G8A

Modulation, overmodulation and link budgets

Tell AM, FM, PM, SSB and digital modulation apart, see what overmodulation does, and work out a link budget and margin.

14 pool questions1 on the exam

Syllabus: Carriers and modulation: AM, FM, and single sideband; modulation envelope; digital modulation; overmodulation; link budgets and link margins

Name the modulation by what the voice changes

The carrier has three things you can vary. Whichever one your signal changes names the mode.

Carrier property changed	Modulation
Height (instantaneous power level)	Amplitude modulation (AM)
Instantaneous frequency	Frequency modulation (FM)
Phase angle	Phase modulation (PM)

A reactance modulator on an RF amplifier stage shifts the signal's phase: that makes phase modulation.

RememberPower level = AM. Frequency = FM. Phase angle = PM.

Overmodulation flat-tops the envelope and widens the signal

The modulation envelope is the outline you get by connecting the peaks of the modulated signal. In clean AM it copies the voice. Too much speech level or drive pushes the peaks into the transmitter's limit: they flatten (flat-topping). The distortion adds extra sidebands, so the signal is excessively wide.

The envelope is the outline of the peaks. Past full drive it flattens and the signal spreads sideways.

Speech drivebelow full

BandwidthNormal

RememberToo much drive: flat tops, distortion, excessive bandwidth.

SSB is the narrowest voice mode

AM sends a carrier plus two sidebands. SSB suppresses the carrier and one sideband, so it takes half the space of AM. FM, PM and vestigial sideband (a TV method) are all wider.

SSB removes the carrier and one sideband: the same voice in half the space.

RememberSSB drops the carrier and one sideband: narrowest of the phone modes.

Digital modes shift frequency or phase to carry bits

Direct binary FSK: a digital signal changes an oscillator's frequency directly, with no audio tone in between. FT8 is 8-tone FSK. QPSK uses phase shifts of 0, 90, 180 and 270 degrees to send pairs of bits.

FSK: the bit picks the frequency. PSK: the bits pick the phase. QPSK packs two bits per phase.

QPSK31 is sideband sensitive, its encoding provides error correction, and its bandwidth is about the same as BPSK31.

RememberFSK = frequency steps. PSK = phase steps. QPSK = 4 phases, 2 bits each.

Link budget is what arrives. Link margin is what's left over.

Add power and antenna gains, subtract losses: that is the link budget, the signal level at the receiver. The link margin is how far that level sits above the minimum the receiver needs.

Budget = power + gains − losses, measured at the receiver. Margin = that level minus the minimum the receiver needs.

Transmit power30 dBmTX antenna gain6 dBRX antenna gain6 dBSystem losses130 dBMinimum needed-100 dBm

Link budget-88 dBm

Link margin12 dB

Budget and marginbudget = TX power + antenna gains − system losses; margin = received level − minimum required level

RememberMargin positive = link works, with room for fades.

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Compendium articles on the ideas in this lesson, beyond what the exam asks.

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Practise the 14 pool questions for this group. Answers are shuffled, and every one comes with a one-line "why".

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