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# Modulation Methods

## Overview

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

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.

### Frequency Modulation

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.