Key clicks
What is the primary effect of extremely short rise or fall time on a CW signal?
The generation of key clicks.
What is the most common method of reducing key clicks?
Increase keying waveform rise and fall times.
A sharp edge is broadband — Fourier again. Clicks are the harmonics of that edge, audible as sharp thumps for kilohertz either side of a signal that is otherwise 50 Hz wide.
The fix is deliberate shaping: a few milliseconds of rise and fall. Too slow and the keying sounds mushy and limits speed; a few milliseconds is the compromise every modern radio implements, and the reason a badly-adjusted one is immediately obvious on the band.
Note this is the same phenomenon as the PSK zero-crossing rule and the sinusoidal pulse shaping from the previous lesson. Sharp transitions cost bandwidth, in every mode.
AFSK distortion
What is a common cause of overmodulation of AFSK signals?
Excessive transmit audio levels.
The most common digital-mode fault there is, and the vertical lines on a waterfall from the General track are its signature.
What parameter evaluates distortion of an AFSK signal caused by excessive input audio levels?
Intermodulation Distortion (IMD)
What is considered an acceptable maximum IMD level for an idling PSK signal?
−30 dB.
A concrete target, which is what makes this actionable. Software such as fldigi measures IMD on a received idling PSK signal and reports it. Worse than −30 dB means reduce your drive, and it is a measurement rather than an opinion.
If you can hear your own signal on a WebSDR, you can measure your own IMD. Few operators do, and the bands would be noticeably cleaner if they did.
Codes, compared
What are some of the differences between the Baudot digital code and ASCII?
Baudot uses 5 data bits per character, ASCII uses 7 or 8; Baudot uses 2 characters as letters/figures shift codes, ASCII has no letters/figures shift code
What is one advantage of using ASCII code for data communications?
It is possible to transmit both uppercase and lowercase text.
Five bits gives 32 symbols — not enough for letters and digits, hence Baudot’s shift codes and its inability to send lowercase. ASCII’s 7 or 8 bits gives 128 or 256, which covers everything.
What is the advantage of including parity bits in ASCII characters?
Some types of errors can be detected.
Some. A single parity bit catches any odd number of bit errors and misses every even number. It is the weakest useful error detection there is — which is why serious protocols use a proper checksum or CRC, and why the pool’s wording is carefully hedged.
Spread spectrum
What spread spectrum communications technique uses a high-speed binary bit stream to shift the phase of an RF carrier?
Direct sequence.
Which describes spread spectrum frequency hopping?
Rapidly varying the frequency of a transmitted signal according to a pseudorandom sequence.
Two ways to spread a signal over far more bandwidth than its information needs:
- Direct sequence multiplies the data by a fast pseudorandom code, spreading the spectrum. The receiver multiplies by the same code to recover it.
- Frequency hopping moves the carrier around a pseudorandom pattern, with the receiver hopping in step.
Why are received spread spectrum signals resistant to interference?
Signals not using the spread spectrum algorithm are suppressed in the receiver.
The de-spreading operation concentrates the wanted signal and spreads everything else. An interfering carrier, however strong, is smeared across the whole bandwidth and drops below the recovered signal. That is why GPS works at signal levels below the noise floor, and why spread spectrum shares spectrum gracefully.
The amateur limit is 10 watts PEP, from the General rules lesson — deliberately low, because a spread-spectrum signal occupies a great deal of spectrum by design.
Check yourself
- Your CW sounds clicky to other operators. What do you adjust, and in which direction?
- Your PSK IMD measures −22 dB. Acceptable?
- Why can a spread-spectrum receiver ignore a strong interfering carrier?
Answers
- Increase the keying waveform rise and fall times. Sharp edges produce the harmonics heard as clicks.
- No. −30 dB is the acceptable maximum for an idling PSK signal, and −22 dB is worse than that. Reduce your drive.
- De-spreading concentrates the wanted signal while spreading the interferer across the whole bandwidth, dropping it below the recovered signal.