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E8D

SIGNALS AND EMISSIONS

Keying defects and overmodulation of digital signals; digital codes; spread spectrum

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E8D011 of 11

Why are received spread spectrum signals resistant to interference?

Why A spread spectrum receiver multiplies the incoming signal by the same spreading code (frequency hopping sequence or direct sequence chip pattern) that the transmitter used. That correlation process collapses the wanted signal back to its narrow bandwidth while any signal that does not follow the same code gets smeared across the wide bandwidth, dropping its power density far below the recovered signal. The result is strong rejection of interference without needing high power or feedback to the transmitter.
Watch out High transmit power is not the mechanism, and in fact Part 97 limits amateur spread spectrum power to 10 watts; error correction and adaptive frequency changing are separate techniques that some systems add, but they are not what makes spread spectrum inherently interference resistant.
Only the matching code gets de-spread; everything else stays spread out and fades into the noise.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D022 of 11

What spread spectrum communications technique uses a high-speed binary bit stream to shift the phase of an RF carrier?

Why Direct sequence spread spectrum multiplies the data by a much faster pseudorandom binary code (the chipping sequence), and each chip flips the phase of the RF carrier. Because the chip rate is far higher than the data rate, the spectrum is spread over a wide bandwidth; the receiver despreads it by correlating with the identical code. The key phrase in the pool is 'high-speed binary bit stream' shifting carrier phase.
Watch out Frequency hopping is the other common spread spectrum method, but it shifts the carrier frequency in a pseudorandom pattern rather than shifting its phase. Plain binary phase-shift keying does shift phase, but only at the data rate, so it is ordinary narrowband modulation and does not spread the signal.
Two spread spectrum flavors: hopping moves the frequency, direct sequence moves the phase.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D033 of 11

Which describes spread spectrum frequency hopping?

Why In frequency hopping spread spectrum (FHSS), the transmitter jumps its carrier from channel to channel many times per second following a pseudorandom hop pattern, and the receiver follows the same pattern in step. Because the sequence is pseudorandom, both ends can generate it from a shared key, so the signal looks like noise to anyone not synchronized. Spreading the energy over many channels also means narrowband interference only corrupts a few hops, which coding can recover.
Watch out The description of a binary bit stream rapidly shifting the carrier's phase is direct sequence spread spectrum (DSSS), the other main spread spectrum technique in the pool. A receiver telling the transmitter to change frequency is just adaptive channel selection, not spread spectrum, and clipping to make harmonics is a keying defect, not error correction.
Hopping = frequency changes; Direct sequence = phase changes. Both driven by a pseudorandom code.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D044 of 11

What is the primary effect of extremely short rise or fall time on a CW signal?

Why A CW signal's occupied bandwidth depends on how fast the envelope switches on and off. Fourier analysis says an abrupt, near-square keying envelope contains high-frequency components, which appear as sidebands spreading well beyond the few hundred hertz a properly shaped CW signal needs. Those broad sidebands are heard on adjacent frequencies as sharp clicks each time the key closes or opens. Good transmitter design shapes the envelope with rise and fall times of roughly 5 ms to keep the signal clean.
Watch out Harmonics are integer multiples of the operating frequency produced by nonlinearity in the amplifier chain, not by envelope shaping; key clicks land just a few hundred hertz to a few kilohertz either side of your own signal.
Fast edges = wide skirts. Sharp keying makes clicks near your signal, not harmonics up at 2f.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D055 of 11

What is the most common method of reducing key clicks?

Why Key clicks are the wideband sidebands created when the CW envelope switches on and off too abruptly. The occupied bandwidth of a keyed signal depends on how fast the envelope changes, so shaping the envelope with a slower, smoother rise and fall (typically on the order of 5 milliseconds) confines the sidebands close to the carrier. This is done in the keying/shaping circuit, not after the final amplifier.
Watch out Low-pass or high-pass filters at the transmitter output only remove energy far from the carrier, such as harmonics; key-click sidebands sit within a few hundred hertz to a few kilohertz of the carrier and pass right through such filters. Shortening rise and fall times makes the clicks worse.
Softer keying edges, cleaner signal: slow the rise/fall (about 5 ms) to kill clicks. Filters can't fix what's next to the carrier.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D066 of 11

What is the advantage of including parity bits in ASCII characters?

