Spread spectrum spreads a signal wide using a pseudorandom pattern
Type
What changes
How
Direct sequence
The carrier's phase
A high-speed binary bit stream shifts the phase
Frequency hopping
The carrier's frequency
Varied rapidly in a pseudorandom sequence
A fast code spreads the signal wide and weak. Only a receiver with the same code collapses it back.
The receiver uses the same pattern. Signals that do not use the spread spectrum algorithm are suppressed in the receiver, which is why it resists interference.
RememberDirect sequence = phase shifted by a fast bit stream. Hopping = frequency jumps by a pseudorandom sequence.
Sharp keying edges cause key clicks. Slow the edges.
A CW signal keyed on and off with extremely short rise or fall times makes key clicks: energy splashing to frequencies far from the carrier. The most common cure is to increase the keying waveform's rise and fall times.
Illustrative model of a 60 ms dot. Slower edges, fewer clicks.
Level 500 Hz from carrier-55.4 dB
RememberShort edges = key clicks. Lengthen rise and fall times.
Too much audio overmodulates AFSK. IMD measures the damage.
Feeding a transmitter excessive audio overmodulates an AFSK signal and distorts it. Intermodulation distortion (IMD) measures that distortion. An idling PSK signal should have IMD of −30 dB or better (lower).
Illustrative two-tone model. Overdrive adds products on both sides of the wanted signal.
IMD-40.3 dB
RememberOvermodulation comes from too much audio. Measure it with IMD: −30 dB or lower for idling PSK.
Baudot has 5 bits and shifts. ASCII has 7 or 8 bits, both cases, and parity.
Baudot: 5 data bits per character and two letters/figures shift codes. ASCII: 7 or 8 bits, no shift codes, and it carries uppercase and lowercase text.
Box width is proportional to the number of data bits.
A parity bit added to an ASCII character lets the receiver detect some types of errors. It does not correct them.
Click bits to flip them. One flip is caught. Two flips cancel and slip through.
RememberBaudot: 5 bits plus 2 shifts. ASCII: 7 or 8 bits, both cases. Parity detects, never corrects.