# Extra Class question of the day: modulation methods; modulation index and deviation ratio; pulse modulation; frequency and time division multiplexing

In FM modulation, the two primary parameters of interest are deviation ratio and modulation index. Deviation ratio is the ratio of the maximum carrier frequency deviation to the highest audio modulating frequency. (E8B09) The deviation ratio of an FM-phone signal having a maximum frequency swing of plus-or-minus 5 kHz when the maximum modulation frequency is 3 kHz is 1.67. (E8B05)The deviation ratio of an FM-phone signal having a maximum frequency swing of plus or minus 7.5 kHz when the maximum modulation frequency is 3.5 kHz is 2.14. (E8B06)

The term for the ratio between the frequency deviation of an RF carrier wave, and the modulating frequency of its corresponding FM-phone signal is modulation index. (E8B01) The modulation index is equal to the ratio of the frequency deviation to the modulating frequency. The modulation index of a phase-modulated emission does not depend on the RF carrier frequency. (E8B02)

The modulation index of an FM-phone signal having a maximum frequency deviation of 3000 Hz either side of the carrier frequency, when the modulating frequency is 1000 Hz is 3. (E8B03) The modulation index of an FM-phone signal having a maximum carrier deviation of plus or minus 6 kHz when modulated with a 2-kHz modulating frequency is 3. (E8B04)

Some amateur radio communications are pulse-width modulated. That is to say that the information being sent is proportional to the time the carrier is on. When using a pulse-width modulation system, the transmitter’s peak power greater than its average power because the signal duty cycle is less than 100%. (E8B07)

Some signals are pulse-position modulated. That is to say, what is significant is when the pulse occurs. The time at which each pulse occurs is the parameter that the modulating signal varies in a pulse-position modulation system. (E8B08)

Frequency division multiplexing is one method that can be used to combine several separate analog information streams into a single analog radio frequency signal. (E8B10) When a system uses frequency division multiplexing, two or more information streams are merged into a “baseband,” which then modulates the transmitter. (E8B11)

When a system uses digital time division multiplexing, two or more signals are arranged to share discrete time slots of a data transmission. (E8B12)

### Comments

1. I don’t remember this question from when I was studying, but the ideas of deviation ratio and modulation index are great. Thanks.