From The RadioReference Wiki
To the scanning hobbyist, the main concern with modulation methods is what method is used for the signal I am trying to receive. For the most part the scanner manufacturers have already set the scanners to default to the proper type of modulation based on the frequency that you program into the scanner. However, it does help if you, the user, has an understanding of what the terms mean and how specifically they impact your reception of some signals. With that target in mind the following is submitted for your use, please understand that this information is general and aimed at the non-technical scanner hobbyist.
Modulation is the addition of intelligence (information) to an electronic signal, in this case Radio Frequency (RF) waves. The main two types of modulation that scanner hobbyists deal with are Amplitude Modulation (AM) and Frequency Modulation (FM) and the derivatives of both of these.
AM is the method of impressing (modulating) an alternating current wave onto a carrier frequency and varying that wave in such a way to carry intelligence that can be demodulated by a receiver to recover the intelligence. In other words, words spoken into a microphone are converted to a AC wave, added to a carrier signal (modulated) by the transmitter and sent out over the air. The receiver picks up the signal, separates the added signal from the the carrier waver (demodulates), converts it back to AC wave and send it to the speaker. This type of transmission is that which is used on the AM Broadcast band of your car, portable, or home stereo radios. The carrier, in an AM signal, actually has no part in the intelligence sent, that intelligence is contained in the two sidebands (Lower and Upper) on either side of the center carrier signal frequency. Indeed, the carrier is a waste of energy as it usually makes up 2/3's of the entire signal being sent. The leads to the best known derivative of AM, that of Single Sideband or SSB. Single Sideband basically is using one side of the AM signal (lower sideband or upper sideband) and then stripping the center carrier signal and the other sideband away. This allows for all of the signal being sent to be contained in one sideband and allows for smaller components as you only need about 1/6 th of the power to send the same signal via SSB as you do with AM. (The carrier being 2/3 of a given signal and the other 1/3 being in the double sidebands.) As a reference, SSB is usually found in the MF and HF portions of the spectrum (say 1.8 MHz to about 30 MHz). AM will be found on the aircraft monitoring frequencies (108 MHz to about 138 MHz), the MilAir frequencies (225 through about 380 MHz), as well as some others areas like CB radio.
FM is the method of impressing data (intelligence) onto an alternating wave by varying the frequency of the wave. It can be used with both analog or digital data. The amount of the variance of the frequency from it's base frequency (think carrier frequency again) is called the deviation. The amount of deviation that is used on the signal determines what type of FM signal it is, more on that in a bit. In analog FM the wave varies in a continuous manner, in digital FM the wave frequency shifts abruptly and the number of possible shifted states us usually a power of 2. For the scanner user P25 Digital Modulation (also called Common Air Interface) is made up of either C4FM or CQPSK, both of these are 4 state digital modulations. In addition to analog and digital, FM signals are generally divided into types of signal by the width of the deviation used. For scanners these can lumped into 3 groups:
- WFM - Wideband FM -- this is used on FM Broadcast radio and is actually wide enough (about 150 KHz) to carry stereo signals, although the signals being sent are actually limited to 15 KHz each.
- In the point-to-point microwave frequencies, of little interest to the casual scanner hobbyist, signals can also be of the SWFM (Super-Wideband FM) and can be of 10 MHz or more in width carrying hundreds of individual signals.
- NFM - Narrowband FM (called FM on most scanners) -- is used on most current analog public safety communications and the total deviation is generally limited to 30 KHz down to about 12 KHz.
- SNFM - Super-Narrowband FM (called NFM on most scanners) -- is used on most digital FM and future analog public safety communications, is generally limited to 11 KHz and below in width.