G-8
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SUBELEMENT G8 – SIGNALS AND EMISSIONS
[3 Exam Questions – 3 Groups]
G8A – Carriers and modulation: AM; FM; single sideband; modulation envelope; digital modulation; overmodulation
Frequency modulation is the name of the process that changes the instantaneous frequency of an RF wave to convey information.
Phase modulation emission is produced by a reactance modulator connected to a transmitter RF amplifier stage. Phase modulation is the name of the process that changes the phase angle of an RF wave to convey information.
An FSK signal is generated by changing an oscillator’s frequency directly with a digital control signal.
Amplitude modulation varies the instantaneous power level of the RF signal. Available transmitter power can be used more effectively is one advantage of carrier suppression in a single sideband phone transmission versus full carrier amplitude modulation. Of the following phone emissions Single sideband, Double sideband, Phase modulation and Frequency modulation; Single sideband uses the narrowest bandwidth.
Excessive bandwidth is an effect of overmodulation. Transmit audio or microphone gain is typically used to adjust a proper ALC setting on an amateur single sideband transceiver. The term flat-topping when referring to a single sideband phone transmission is signal distortion caused by excessive drive. The waveform created by connecting the peak values of the modulated signal is the modulation envelope of an AM signal.
G8B – Frequency mixing; multiplication; bandwidths of various modes; deviation; duty cycle
Mixer receiver stage combines a 14.250 MHz input signal with a 13.795 MHz oscillator signal to produce a 455 kHz intermediate frequency (IF) signal. If a receiver mixes a 13.800 MHz VFO with a 14.255 MHz received signal to produce a 455 kHz intermediate frequency (IF) signal, image response interference will produce a 13.345 MHz signal in the receiver. Heterodyning is another term for the mixing of two RF signals.
Multiplier is the stage in a VHF FM transmitter that generates a harmonic of a lower frequency signal to reach the desired operating frequency. 2300 Hz is the approximate bandwidth of a PACTOR3 signal at maximum data rate. 16 kHz is the total bandwidth of an FM phone transmission having 5 kHz deviation and 3 kHz modulating frequency. 416.7 Hz is the frequency deviation for a 12.21 MHz reactance modulated oscillator in a 5 kHz deviation, 146.52 MHz FM phone transmitter.
Some modes have high duty cycles which could exceed the transmitter’s average power rating is why it important to know the duty cycle of the mode you are using when transmitting. Matching receiver bandwidth to the bandwidth of the operating mode results in the best signal to noise ratio. Higher symbol rates require wider bandwidth is the relationship between transmitted symbol rate and bandwidth.
G8C – Digital emission modes
JT9 and JT65 are designed to operate at extremely low signal strength on the HF
Header is the part of a data packet containing the routing and handling information.
Baudot code is a 5-bit code with additional start and stop bits. The receiving station requests the packet be retransmitted if an ARQ data mode packet containing errors.
In the PACTOR protocol, an NAK means the receiver is requesting the packet be retransmitted. When using PACTOR or WINMOR, the connection is dropped as a result from a failure to exchange information due to excessive transmission attempts.
Varicode type is the code used for sending characters in a PSK31 signal. The number bits varies when sent in a single PSK31 character. When using PSK31, upper case letters have longer varicode signals and will slow down the transmission. The number 31 represents in “PSK31” the approximate transmitted symbol rate. Forward Error Correction (FEC) allows the receiver to correct errors in received data packets by transmitting redundant information with the data. Mark and Space are the two separate frequencies of a Frequency Shift Keyed (FSK) signal.
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