General · correct answers only, in question-pool order.
Direct binary FSK changes the oscillator frequency directly according to a digital control signal, switching between two discrete frequencies to represent binary data.
Phase modulation conveys information by changing the phase angle of the RF carrier.
Frequency modulation conveys information by varying the instantaneous frequency of the RF wave.
A reactance modulator connected to an RF amplifier stage produces phase modulation because the varying reactance shifts the signal's phase.
Amplitude modulation varies the instantaneous amplitude, and therefore the instantaneous power level, of the RF signal.
The official G8A06 source identifies sideband sensitivity as one characteristic of QPSK31.
The official G8A06 source identifies built-in error correction as one characteristic of QPSK31.
The official G8A06 source states that QPSK31 occupies approximately the same bandwidth as BPSK31.
Single sideband uses less occupied bandwidth than the other listed analog phone emissions because it suppresses the carrier and one sideband.
Overmodulation drives the transmitter beyond the intended modulation range and produces unwanted sidebands, increasing occupied bandwidth.
FT8 uses eight-tone frequency-shift keying, with information represented by transitions among eight closely spaced tones.
Flat-topping occurs when excessive drive or speech level causes the tops of the modulation envelope to clip or flatten, producing distortion.
The modulation envelope is the curve traced by connecting the peak values of the modulated RF waveform.
QPSK sends two bits per symbol by selecting among four carrier phases: 0, 90, 180, and 270 degrees.
A link budget accounts for transmitter power, antenna gains, and system losses to estimate the signal power that arrives at the receiver.
Link margin is the amount by which the received signal exceeds the minimum level required by the receiver.
In a superheterodyne receiver, the local oscillator is varied so different incoming RF signals are converted to the same intermediate frequency.
An image response occurs when an undesired signal, separated from the desired signal by twice the IF, also mixes to the same intermediate frequency.
Heterodyning is the mixing of two RF signals to produce new sum and difference frequencies.
A frequency multiplier generates harmonics of a lower-frequency signal and selects one of them to reach a higher operating frequency.
Odd-order intermodulation products tend to fall closest to the original signal frequencies, which makes them especially troublesome in communication receivers and transmitters.
Carson's rule estimates FM bandwidth as 2 × (deviation + highest modulating frequency). With 5 kHz deviation and 3 kHz audio, the result is 16 kHz.
If a 12.21 MHz oscillator is multiplied by 12 to reach 146.52 MHz, its deviation is also multiplied by 12. A final 5 kHz deviation therefore requires about 416.7 Hz at the oscillator.
High-duty-cycle modes keep the transmitter producing substantial power for long periods, so the average heating load can exceed the transmitter's rating even when peak power is within limits.
Matching receiver bandwidth to the signal admits the wanted modulation while excluding unnecessary noise, producing the best signal-to-noise ratio.
Higher symbol rates require the system to change states more quickly, which needs wider occupied bandwidth.
A mixer produces output components at the sum and difference of its local-oscillator and RF input frequencies.
Intermodulation occurs when two or more signals pass through a nonlinear device and combine to create unwanted new frequencies.
The order of an intermodulation product is the sum of the absolute values of its coefficients. 2F1 − F2 is third order, so it is an odd-order product.
WSPR is designed for very low-power beacon-style transmissions used to study and assess HF propagation paths.
The header of a packet-radio frame contains addressing, routing, and handling information needed to deliver the packet.
Baudot is a five-bit character code used with additional start and stop bits in traditional RTTY systems.
In ARQ, NAK means negative acknowledgment: the receiver is asking for the packet to be sent again.
When repeated ARQ attempts fail beyond the protocol's retry limit, the connection is dropped.
FT8 is designed to decode narrow-band signals at very low signal-to-noise ratios.
PSK31 uses Varicode, where uppercase letters have longer bit sequences than lowercase letters. Using uppercase therefore slows transmission.
Mesh networks can route around a failed node by sending packets through alternate nodes when another viable path exists.
Forward error correction transmits redundant information along with the payload so the receiver can reconstruct some damaged or missing data without asking for a retransmission.
The two frequencies in an FSK signal are traditionally called mark and space.
PSK31 uses Varicode, a variable-length character coding scheme optimized for common text characters.
Vertical lines beside an RTTY or other data signal on a waterfall often indicate splatter from overmodulation.
On a typical waterfall, frequency runs horizontally, signal strength is shown by intensity or color, and time progresses vertically.
An FT8 report of +3 means the received signal-to-noise ratio is equivalent to +3 dB in a 2.5 kHz reference bandwidth.
DMR, D-STAR, and System Fusion are digital voice systems used in amateur radio.