Two ratios, same arithmetic, different inputs
This is the distinction the exam tests, and the only reliable way to get it right is to notice which word — actual or maximum — appears in the question.
What is the modulation index of an FM signal?
The ratio of frequency deviation to modulating signal frequency.
What is deviation ratio?
The ratio of the maximum carrier frequency deviation to the highest audio modulating frequency.
Modulation index uses the actual values at this instant, so it changes as the audio changes — a given deviation with a 500 Hz tone gives four times the index it gives with a 2 kHz tone.
Deviation ratio uses the maximums, so it is a fixed property of the transmitter — the worst case, which is what bandwidth calculations need.
Four worked answers, exactly as the pool asks:
| Question | Computation | Answer |
|---|---|---|
| Index: 6000 Hz deviation, 2000 Hz modulating | 6000 / 2000 | 3 |
| Index: 3000 Hz deviation, 1000 Hz modulating | 3000 / 1000 | 3 |
| Deviation ratio: 5000 Hz max deviation, 3000 Hz max audio | 5000 / 3000 | 1.67 |
| Deviation ratio: 7500 Hz max deviation, 3500 Hz max audio | 7500 / 3500 | 2.14 |
And one conceptual question that catches people:
How does the modulation index of a phase-modulated emission vary with RF carrier frequency?
It does not depend on the RF carrier frequency.
Neither index nor deviation ratio involves the carrier at all. Both are ratios of audio-related quantities. The multiplier lesson in the General track is the related fact from the other side: multiplying the carrier multiplies the deviation too, so the index survives multiplication unchanged.
Multiplexing
Three ways to fit several signals into one channel, and the pool distinguishes all three.
What is frequency division multiplexing (FDM)?
Dividing the transmitted signal into separate frequency bands that each carry a different data stream
Different signals on different frequencies, simultaneously. The whole radio spectrum is FDM.
What is digital time division multiplexing?
Two or more signals are arranged to share discrete time slots of a data transmission.
Different signals at different times, taking turns fast enough that each appears continuous. DMR is TDM — the two time slots from the Technician track.
What describes orthogonal frequency-division multiplexing (OFDM)?
A digital modulation technique using subcarriers at frequencies chosen to avoid intersymbol interference
Orthogonal frequency-division multiplexing (OFDM) is a technique used for which types of amateur communication?
Digital modes
OFDM is FDM done cleverly. Many closely spaced subcarriers, each carrying a slow data stream, with their frequencies chosen so that each subcarrier’s spectrum has a null exactly where every other subcarrier sits — that is what “orthogonal” means here. They can therefore overlap without interfering.
Why it wins: each subcarrier is slow, so each symbol is long, and a long symbol tolerates the multipath delay spread that would smear a fast single-carrier signal. That is the multipath problem from the Technician propagation lesson, solved by design.
OFDM is what Wi-Fi, LTE, digital television, and the faster amateur data modes all use.
Check yourself
- A transmitter deviates 4 kHz on a 1 kHz tone. Modulation index?
- The same transmitter has a 5 kHz maximum deviation and 3 kHz maximum audio. Deviation ratio?
- Why does OFDM cope with multipath better than a single fast carrier?
Answers
- 4000 / 1000 = 4.
- 5000 / 3000 = 1.67. Note the different inputs — actual for index, maximum for deviation ratio.
- Each subcarrier is slow, so each symbol is long compared with the multipath delay spread, and the echoes do not smear one symbol into the next.