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E8B

SIGNALS AND EMISSIONS

Modulation and demodulation: modulation methods; modulation index and deviation ratio; frequency- and time-division multiplexing; orthogonal frequency-division multiplexing (OFDM)

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E8B011 of 11

What is the modulation index of an FM signal?

Why In FM the modulation index is defined as the peak frequency deviation divided by the frequency of the modulating tone, m = Δf / fm. For example, 5 kHz of deviation produced by a 1 kHz tone gives an index of 5. Because it depends on the instantaneous modulating frequency, the index changes as the audio pitch changes, even if deviation stays the same. The related quantity called deviation ratio uses the maximum deviation over the maximum modulating frequency, and it is a fixed number for a given system.
Watch out The choice dividing transmitted bandwidth by modulating frequency looks similar, but bandwidth is a result of modulation (Carson's rule), not part of the definition; amplitude of the modulating signal only matters because it sets the deviation, not directly.
Modulation index = deviation over tone frequency. Deviation on top, always.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B022 of 11

How does the modulation index of a phase-modulated emission vary with RF carrier frequency?

Why Modulation index for angle modulation is the peak carrier phase deviation in radians, which equals peak frequency deviation divided by the modulating frequency. Both of those quantities are set by the modulating audio (its amplitude and its frequency), so the RF carrier frequency never enters the equation. In a transmitter that uses frequency multiplier stages, the deviation is multiplied by the same factor as the carrier, so the ratio, and hence the index, stays the same.
Watch out The idea that the index drops as carrier frequency rises comes from thinking a given deviation is a smaller percentage of a higher carrier, but modulation index compares deviation to the modulating frequency, not to the carrier.
Index = deviation / modulating frequency. The carrier frequency is not in the formula, so it cannot matter.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B033 of 11

What is the modulation index of an FM phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency if the highest modulating frequency is 1000 Hz?

Why Modulation index for FM is simply the peak deviation divided by the modulating frequency at that instant. Here 3000 Hz of deviation divided by a 1000 Hz modulating tone gives an index of 3. Note that the index changes with the modulating frequency: the same deviation with a higher audio tone yields a smaller index. (Deviation ratio is the related term that uses maximum deviation divided by the highest modulating frequency permitted in the system.)
Watch out The value of 0.3 comes from dividing the wrong way, modulating frequency over deviation, and 6 comes from using peak-to-peak deviation (6000 Hz) instead of deviation to one side of the carrier.
Index = deviation / audio frequency. Divide the big number by the small one: 3000/1000 = 3.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B044 of 11

What is the modulation index of an FM phone signal having a maximum carrier deviation of plus or minus 6 kHz if the highest modulating frequency is 2 kHz?

Why For FM, the modulation index is the peak carrier deviation divided by the frequency of the modulating tone at that instant. Here 6 kHz of deviation divided by a 2 kHz modulating frequency gives an index of 3. Note this is a ratio of two frequencies, so it has no units, and it changes as the modulating frequency changes.
Watch out The value 0.3 comes from dividing in the wrong order (2 divided by 6, then misplacing the decimal), and 6 is just the deviation in kHz with no division done at all.
Index = deviation / modulating frequency. Big deviation over small tone = big index. 6/2 = 3.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B055 of 11

What is the deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 5 kHz if the highest modulation frequency is 3 kHz?

Why Deviation ratio is defined as the maximum carrier frequency swing divided by the highest modulating (audio) frequency. Here that is 5 kHz divided by 3 kHz, which gives about 1.67. Note this is a fixed design figure for the system, using the peak allowed deviation and the top audio frequency, unlike modulation index which changes with the instantaneous audio frequency.
Watch out The 0.6 value comes from dividing the two numbers the wrong way (3 divided by 5); deviation always goes on top.
Deviation ratio = swing over highest audio. Deviation first, divided by audio: 5/3 = 1.67.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B066 of 11

What is the deviation ratio of an FM phone signal having a maximum frequency swing of plus or minus 7.5 kHz if the highest modulation frequency is 3.5 kHz?

