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Subelement E8
SIGNALS AND EMISSIONS
Section E8D
Keying defects and overmodulation of digital signals; digital codes; spread spectrum
Why are received spread spectrum signals resistant to interference?
  • Signals not using the spread spectrum algorithm are suppressed in the receiver
  • The high power used by a spread spectrum transmitter keeps its signal from being easily overpowered
  • The receiver is always equipped with a digital blanker
  • If interference is detected by the receiver it will signal the transmitter to change frequencies

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.

KM4ARR

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What spread spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier?
  • Frequency hopping
  • Direct sequence
  • Binary phase-shift keying
  • Phase compandored spread spectrum

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 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 re-stored 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 with out 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.

-KE0IPR

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How does the spread spectrum technique of frequency hopping work?
  • If interference is detected by the receiver it will signal the transmitter to change frequencies
  • If interference is detected by the receiver it will signal the transmitter to wait until the frequency is clear
  • A pseudo-random binary bit stream is used to shift the phase of an RF carrier very rapidly in a particular sequence
  • The frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station

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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What is the primary effect of extremely short rise or fall time on a CW signal?
  • More difficult to copy
  • The generation of RF harmonics
  • The generation of key clicks
  • Limits data speed

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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What is the most common method of reducing key clicks?
  • Increase keying waveform rise and fall times
  • Low-pass filters at the transmitter output
  • Reduce keying waveform rise and fall times
  • High-pass filters at the transmitter output

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.

-KE0IPR

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

-WA5OFZ

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Which of the following indicates likely overmodulation of an AFSK signal such as PSK or MFSK?
  • High reflected power
  • Strong ALC action
  • Harmonics on higher bands
  • Rapid signal fading

Using your sound card in digital mode power from your transmitter should require about 25% of the available power. Keeping the audio drive levels down below the level that triggers strong automatic level control (ALC) action will decrease your chances of overmodulation.

-KE0IPR (corrected --//Shri)

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What is a common cause of overmodulation of AFSK signals?
  • Excessive numbers of retries
  • Ground loops
  • Bit errors in the modem
  • Excessive transmit audio levels

AFSK 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.

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What parameter might indicate that excessively high input levels are causing distortion in an AFSK signal?
  • Signal to noise ratio
  • Baud rate
  • Repeat Request Rate (RRR)
  • Intermodulation Distortion (IMD)

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.

-KE0IPR

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What is considered a good minimum IMD level for an idling PSK signal?
  • +10 dB
  • +15 dB
  • -20 dB
  • -30 dB

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 \(-10\:\text{dB}\).

-KE0IPR

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

-K3ABE

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What are some of the differences between the Baudot digital code and ASCII?
  • Baudot uses 4 data bits per character, ASCII uses 7 or 8; Baudot uses 1 character as a letters/figures shift code, ASCII has no letters/figures code
  • Baudot uses 5 data bits per character, ASCII uses 7 or 8; Baudot uses 2 characters as letters/figures shift codes, ASCII has no letters/figures shift code
  • Baudot uses 6 data bits per character, ASCII uses 7 or 8; Baudot has no letters/figures shift code, ASCII uses 2 letters/figures shift codes
  • Baudot uses 7 data bits per character, ASCII uses 8; Baudot has no letters/figures shift code, ASCII uses 2 letters/figures shift codes

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" -KM6PNZ

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What is one advantage of using ASCII code for data communications?
  • It includes built in error correction features
  • It contains fewer information bits per character than any other code
  • It is possible to transmit both upper and lower case text
  • It uses one character as a shift code to send numeric and special characters

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

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

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

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What is the advantage of including a parity bit with an ASCII character stream?
  • Faster transmission rate
  • The signal can overpower interfering signals
  • Foreign language characters can be sent
  • Some types of errors can be detected

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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