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G8A

SIGNALS AND EMISSIONS

- Carriers and modulation: AM, FM, and single sideband; modulation envelope; digital modulation; overmodulation; link budgets and link margins

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G8A011 of 14

How is direct binary FSK modulation generated?

Why Frequency Shift Keying sends binary data by shifting the transmitted carrier between two frequencies, one for mark and one for space. In direct FSK the digital data stream is applied to the oscillator itself, pulling a VFO or commanding a DDS/PLL so the RF output jumps between the two frequencies. This is different from AFSK, where audio tones representing the bits are fed into an SSB or FM transmitter and the frequency shift happens indirectly in the modulator.
Watch out Keying an FM transmitter with a sub-audible tone describes an audio-based scheme (AFSK-style), not direct keying of the oscillator, and sub-audible tones are used for CTCSS repeater access rather than data.
Direct FSK = the data wiggles the oscillator. AFSK = audio tones wiggle the modulator.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A022 of 14

What is the name of the process that changes the phase angle of an RF signal to convey information?

Why Modulation means varying some property of an RF carrier to carry information. Vary the amplitude and you get AM, vary the frequency and you get FM, and vary the phase angle and you get phase modulation. Phase modulation is closely related to FM, since shifting phase over time is the same as a momentary frequency change, which is why a phase modulator driven through an audio shaping network can produce an FM signal.
Watch out Phase inversion is a real term but it just means flipping a signal 180 degrees, as an inverting amplifier does, and it carries no information by itself. Phase convolution and phase transformation are not standard radio terms.
Change amplitude = AM, change frequency = FM, change phase = PM. The word is always modulation.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A033 of 14

What is the name of the process that changes the instantaneous frequency of an RF wave to convey information?

Why Modulation means varying some property of an RF carrier in step with an information signal. When the property being varied is the instantaneous frequency of the carrier, the process is frequency modulation (FM); the amount of frequency shift follows the amplitude of the modulating audio. Its close cousin, phase modulation, varies the instantaneous phase and produces a similar result on the air.
Watch out Frequency conversion is the tempting one, but that is mixing a signal with a local oscillator to shift it to a new frequency, as in a superheterodyne receiver, and it carries no new information.
Modulation adds information, conversion just moves the signal. Frequency + information = FM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A044 of 14

What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage?

Why A reactance modulator makes a varying capacitance or inductance appear across a tuned circuit, shifting its resonance in step with the audio. When that tuned circuit is in an amplifier stage after the oscillator, the carrier frequency itself is fixed, so the varying tuned circuit shifts the signal's timing instead, producing phase modulation. Phase and frequency modulation are closely related (both are angle modulation), and PM can be converted to equivalent FM by pre-shaping the audio.
Watch out The choice naming frequency modulation, when it appears, describes what the same reactance modulator would produce if it were connected directly to the oscillator; connecting it to a later amplifier stage gives phase modulation instead. Amplitude modulation requires varying the signal's amplitude, which a reactance modulator does not do.
Reactance modulator on the oscillator = FM; on an amplifier stage after it = PM.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A055 of 14

What type of modulation varies the instantaneous power level of the RF signal?

Why Instantaneous RF power is proportional to the square of the signal's amplitude, so any scheme that changes the envelope amplitude changes the power level moment to moment. Amplitude modulation (including SSB, which is a form of AM) does exactly that: the modulating audio rides on the envelope, and power rises and falls with the voice peaks. That is why AM and SSB transmitters are rated in peak envelope power rather than a steady carrier power.
Watch out Frequency and phase modulation are tempting because they do carry information, but both are constant-envelope modes: only the timing of the wave changes, and the amplitude, and therefore the power, stays the same. There is no standard mode called power modulation.
Power follows amplitude squared. FM/PM keep the envelope flat; only AM makes the needle swing.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A066 of 14

Which of the following is characteristic of QPSK31?

Why QPSK31 uses the same 31.25 baud symbol rate and shaping as BPSK31, so it occupies roughly the same narrow bandwidth of about 31 Hz. The difference is that it uses four phase states instead of two, and the extra bits carry a convolutional code that a Viterbi decoder uses to correct errors. Because the receiver must track the direction of phase rotation, QPSK31 is sideband sensitive: pick the wrong sideband and the decode fails, unlike BPSK31 which works either way.
Watch out Each individual statement is true, so choosing only one of them, such as the bandwidth statement, leaves out the error correction and sideband sensitivity that also define the mode.
QPSK31 = BPSK31 plus two extras: error correction and sideband sensitivity, same 31 Hz width.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A077 of 14

Which of the following phone emissions uses the narrowest bandwidth?

Why Single sideband transmits only one sideband and suppresses the carrier, so its bandwidth is just the audio bandwidth used, typically about 2.4 to 3 kHz. Full-carrier AM needs both sidebands, roughly twice that. Angle modulation (FM or PM) for voice spreads energy over several kHz of deviation plus sidebands, so a typical narrowband FM voice channel runs around 10 to 16 kHz. That makes SSB the most spectrum-efficient of the phone modes listed.
Watch out Vestigial sideband is the closest contender, but it keeps one full sideband plus a leftover piece of the other (it is used in analog TV video), so it is always wider than true single sideband.
SSB about 3 kHz, AM about 6 kHz, FM about 10-16 kHz. One sideband beats one and a fraction.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A088 of 14

Which of the following is an effect of overmodulation?

