) use intelligence frequencies ranging from 100 Hz to 3 kHz
C) use a maximum deviation of 10 kHz
D) all of the above

Question 25

Which of the following is not an example of a direct FM modulator?

A) reactance modulator
B) 566 VCO modulator
C) varactor diode modulator
D) Armstrong modulator

Question 26

Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a maximum deviation of 75 kHz and a maximum intelligence frequency of 15 kHz if the input signal-to-noise ratio is 2:1.

A) 5:1
B) 14.7:1
C) 10:1
D) 3:1

Question 27

The most important advantage of FM over AM is:

A) its conservation of energy
B) its frequency stability
C) its limited bandwidth
D) its superior noise characteristics

Question 28

The main purpose of pre-emphasis and de-emphasis networks in FM communication systems is to:

A) allow for a reduction in bandwidth of the FM communication channel
B) provide a near constant noise reduction capability between low and high frequency intelligence signals
C) filter out noise produced by the FM transmitter's modulator stage
D) allow for stereo broadcasts to be received by a monaural receiver

Question 29

The worst case signal-to-noise ratio at the output of an FM detector occurs when:

A) the desired signal is 90 degrees out of phase with the intelligence signal.
B) the desired signal is 90 degrees out of phase with the resultant signal of adding the signal to the noise.
C) the noise signal is 90 degrees out of phase with the resultant signal of adding the signal to the noise.
D) the desired signal is 90 degrees out of phase with the noise signal.

Question 30

The main disadvantage of direct FM modulators is:

A) they can only work at low radio frequencies
B) they have very limited frequency stability
C) they have insufficient frequency deviation
D) they work reliably only with low-level intelligence signals

Question 31

A stage that produces a specific dc output voltage level based on the exact frequency of the input signal is known as:

A) an indirect FM modulator
B) a frequency multiplier
C) a direct FM modulator
D) a discriminator

Question 32

A de-emphasis network has:

A) an upper cutoff frequency of 2.120 kHz.
B) a high-frequency roll-off rate of -20 db per decade.
C) a time constant of 75 microseconds.
D) all of the above

Question 33

The modulation index that occurs when the deviation and intelligence frequencies are maximum is called:

A) the maximum modulation index
B) the maximum bandwidth
C) the deviation ratio
D) the maximum side frequency component

Question 34

De-emphasis is:

A) increasing the relative strength of high frequency components before being fed into the modulator of an FM transmitter.
B) increasing the relative strength of low-frequency components before being fed into the modulator of an FM transmitter.
C) decreasing the relative strength of low-frequency components of the output signal of an FM detector of an FM receiver.
D) decreasing the relative strength of high-frequency components of the output signal of an FM detector in an FM receiver.

Question 35

FM systems utilizing a direct FM modulator with an automatic frequency control (AFC) are known as:

A) PLL systems
B) Crosby systems
C) Armstrong systems
D) Hartley systems

Question 36

FM systems that utilize an indirect FM modulator consisting of a phase modulator stage are known as:

A) Crosby systems
B) Hartley systems
C) Armstrong systems
D) PLL systems

Question 37

Which best describes how a varactor diode modulator creates FM?

A) The intelligence signal causes the diode to create phase shift which indirectly creates FM
B) The intelligence signal alters the capacitance of the diode to shift the resonant frequency of a tank circuit.
C) The intelligence signal creates mixing action in the nonlinear varactor diode to create an FM signal.
D) The intelligence signal alters the amount of forward bias of the varactor diode to create an FM signal.

Question 38

Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a maximum deviation of 75 kHz and a maximum intelligence frequency of 10 kHz if the input signal-to-noise ratio is 2:1

A) 30:1
B) 15:1
C) 2:1
D) 10:1

Question 39

The noise characteristics of an FM communication system are mainly due to:

A) its limiter and detector stages
B) its narrow bandwidth characteristics
C) its low level of modulation index
D) its modulator stage

Question 40

Determine the worst case output signal-to-noise ratio for a broadcast FM receiver with a maximum deviation of 75 kHz and a maximum intelligence frequency of 15 kHz if the input signal-to-noise ratio is 3:1.

A) 3:1
B) 10:1
C) 5:1
D) 14.7:1

Question 41

In an FM stereo transmitter, the L+R audio signal applied to the FM modulator stage

A) extends only from 23 kHz to 38 kHz
B) extends from 23 kHz to 53 kHz
C) extends from 0 to 15 kHz
D) extends only from 38 kHz to 53 kHz

Question 42

The deviation ratio for broadcast FM radio is

A) 5
B) 1.67
C) 6.71
D) 2.5
E) )5

Question 43

The deviation return for NTSC broadcast television is

A) )5
B) 2.5
C) 6.71
D) 1.67
E) 5

Question 44

A phase modulator can be made to create FM indirectly by:

A) ensuring that the audio signal has an amplitude that varies inversely proportional to its frequency.
B) integrating the intelligence signal prior to phase modulation
C) feeding the intelligence signal through a frequency correcting network
D) all of the above

Question 45

Why are stereo FM broadcasts more prone to noise than are monophonic FM broadcasts?

A) Stereo has two separate channels that can become covered by noise rather than the single channel of the monophonic broadcast.
B) The L-R signal is weaker than the L+R signal.
C) The L-R signal is at higher modulating frequencies than the L+R signal.
D) all of the above

Question 46

In a stereo FM broadcast transmitter, when the 38 kHz pilot carrier is mixed with the L-R signal, what is produced?

A) an L-R double sideband suppressed carrier signal
B) an FM signal containing L+R and L-R sidebands
C) a 19 kHz pilot carrier
D) a 38 kHz AM signal

Question 47

An FM stereo broadcast is an example of

A) frequency-division multiplexing
B) amplitude-division multiplexing
C) time-division multiplexing
D) phase-division multiplexing

Question 48

Multiplex transmission is:

A) using two transmitters to transmit two separate channels of information
B) the technique of separating the L and R signals into L+R and L-R in a stereo FM transmitter.
C) the simultaneous transmission of two or more signals on one carrier
D) using two carriers to transmit two separate channels of information

Question 49

The disadvantage of indirect FM modulators is

A) they can only work at low intelligence frequencies
B) they have insufficient frequency deviation
C) they only work with large signal amplitudes
D) they have very limited frequency stability

Question 50

Why can low-level modulated FM signals be amplified by class C power amplifier stages without distorting unlike AM or SSB signals?

A) These FM signals have only one upper and lower sideband frequency components.
B) These FM signals do not vary in amplitude.
C) The FM signals do not have a carrier frequency component.
D) These FM signals have larger bandwidths than do AM or SSB signals.

Question 51

An indirect FM wideband transmitter can have its output signal be at the proper carrier frequency and at the same time have the required deviation by:

A) using a balanced modulator and a 90 degree phase shifter
B) using a reactance modulator
C) using combinations of mixers and multiplier stages
D) using a frequency correcting network

Question 52

In FM stereo broadcasts, why are the left and right audio signals first converted to L+R and L-R signals before being frequency modulated?

A) This permits higher intelligence frequencies to be used
B) This permits compatibility between monaural and stereo systems
C) This is necessary to be able to be multiplexed properly
D) This produces higher signal-to-noise ratios