Intentional or unintentional interference and jamming signals are rejected because they do not contain the spread-spectrum key. Only the desired signal, which has the key, will be seen at the receiver when the despreading operation is exercised.

Hint: Question and answer have Receive and Spread Spectrum in them.

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In some respects, the best way to know the answer to this is to understand why each of the wrong answers can be eliminated.

In the case of any frequency hopping, there isn't a single RF carrier.

In binary phase-shift keying, there are only two (binary) phases used at all times, and the operation can only flip between one and the other as needed, so it can't support a high-speed bit stream.

In phase compandored spread spectrum, the process of phase companding takes place. That's a compression process, where the amount of phase variation of the original signal is compressed into a smaller phase range at the transmitter, then restored at the receiver. Naturally that means a lower bit rate is used to send a higher bit rate signal, but compression is not possible for all source signals without deliberately discarding data, so either phase compandored spread spectrum is lossy to achieve a given bit rate, or the bit rate must vary depending on the source data -- which doesn't directly eliminate phase compandored spread spectrum, but it does make it less probable than direct sequence spread spectrum.

In direct sequence spread spectrum, there is a single carrier, and more than two phase positions are used so that multiple bits can be represented at one time, and no compression is used, allowing for a high-speed bit stream at the full, raw rate required by the source signal.

Memory Trick: High-Speed to get there Directly

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HINT: For "hopping" to work, they have to "hop together" (also use by)

HINT: Rabbits Hop Rapidly varying their path.

Spread spectrum techniques try to improve the signal-to-noise ratio and jamming immunity of a signal moving between transmitter and receiver, by using some kind of non-random, deterministic variation in frequency that is known to both the sender and receiver (but not to others; hence, jamming immunity). The improvement in detectability comes from the fact that other signals you don't want - i.e. atmospheric, equipment, or cosmic noise - have random frequency components, and therefore the signal you want can be more easily separated from them. ("It's easier to find something if you know what you are looking for.")

Frequency hopping spread spectrum uses tables of rapid frequency variation, changing on a known time cadence, that are known to both sides of the communications channel.

Interesting historical tidbit: the actress Hedy Lamarr was a co-developer and patent holder of the frequency-hopping spread spectrum technique, invented during World War II as a means to improve the jamming resistance of radio controlled torpedoes! Read more at https://en.wikipedia.org/wiki/Hedy_Lamarr .

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A short rise time on a CW signal means that basically a square wave is generated when the key is depressed (rise time) or released (fall time). A square wave is "rich" in harmonics. When the harmonics are heard in the audio range, it manifests itself in what sounds like clicks. Rather, a longer rise and fall time is "gentler" to the ear and there are no key clicks or at least they are so minor they are not heard.

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Hearing the clicks indicates there is some reverberation (band splatter) occurring when the key is let off and when the key is pressed.

The tones are too abrupt with the stop and start of the tone, hence the clicks.

Increasing the keying waveform rise and fall times reduces the reverberation.

"Increasing" means less abrupt or longer rise and fall times unlike a square wave.

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A true parity bit is a Single Error Detecting Code. It can detect when one bit has been changed, but conveys nothing about which bit. Parity is also a very simple error detecting code that provides rather poor characteristics at dealing with double errors and beyond.

Simple parity rules have to be agreed-upon by the sender and receiver. Typically, parity is either ignored or set to either an "ODD" or "EVEN" state. The state of the parity chosen for the communication tells the sender the format for how the parity bit will be computed and appended.

If ODD parity is selected, the transmitter will append a parity bit set so that the quantity of "Ones" in the transmission is an odd number.

If EVEN parity is selected, the transmitter will append a parity bit set so that the quantity of "Ones" in the transmission is an even number.

The receiver then independently computes parity for the received value and compares it to what it expects.

Example: ODD PARITY

7-byte value to be transmitted: 100100

This contains an EVEN number of 1's (there are two).

Sender appends a "1" as the parity bit, forcing the count to three (ODD number).

Transmitted value becomes: 1001001 (the right-most bit is the parity bit).

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AFSK (Audio Frequency-Shift Keying) produces audio that is then sent to a FM transmitter. A FM transmitter modulates its carrier frequency with this audio.

If the audio levels are too high, it may deviate more than the allowed/specified amount from its carrier frequency. This is called overmodulation.

The effect is to create "splatter" across the band which is visible to (annoyed) receivers on a waterfall plot and interferes with their signals, so try to avoid overmodulation as much as possible!

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When a signal containing energy on multiple frequencies is subjected to a nonlinear process, sum and difference frequencies are created. If this is undesired, it is called intermodulation distortion.

If excessive signal levels are presented to equipment, clipping can occur. That's when the top of a waveform is cut off / flattened. Clipping is a nonlinear process that both creates harmonics and causes intermodulation.

HINT: Only one answer has the word "Distortion" in it.

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During a phase shift keying (PSK) QSO an Inter-Modulation Distortion (IMD) report is often exchanged as a value to denote the received signal. A very good IMD report for an idling signal is around \(-30\:\text{dB}\), a poor report around \(-20\:\text{dB}\) with the worst possible at \(-10\:\text{dB}\).

This value means the spurious emissions from the PSK transmitter are 1000 times weaker than the main PSK signal. Lower numbers indicate a cleaner signal.

Memory Aid: One of these things is not like the others. The correct choice is the only answer with a negative dB value.

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Baudot uses five data bits per character, ASCII uses seven or eight; Baudot uses two characters as shift codes, ASCII has no shift code.

More information can be found below:

ASCII

Baudot

Hint: Baudot is a five-bit code. Only one answer has this.

One Word Key "5".

\(2^5 = 32\) is the smallest size to represent 26 letters.

Silly Hint: if Baudot is a French word with the T being silent; only 5 letters would be enunciated. 5 letters, 5-bits.

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ASCII, or "American Standard Code for Information Interchange" uses numbers in place of all characters, both uppercase and lowercase.

65-90 are A-Z, 97-122 are a-z

ASCII is an 8-bit code providing a potential for 256 possible characters. As such, it does not require a shift code.

In contrast, BAUDOT code is a 5-bit code only allowing 32 possible combinations. BAUDOT therefore can only support upper case letters.

To see more info visit: http://en.wikipedia.org/wiki/ASCII http://en.wikipedia.org/wiki/Baudot_code

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