Why Overmodulation drives the modulator past its linear range, so the audio is clipped and distorted. Clipping creates harmonics and intermodulation products of the audio signal, and those extra frequencies appear as additional sidebands far from the carrier. The result is splatter, interference to stations on adjacent frequencies, and occupied bandwidth much wider than the transmission needs.
Watch out Insufficient audio is the opposite condition, undermodulation, which just makes your signal weak and hard to copy; frequency drift comes from an unstable oscillator or thermal effects, not from audio level.
Too much modulation equals too much bandwidth: overdriving audio splatters onto the neighbors.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A099 of 14

What type of modulation is used by FT8?

Why FT8 is one of the WSJT-X modes; the "8" in the name refers to 8-tone frequency shift keying (8-FSK). The transmitter steps among 8 tones spaced 6.25 Hz apart, giving a signal only about 50 Hz wide, and each 15-second transmission carries a short structured message. Because it is constant-envelope FSK, it can be sent through a transmitter run at full output without distortion concerns.
Watch out Spread spectrum is tempting because it also mentions 8, but FT8 is extremely narrowband, the opposite of spreading energy over a wide bandwidth; vestigial sideband is a legacy analog TV video technique.
The 8 in FT8 = 8 tones. 8-FSK, 8 tones, 6.25 Hz apart, 15-second slots.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A1010 of 14

What is meant by the term "flat-topping," when referring to an amplitude-modulated phone signal?

Why In an AM or SSB phone transmitter the RF envelope should follow the audio waveform. If the drive or microphone/speech gain is too high, the final amplifier runs out of headroom and the peaks of the envelope are clipped off, giving the envelope a flattened top. That clipping creates harmonic and intermodulation distortion products, which show up on the air as splatter in adjacent channels and poor audio quality. The cure is to back off the mic gain or drive so that ALC action is light and the envelope stays undistorted.
Watch out The choice about ALC being properly adjusted is the opposite situation: correct ALC keeps the envelope out of clipping, and flat-topping is what happens when it is overdriven. Insufficient collector current would cause low output or other distortion, not the classic clipped-envelope pattern.
Flat top = too much drive. Squash the peaks, splatter the neighbors.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A1111 of 14

What is the modulation envelope of an AM signal?

Why In amplitude modulation the RF carrier's amplitude rises and falls in step with the modulating audio. If you trace a line through the tips of the successive RF cycles, that outline reproduces the shape of the modulating waveform, and that outline is what is called the modulation envelope. An oscilloscope display of an AM signal shows this envelope directly, which is why you can spot overmodulation (flat-topping or pinching off at the zero line) by watching it.
Watch out Calling it the bandwidth confuses the envelope with occupied spectrum: bandwidth is a frequency-domain measurement, while the envelope is a time-domain shape. The carrier itself is just the constant-amplitude RF being modulated, not the outline traced by the peaks.
Envelope = the outline you'd draw around the RF peaks on a scope, and it looks like the audio.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A1212 of 14

What is QPSK modulation?

Why QPSK stands for Quadrature Phase Shift Keying. The carrier amplitude stays constant and the information is carried entirely in the carrier's phase, which is switched among four states spaced 90 degrees apart: 0, 90, 180 and 270 degrees. Because there are four distinct states, each symbol carries 2 bits (4 = 2^2), so QPSK sends twice the data rate of plain BPSK in the same bandwidth. It shows up in ham digital modes such as PSK31's QPSK variant and in many satellite and data links.
Watch out The choices mentioning spread spectrum, harmonics generated by FFTs, or parallel-to-serial conversion are invented word salad; nothing in QPSK involves harmonics of the carrier or spreading the signal.
Quadrature = quarter turns: 0, 90, 180, 270 degrees, four phases, 2 bits per symbol.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A1313 of 14

What is a link budget?

Why A link budget is an accounting of all the gains and losses in a communication path, starting with transmitter output power, adding transmit and receive antenna gains, and subtracting feedline loss, path loss, and other system losses. The result is the signal power arriving at the receiver input, which you then compare to the receiver's sensitivity or noise floor. The amount by which the received signal exceeds the minimum usable level is the link margin.
Watch out The choice listing only antenna gains minus losses leaves out transmit power, which is the starting point of the whole calculation; the choice about transmit power minus receiver sensitivity skips the path and system losses entirely and describes something closer to a crude margin estimate.
Budget = power in, plus gains, minus losses, measured at the receiver. What's left over is your link margin.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
G8A1414 of 14

What is link margin?

Why A link budget adds up transmit power plus antenna gains minus all the losses (feed line, path loss, pointing, atmospheric) to predict how much signal actually arrives at the receiver input. Link margin is what is left over: the received power level minus the minimum signal level the receiver needs to decode the signal, expressed in dB. A positive margin means the link works and has some reserve for fading; a margin near zero means any fade drops you below threshold.
Watch out Transmit power minus receiver sensitivity ignores every gain and loss between the two ends, which is the whole point of the link budget calculation. Fade margin is essentially the same idea as reserve above threshold, not the opposite of link margin.
Margin = what you get minus what you need, in dB. Positive margin means the link closes.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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