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2016-E8D: Keying defects and overmodulation of digital signals; digital codes; spread spectrum

2016-E8D01: 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



2016-E8D02: What spread spectrum communications technique uses a high speed binary bit stream to shift the phase of an RF carrier?

Direct sequence

Frequency hopping

Binary phase-shift keying

Phase compandored spread spectrum



2016-E8D03: How does the spread spectrum technique of frequency hopping work?

The frequency of the transmitted signal is changed very rapidly according to a particular sequence also used by the receiving station

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



2016-E8D04: What is the primary effect of extremely short rise or fall time on a CW signal?

The generation of key clicks

More difficult to copy

The generation of RF harmonics

Limits data speed



2016-E8D05: 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



2016-E8D06: Which of the following indicates likely overmodulation of an AFSK signal such as PSK or MFSK?

Strong ALC action

High reflected power

Harmonics on higher bands

Rapid signal fading



2016-E8D07: What is a common cause of overmodulation of AFSK signals?

Excessive transmit audio levels

Excessive numbers of retries

Ground loops

Bit errors in the modem



2016-E8D08: What parameter might indicate that excessively high input levels are causing distortion in an AFSK signal?

Intermodulation Distortion (IMD)

Signal to noise ratio

Baud rate

Repeat Request Rate (RRR)



2016-E8D09: What is considered a good minimum IMD level for an idling PSK signal?

-30 dB

+10 dB

+15 dB

-20 dB



2016-E8D10: What are some of the differences between the Baudot digital code and ASCII?

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 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 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



2016-E8D11: What is one advantage of using ASCII code for data communications?

It is possible to transmit both upper and lower case text

It includes built in error correction features

It contains fewer information bits per character than any other code

It uses one character as a shift code to send numeric and special characters



2016-E8D12: What is the advantage of including a parity bit with an ASCII character stream?

Some types of errors can be detected

Faster transmission rate

The signal can overpower interfering signals

Foreign language characters can be sent





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