E8D: Keying Defects and Spread Spectrum

Even a well-designed signal can cause interference if its keying characteristics are poor, and understanding both the defects and the digital codes used to transmit information is essential for Extra class operators. This group also covers spread spectrum — a technique that resists interference by distributing signal energy across a wide band.

This lesson covers key clicks and their cause, how to reduce them, spread spectrum techniques (direct sequence and frequency hopping), AFSK overmodulation, IMD, parity bits, acceptable IMD limits for PSK, and the differences between Baudot and ASCII codes.

Key Clicks: Cause and Cure

A key click is a broadband interference artifact produced by CW transmitters with excessively abrupt keying. When the transmitter switches on or off with an extremely short rise or fall time — essentially a rectangular keying envelope — the fast transition generates a broad burst of RF energy spread across a wide range of frequencies, causing interference to nearby stations.

The most common method of reducing key clicks is to increase the keying waveform rise and fall times. By shaping the transmitted envelope so that it ramps up and down more gradually, the transient energy is confined to a narrow bandwidth near the carrier and key clicks are eliminated. Most modern transceivers include adjustable keying waveform shaping for this purpose.

Spread Spectrum Fundamentals

Spread spectrum is a class of transmission techniques that deliberately spread the transmitted signal's energy over a much wider bandwidth than the minimum required to carry the information. This spreading provides resistance to interference: a receiver using the same spreading algorithm can recover the signal, while signals not using the spread spectrum algorithm are suppressed in the receiver. To the spread spectrum receiver, conventional narrowband interference appears as low-level broadband noise, which is easily overcome.

Direct Sequence Spread Spectrum

In direct sequence spread spectrum (DSSS), a high-speed binary bit stream (called a pseudorandom noise or PN code) is used to shift the phase of the RF carrier at a rate far higher than the data rate. This phase-shifting spreads the signal energy across a wide bandwidth. The receiver uses an identical PN code, synchronized with the transmitter, to despread and recover the original data.

Frequency Hopping Spread Spectrum

In frequency hopping spread spectrum (FHSS), the transmitted signal rapidly varies its frequency according to a pseudorandom sequence. The transmitter and receiver are synchronized to the same hopping sequence, so they always use the same frequency at the same time. Any narrowband interferer only disrupts the signal during the fraction of time when the hop lands on that frequency — the effect is minimal compared to staying on a fixed frequency.

AFSK Overmodulation and IMD

Audio frequency shift keying (AFSK) is used in many digital modes where a sound card generates audio tones that are fed into a transceiver's microphone input. A common problem is overmodulation caused by excessive transmit audio levels. When the audio level is set too high, the transmitter is driven beyond its linear range, distorting the signal and spreading energy into adjacent frequencies.

The parameter used to evaluate this distortion is intermodulation distortion (IMD). IMD measures how much unwanted mixing product is generated when two or more audio tones are processed by a nonlinear system. For an idling PSK signal, the acceptable maximum IMD level is −30 dB — 30 dB below the desired signal level. Values better (more negative) than −30 dB are acceptable; values worse (less negative) indicate a problem with the audio drive level or transmitter linearity.

Parity Bits

A parity bit is an extra bit added to a character or data word that makes the total number of 1-bits either always even (even parity) or always odd (odd parity). The advantage of parity is that some types of errors can be detected — specifically, any single-bit error changes the parity, signaling a problem. However, parity cannot correct errors or detect even numbers of simultaneous bit errors, so it is a limited but simple form of error detection.

Baudot vs ASCII

Two character encoding systems appear on the Extra exam:

• Baudot: Uses 5 data bits per character, allowing 32 code combinations. To represent more than 32 characters, Baudot uses two shift codes — a letters shift and a figures shift — to switch between two character sets. This doubles the effective character set but requires the receiver to track which mode is active.
• ASCII: Uses 7 or 8 data bits per character, providing 128 or 256 unique codes. ASCII has no letters/figures shift codes — every character has a unique code. The key practical advantage of ASCII is that it can represent both uppercase and lowercase text, along with digits, punctuation, and control characters, all without mode-shifting.

E8D Practice Questions