Why Deviation ratio is defined as the maximum carrier frequency swing divided by the highest modulating frequency. Here that is 7.5 kHz divided by 3.5 kHz, which gives about 2.14. Note the difference from modulation index, which uses the actual modulating frequency in use at the moment; deviation ratio uses the maximum values the system allows.
Watch out The value near 0.47 comes from dividing the numbers the wrong way (3.5 / 7.5), and the choices differing only by a factor of ten are decimal-point traps.
Deviation ratio = swing over audio: big number on top. 7.5/3.5 = 2.14.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B077 of 11

Orthogonal frequency-division multiplexing (OFDM) is a technique used for which types of amateur communication?

Why OFDM splits a data stream across many closely spaced subcarriers whose spacing is chosen so each one's spectrum nulls fall on its neighbors' centers, meaning they do not interfere despite overlapping. Each subcarrier carries a slow, low-rate portion of the data, which makes the signal very resistant to multipath and selective fading. In amateur service it shows up in high-throughput digital modes and is the basis of Wi-Fi, DSL and digital broadcast systems.
Watch out OFDM is perfectly legal on the amateur bands as long as the emission fits the band's rules, so the choice claiming it is prohibited is simply wrong. It is also not tied to weak-signal work like EME or QRP, where narrow single-carrier modes dominate.
OFDM = many parallel subcarriers = data. Think Wi-Fi and DSL: it is a digital mode technique.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B088 of 11

What describes orthogonal frequency-division multiplexing (OFDM)?

Why OFDM splits a high-rate data stream across many closely spaced subcarriers, each carrying a slow, narrow-band piece of the data. The subcarrier spacing is chosen so each one's spectral nulls fall on its neighbors' center frequencies, which is what 'orthogonal' means, so they do not interfere with each other. Because each symbol lasts much longer at the reduced per-subcarrier rate, multipath echoes fall within a guard interval and intersymbol interference is avoided. This is the modulation behind Wi-Fi, DVB-T, LTE and ham modes like FreeDV 2020 and some HF data systems.
Watch out The Fourier transform answer is tempting because OFDM is implemented with the FFT/IFFT, but OFDM is a modulation scheme, not a bandwidth compression technique, and it generally occupies a wide channel. The narrow-band, slow-speed description fits modes like PSK31 or WSPR, the opposite of OFDM's wide, high-throughput character.
OFDM = Orthogonal, so subcarriers are spaced to avoid interference between symbols. Many slow carriers beat one fast one.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B099 of 11

What is deviation ratio?

Why In FM, the deviation ratio is the maximum permitted carrier frequency deviation divided by the highest audio modulating frequency the system will pass. For a typical narrowband amateur FM repeater channel, 5 kHz deviation over a 3 kHz maximum audio frequency gives a deviation ratio of about 1.67. It is essentially the modulation index evaluated at the worst case: peak deviation and the top of the audio passband, so it describes the whole system rather than one instant.
Watch out The choices involving the carrier center frequency are a trap because the carrier frequency has nothing to do with it. What matters is how far the carrier swings, not where it sits, which is why an FM signal's bandwidth is the same on 29 MHz as on 440 MHz.
Deviation ratio = max deviation over max audio frequency. Both numbers are maximums, and the carrier frequency never appears.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B1010 of 11

What is frequency division multiplexing (FDM)?

Why Frequency division multiplexing splits the available bandwidth into several non-overlapping frequency slots, and each slot carries its own independent information stream simultaneously. All the channels are on the air at the same time, separated in frequency rather than in time. This is the basic idea behind broadcast band allocation, subcarriers in an FM stereo signal, and the many carriers used in OFDM.
Watch out The choice about merging streams into a digital combiner that pulse position modulates the transmitter describes time division multiplexing, where streams share one frequency by taking turns in time; the band-hopping choice describes frequency hopping spread spectrum.
FDM = separated in Frequency, all at once. TDM = separated in Time, taking turns.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8B1111 of 11

What is digital time division multiplexing?

Why Time division multiplexing lets several independent signals share one channel by taking turns: each signal gets its own repeating time slot, and the receiver reassembles the streams by keeping sync with that slot timing. Because only one stream occupies the channel at any instant, no extra frequency space is needed, just fast enough sampling and switching. D-STAR voice plus data and TDMA cellular systems work this way.
Watch out Assigning data streams to discrete sub-carriers describes frequency division multiplexing (as in OFDM), where the streams are separated in frequency rather than in time; quadrature modulation is a modulation scheme, not a multiplexing method.
Time division = take turns in time slots; frequency division = each stream gets its own carrier.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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