Why A parity bit is one extra bit appended to each character, set so the total number of 1 bits is always even (or always odd, depending on the convention). The receiver counts the bits and flags the character if the count no longer matches the expected parity, which reveals a single bit flipped in transit. That is error detection only, not correction, and it misses errors that flip an even number of bits.
Watch out Adding a parity bit actually slows throughput slightly because you send 8 bits instead of 7 for the same character, and it does nothing to the signal-to-noise ratio or the size of the character set.
Parity = one spare bit that says 'count my ones.' It detects errors, it does not fix them or add speed.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D077 of 11

What is a common cause of overmodulation of AFSK signals?

Why AFSK (audio frequency shift keying) works by feeding audio tones from a soundcard or modem into the microphone or line input of an SSB transmitter, so the audio level directly sets the drive level. Too much audio pushes the transmitter past its linear range, causing ALC action, flat-topping, splatter, and strong intermodulation products either side of the signal. The fix is to reduce the soundcard or sound device output until the ALC barely moves and the RF output is well below maximum.
Watch out Excessive frequency deviation describes overmodulating an FM transmitter, a different mechanism; with AFSK on SSB there is no deviation control, only audio gain. Retries and modem bit errors affect throughput and data integrity, not the transmitted waveform's linearity.
AFSK rides in on audio: too much audio in, splatter out. Turn the soundcard down until ALC barely twitches.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D088 of 11

What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?

Why When audio tones are fed too hot into an SSB transmitter, the transmitter is driven out of its linear range and the tones mix with each other, producing extra unwanted sum and difference products. That is intermodulation distortion, and software such as PSK31 monitors reports it in dB below the desired signal (roughly -30 dB or better is considered clean). Backing off the audio drive or ALC action reduces IMD and keeps the signal narrow.
Watch out Signal-to-noise ratio measures how far a signal stands above the noise floor at the receiver, not how badly the transmitter is being overdriven; it can look fine while the signal is badly splattering.
Too much audio equals too much IMD. Watch the IMD number in dB, aim for -30 dB or better.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D099 of 11

What is considered an acceptable maximum IMD level for an idling PSK signal?

Why PSK31 software (and the common IMD meters built into digital mode programs) measures intermodulation products of an idling signal relative to the main tone pair, and the number is negative because the IMD products must sit below the desired signal. A level of -30 dB or lower means the distortion products are at least 30 dB down, which is the generally accepted threshold for a clean PSK signal. If you drive the transmitter too hard, the ALC acts and the IMD figure climbs toward -20 dB or worse, splattering into adjacent frequencies.
Watch out The positive values are physically nonsensical here, since they would mean the distortion products are stronger than the signal itself; they are just there to catch someone who ignores the sign.
IMD is measured downward: only the negative number can be right, and -30 dB is the clean-signal target.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D1010 of 11

What are some of the differences between the Baudot digital code and ASCII?

Why Baudot is a 5-bit code, so it has only 2^5 = 32 possible combinations, not enough for letters, numbers and punctuation at once. To get around that, it reserves two of those combinations as shift codes, LTRS and FIGS, which switch the receiver between the letters case and the figures/punctuation case. ASCII uses 7 bits (128 combinations, or 8 bits in extended versions), which is plenty to give every letter, digit and symbol its own unique code, so no shift characters are needed.
Watch out The choices that put the letters/figures shift in ASCII have it backwards, and the 4-, 6- and 7-bit counts for Baudot are simply wrong; classic RTTY Baudot is always 5 bits.
Baudot = 5 bits + 2 shifts (LTRS/FIGS); ASCII = 7 bits, no shifting needed.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
E8D1111 of 11

What is one advantage of using ASCII code for data communications?

Why ASCII is a 7-bit code giving 128 distinct characters, enough room for the full uppercase and lowercase alphabets plus digits, punctuation and control codes. That is its main advantage over the older 5-bit Baudot/ITA2 code used in classic RTTY, which only has 32 combinations and therefore sends uppercase letters only.
Watch out The shift-code answer describes Baudot, which needs LTRS and FIGS shift characters to reach numbers and symbols; and ASCII is a larger code with no built-in error correction, so both of those choices apply to something else.
ASCII = 7 bits, 128 characters, so it has room for lowercase. Baudot = 5 bits and needs shift codes.
HamSandwich explanation, first draft. The question and answers are the NCVEC text.
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