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Subelement E8
SIGNALS AND EMISSIONS
Section E8B
Modulation and demodulation: modulation methods; modulation index and deviation ratio; frequency and time division multiplexing; Orthogonal Frequency Division Multiplexing
What is the term for the ratio between the frequency deviation of an RF carrier wave and the modulating frequency of its corresponding FM-phone signal?
  • FM compressibility
  • Quieting index
  • Percentage of modulation
  • Correct Answer
    Modulation index

Modulation index is used to describe the ratio of maximum frequency deviation of the RF carrier to the maximum frequency deviation of the modulating signal.

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How does the modulation index of a phase-modulated emission vary with RF carrier frequency (the modulated frequency)?
  • It increases as the RF carrier frequency increases
  • It decreases as the RF carrier frequency increases
  • It varies with the square root of the RF carrier frequency
  • Correct Answer
    It does not depend on the RF carrier frequency

The modulation index for phase modulation is simply the peak phase deviation.

The maximum deviation is +/- 90 degrees, regardless of frequency.

https://en.m.wikipedia.org/wiki/Phase_modulation

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What is the modulation index of an FM-phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency when the modulating frequency is 1000 Hz?
  • Correct Answer
    3
  • 0.3
  • 3000
  • 1000

Simply divide the deviation by the modulation frequency to arrive at the correct answer:

\begin{align} \text{modulation index} &= \frac{\text{deviation}}{\text{modulation frequency}} \\ &= \frac{3000 \text{ Hz}}{1000 \text{ Hz}} \\ &= 3 \end{align} --wileyj2956

In other words, the modulation index tells you how efficient the modulation is. An index of 1 would mean that a 1000 Hz signal would cause a deviation of 1000 Hz on either side of the carrier. In this example, it takes a bandwidth of three times the modulating frequency on each side of the carrier to carry the information, a modulation index of 3.

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What is the modulation index of an FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz when modulated with a 2 kHz modulating frequency?
  • 6000
  • Correct Answer
    3
  • 2000
  • 1/3

The FM modulation index is equal to the ratio of the frequency deviation to the modulating frequency.

\begin{align} \text{modulation index} &= \frac{\text{frequency deviation}}{\text{modulation frequency}} \\ &= \frac{6\text{ kHz}}{2 \text{ kHz}}\\ &= 3 \end{align}

See: https://www.radio-electronics.com/info/rf-technology-design/fm-frequency-modulation/modulation-index-deviation-ratio.php

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What is the deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz?
  • 60
  • 0.167
  • 0.6
  • Correct Answer
    1.67

The deviation ratio is the maximum carrier frequency deviation divided by the highest (maximum) audio modulating frequency: \[\text{deviation ratio}=\frac{\text{max. carrier frequency swing}}{\text{max. modulation frequency}}\]

The carrier frequency deviation is defined by the frequency swing relative to the carrier, in this case \(+5\) kHz or \(-5\) kHz, the maximum being \(5\) kHz. Therefore, the deviation ratio is:

\begin{align} \text{deviation ratio}&=\frac{5 \text{ kHz}}{3\text{ kHz}}\\ &=1.666...\approx1.67 \end{align}

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What is the deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz?
  • Correct Answer
    2.14
  • 0.214
  • 0.47
  • 47

\begin{align} \text{deviation} &= \frac{\text{frequency swing}}{\text{frequency}}\\ &= \frac{7.5 \text{ kHz}}{3.5 \text{ kHz}}=2.142857...\\ &\approx 2.14 \end{align}

Thus the deviation ratio is calculated by dividing the maximum deviation by the maximum modulation: \[ \frac{\text{maximum deviation}}{\text{maximum modulation frequency}}\]

The deviation ratio of an FM-phone signal having a maximum frequency swing of \(\pm\) 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14.

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Orthogonal Frequency Division Multiplexing is a technique used for which type of amateur communication?
  • Correct Answer
    High speed digital modes
  • Extremely low-power contacts
  • EME
  • OFDM signals are not allowed on amateur bands

OFDM is capable of such high speeds by effectively combining the speeds of several digital streams. The individual signals work in parallel to communicate the information. Any type of modulation can be used on the individual signal members, known as subcarriers.

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What describes Orthogonal Frequency Division Multiplexing?
  • A frequency modulation technique which uses non-harmonically related frequencies
  • A bandwidth compression technique using Fourier transforms
  • A digital mode for narrow band, slow speed transmissions
  • Correct Answer
    A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference

Orthogonal Frequency Division Multiplexing is a digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference.


Orthogonal Frequency Division Multiplexing (OFDM) is a frequency-division multiplexing (FDM) scheme used as a digital multi-carrier modulation method. A large number of closely spaced orthogonal sub-carrier signals are used to carry data on several parallel data streams or channels.

Hint: The correct answer is the one that deals with subcarriers and frequencies

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What is meant by deviation ratio?
  • The ratio of the audio modulating frequency to the center carrier frequency
  • Correct Answer
    The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency
  • The ratio of the carrier center frequency to the audio modulating frequency
  • The ratio of the highest audio modulating frequency to the average audio modulating frequency

TIP: It is the only answer that has 'deviation' in it.

In FM modulation, the two primary parameters of interest are deviation ratio and modulation index. Deviation ratio is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency.

The deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz is 1.67.

The deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14.

K4AGO

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What describes frequency division multiplexing?
  • The transmitted signal jumps from band to band at a predetermined rate
  • Correct Answer
    Two or more information streams are merged into a baseband, which then modulates the transmitter
  • The transmitted signal is divided into packets of information
  • Two or more information streams are merged into a digital combiner, which then pulse position modulates the transmitter

http://en.wikipedia.org/wiki/Frequency-division_multiplexing

In frequency division multiplexing, a communication medium is divided into non-overlapping frequency sub-bands, which can be used to independently transmit separate information streams, or to dependently transmit a single information stream in parallel.

--ki60qj

Although a technically correct answer, the option looks at the question from the reverse of the way Frequency Division Multiplexing is normally thought of - from the result, which is that the frequency band has been divided into separate streams with each stream carrying part of the overall message. Describing it as "Two or more information streams are merged into a baseband, which then modulates the transmitter" is intended to be confusing, but it is technically accurate.

At first blush, none of the answers appear correct, especially if you are already familiar with the encoding technique from data modems. Remember that this question requires reversing the thought process. Ignore the name of the method and realize that the way you get to the end result is to first split the information being sent (not stated in the question) into two or (often many) more data streams then modulating each of the multiple streams into its own frequency segment of the transmitted signal.

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What is digital time division multiplexing?
  • Two or more data streams are assigned to discrete sub-carriers on an FM transmitter
  • Correct Answer
    Two or more signals are arranged to share discrete time slots of a data transmission
  • Two or more data streams share the same channel by transmitting time of transmission as the sub-carrier
  • Two or more signals are quadrature modulated to increase bandwidth efficiency

Time division multiplexing (TDM) is a communications process that transmits two or more streaming digital signals over a common channel. In TDM, incoming signals are divided into equal fixed-length time slots. After multiplexing, these signals are transmitted over a shared medium and reassembled into their original format after de-multiplexing. Time slot selection is directly proportional to overall system efficiency.

Time division multiplexing (TDM) is also known as a digital circuit switched.

per https://www.techopedia.com/definition/9669/time-division-multiplexing-tdm

W5ZAP


Hint: The answer has time slot in it.

-KE0IPR

Hint: time division --> time slots

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