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Subelement G8
Signals and Emissions
Section G8A
Carriers and modulation: AM; FM; single sideband; modulation envelope; digital modulation; overmodulation
How is an FSK signal generated?
  • By keying an FM transmitter with a sub-audible tone
  • Correct Answer
    By changing an oscillator's frequency directly with a digital control signal
  • By using a transceiver's computer data interface protocol to change frequencies
  • By reconfiguring the CW keying input to act as a tone generator

Hint: FSK (Frequency Shift Keying) = digital control

FSK is a method of rapidly changing the carrier frequency of a transmitter between two different frequencies. These two carrier frequencies are often referred to as the Mark frequency and the Space frequency, or simply Mark and Space. FSK is commonly used for RTTY (radioteletype) as well as ASCII-based transmissions like packet. Since there are only two frequencies to generate, a digital control signal is used. The terms digital, binary, ON-OFF, and two-state are all basically equivalent. This two-state control signal can be used to shift the frequency of an oscillator back and forth between a Mark frequency and a Space frequency. After appropriate amplification, the carrier signal generated by the oscillator can leave the transmitter to energize an antenna.

Quick and dirty mnemonic: "F"requency "S"ignal "K"ontrol.

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What is the name of the process that changes the phase angle of an RF signal to convey information?
  • Phase convolution
  • Correct Answer
    Phase modulation
  • Phase transformation
  • Phase inversion

(B). Phase modulation is the name of the process that changes the phase angle of an RF wave to convey information. Phase modulation changes the signal by modifying the instantaneous phase angle of the carrier. PM is often used for data transmission methods such as PSK (phase-shift keying).

For more info see Wikipedia: Phase modulation

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What is the name of the process that changes the instantaneous frequency of an RF wave to convey information?
  • Frequency convolution
  • Frequency transformation
  • Frequency conversion
  • Correct Answer
    Frequency modulation

Hint: modulate CONVEYs & controls

(D). Frequency modulation (FM) is the process which changes the frequency of an RF wave to convey information. The instantaneous frequency of the carrier is altered to convey the information, while the amplitude remains constant. FM is used for frequency-shift keying, radar in addition to the radio broadcasting uses.

For more info see Wikipedia: Frequency modulation

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What emission is produced by a reactance modulator connected to a transmitter RF amplifier stage?
  • Multiplex modulation
  • Correct Answer
    Phase modulation
  • Amplitude modulation
  • Pulse modulation

Hint: PHASE MODULATION is produced by REACTANCE

(B). Phase modulation is the emission produced by a reactance modulator connected to an RF power amplifier. The reactance modulator changes the instantaneous phase angle of the carrier, generating phase modulated signals.

For more info see Wikipedia: Phase modulation

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What type of modulation varies the instantaneous power level of the RF signal?
  • Frequency shift keying
  • Phase modulation
  • Frequency modulation
  • Correct Answer
    Amplitude modulation

Hint: “Amplitude” is the power level of RF.

(D). Amplitude modulation (AM) is the type of modulation that varies the instantaneous power level of the RF signal. The power or signal strength of the wave is measured as the wave amplitude (may also be called the envelope).

For more info see Wikipedia: Amplitude modulation (AM)

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Which of the following is characteristic of QPSK31?
  • It is sideband sensitive
  • Its encoding provides error correction
  • Its bandwidth is approximately the same as BPSK31
  • Correct Answer
    All these choices are correct

Hint: Quick Provide Sideband = ALL

QPSK31 stands for Quadrature Phase Shift Keying, 31-baud

From http://www.podxs070.com/frequently-asked-questions/introduction-to-psk, here's an explanation of QPSK31: So what's different about QPSK31?

The QPSK31 format transmits two bits per phase state, and transmits one of four phase states each time. (If you think about it, there are four possible combinations of two bits, and each of these combinations is assigned a unique phase.) This allows twice as many bits to be sent in the same period of time (and bandwidth) as with BPSK. Martinez coupled this with a rate-1/2, constraint length 5 convolutional code to improve the power efficiency of the modulation (defined as the signal-to-noise ratio required to deliver a specified bit-error-rate, or BER).

This adds a step in the transmission process - generation of the coded bitstream - after the insertion of fill bits. It also adds a corresponding step in the receive process. After the bits are recovered from the phase transition, a Viterbi decoder removes the redundancy inserted by the code, corrects bit errors (up to a point) and recovers the original bit stream for translation of the Varicode.

One interesting note is that while the BPSK31 signal is phase-symmetric, QPSK31 is not. This means that if you are using BPSK, it doesn't matter if you are using USB or LSB - the signal is the same. If you are using QPSK, both stations must be using the same sideband, or one must use the "invert" function in their software.

