Understanding AM, FM and SSB

What carries the information

In amplitude modulationAM×Amplitude modulation varies a carrier’s amplitude with the information signal. Conventional AM contains the carrier and two sidebands.Example & full explanation →, the wanted information varies the carrier’s amplitude. Conventional AM contains a carrier and two sidebands. In frequency modulationFM×Frequency modulation varies the carrier’s instantaneous frequency. Deviation and the highest modulation frequency influence the required bandwidth.Example & full explanation →, information varies instantaneous frequency. An AM demodulator therefore cannot simply recover correct speech from FM, even when the receiver clearly displays its energy.

SSBSSB · USB / LSB×Single Sideband transmits one sideband, usually with the carrier suppressed. USB is the upper and LSB the lower sideband relative to the carrier reference.Example & full explanation → transmits one sideband and normally suppresses the carrier substantially. USB means upper sideband and LSB lower sideband. The receiver supplies its own demodulation reference. Choosing the wrong sideband or tuning incorrectly therefore produces unintelligible speech or altered pitch.

The filter belongs with the mode

A receive filter limits the frequencies reaching the demodulator. Too wide, it admits extra noise and adjacent signals. Too narrow, it removes wanted information. For a comparison using SSB speech, 2.4 kHz and 1.8 kHz are useful experimental widths, not universal prescriptions.

With FM, the required bandwidthBandwidth×Bandwidth is the width of a frequency range. A receiver filter limits the analysed range; the signal’s occupied bandwidth is a separate quantity.Example & full explanation → and appropriate demodulator also depend on frequency deviation. Wideband broadcast FM and narrowband voice radio are therefore different reception tasks. Also distinguish audio filtering after demodulation from RF or intermediate-frequency filtering before it.

A repeatable listening comparison

Find a sustained voice signal and keep listening volume roughly constant. Select the correct mode, fine-tune, then narrow the filter gradually. Note when unwanted noise decreases and when consonants or natural voice quality disappear. The best setting reliably preserves the information.