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Which of the following phone emissions uses the narrowest bandwidth?
  • Correct Answer
    Single sideband
  • Double sideband
  • Phase modulation
  • Frequency modulation

(A). Single sideband is the phone emission which uses the narrowest frequency bandwidth. For each part of the signal that is suppressed, the bandwidth will be reduced. By supressing the carrier and filtering the unwanted sideband, the typical bandwidth of a single-sideband signal is reduced to only about 3 kHz.

For more info see Wikipedia: single-sideband (SSB)

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Which of the following is an effect of overmodulation?
  • Insufficient audio
  • Insufficient bandwidth
  • Frequency drift
  • Correct Answer
    Excessive bandwidth

Hint: Over-modulation = EXCESSIVE BANDWIDTH

(D). Excessive bandwidth is an effect of over-modulation. Overmodulating the signal causes increased distortion or deviation of the waveform. This increases the formation of spurious emissions with distortion outside the normal bandwidth.

For more info see Wikipedia: Overmodulation

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What type of modulation is used by the FT8 digital mode?
  • Correct Answer
    8-tone frequency shift keying
  • Vestigial sideband
  • Amplitude compressed AM
  • Direct sequence spread spectrum

Hint: FT8 means Frequency Tone 8. Bingo!

FT8 uses 8 tone FSK (frequency shift keying) to encode time-sequenced digital data transmissions.

From http://physics.princeton.edu/pulsar/K1JT/Release_Notes_1.8.0.txt

Brief Description of the FT8 Protocol -------------------------------------

WSJT-X Version 1.8.0 includes a new mode called FT8, developed by K9AN and K1JT. The mode name "FT8" stands for "Franke and Taylor, 8-FSK modulation". FT8 uses 15-second T/R sequences and provides 50% or better decoding probability down to -20 dB on an AWGN channel. An auto-sequencing facility includes an option to respond automatically to the first decoded reply to your CQ. FT8 QSOs are 4 times faster than those made with JT65 or JT9. FT8 is an excellent mode for HF DXing and for situations like multi-hop E_s on 6 meters, where deep QSB may make fast and reliable completion of QSOs desirable.

Some important characteristics of FT8:

  • T/R sequence length: 15 s
  • Message length: 75 bits + 12-bit CRC
  • FEC code: LDPC(174,87)
  • Modulation: 8-FSK, tone spacing 6.25 Hz
  • Constant-envelope waveform
  • Occupied bandwidth: 50 Hz
  • Synchronization: 7x7 Costas arrays at start, middle, and end
  • Transmission duration: 79*1920/12000 = 12.64 s
  • Decoding threshold: -20 dB; several dB lower with AP decoding
  • Multi-decoder finds and decodes all FT8 signals in passband
  • Optional auto-sequencing and auto-reply to a CQ response
  • Operational behavior similar to JT9, JT65

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What is meant by the term "flat-topping," when referring to a single sideband phone transmission?
  • Signal distortion caused by insufficient collector current
  • The transmitter's automatic level control (ALC) is properly adjusted
  • Correct Answer
    Signal distortion caused by excessive drive
  • The transmitter's carrier is properly suppressed

Flat-topping (or clipping) is where overmodulation causes a distortion of the waveform in which the amplitude appears flattened on the oscilloscope, because the maximum levels of output voltage or current are exceeded.

For more info see Wikipedia: Flat-topping or Clipping

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What is the modulation envelope of an AM signal?
  • Correct Answer
    The waveform created by connecting the peak values of the modulated signal
  • The carrier frequency that contains the signal
  • Spurious signals that envelop nearby frequencies
  • The bandwidth of the modulated signal

Hint: Think of an envelope's four corners, these would be the peak values.

In AM, the frequency is stable and only the amplitude modulates (Amplitude Modulation). When this happens, the carrier signal's peak value will change during modulation. In FM, the signal's peak value remains constant.

Think of an envelope's four corners, these would be the peak values.

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Which of the following narrow-band digital modes can receive signals with very low signal-to-noise ratios?
  • MSK144
  • Correct Answer
    FT8
  • AMTOR
  • MFSK32

Hint: FT8 is very narrow. note “narrow-band”

FT8 is the correct answer - note narrow-band

MSK144 is designed for meteor scatter, would need to worry about low signal-to-noise ratios, but is a wide band protocol designed to move data quickly, given the short windows in which meteor scatter propogation function. It generally used on VHF or higher frequencies, given the width.

AMTOR & MFSK32 are not good protocols for very low signal-to-noise ratio HF transmission.

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