FM APPLICATIONS

 

 

FM applications are divided into two broad categories:

            Wideband FM (WFM)

            Narrowband FM (NBFM)

 

The primary difference between the two types of FM is the number of sidebands in the modulated signal. Wideband FM has a large number  (theoretically infinite) number of sidebands. Narrowband FM has only a single pair of significant sidebands. 

 

It is possible to determine if a particular FM signal will be wide or narrow band by looking at a quantity called the Deviation Ratio (DR). It is defined as the ratio of the maximum deviation of the FM signal to the maximum modulating frequency:

 

The DR is also the modulation index of the highest modulating frequency. If the DR ≥ 1.0 the modulation is called wideband FM (WFM). If the DR < 1.0 the modulation is narrow band FM (NBFM). 

 

One of the drawbacks of wideband FM is the large bandwidth required. Commercial FM broadcasting requires 150 KHz of bandwidth  to transmit a15 KHz audio signal, 5 times the bandwidth required for an AM signal.

 

The figure below compares the spectra of a WFM signal (DR = 5) and a NBFM signal (DR = 0.5). The separation between sidebands is equal to the modulating frequency. Thus the bandwidth for NBFM is 2*fm , which is the same as for AM. However, for WFM, the bandwidth is approximately 2N* fm , where N = the number of sidebands.

   

 

 

 

FM is used in the following applications:

 

  1. Non-commercial broadcasting from 88 – 90 MHz (WFM)

  2. Commercial broadcasting from 90 – 108 MHz (WFM)

  3. Television audio (WFM)

  4. Public Service communications (police, fire departments, etc.) from 30 – 50 MHz, 136-174 MHz, 450-470 MHz, and 800 MHz (NBFM)

  5. Amateur Radio Service Communications 29.5 – 29.7 MHZ, 52 – 54 MHz, 144 – 148 MHz, 222 – 225 MHz, 440 – 450 MHz, 902 MHz, 1240 – 1300 MHz, and other frequencies above 2.3 GHz (NBFM)

  6. Point-to-point microwave links used by telecommunications companies (this is very wideband FM – the deviation of the carrier can be 10 MHz or more)

 

The output powers used can range from 10 mW to 100 kW.  Note that FM is not used at frequencies below approximately 30 MHz.  There are two reasons for this:

 

  1. Over the horizon propagation in this frequency range (the result of ionospheric refraction) can introduce significant phase distortion into a  wideband FM signal.

  2. The bandwidth available in this region of the radio spectrum is limited, so other modes such as SSB (single sideband suppressed carrier) are used that are more spectrum efficient. (SSB requires 3 KHz of bandwidth for a 3KHz audio signal, vs. 6 KHz for NBFM and > 12 KHz for WFM).

 

WIDEBAND FM (WFM)

 

    FM BROADCASTING

 

Commercial FM broadcasts occur in the VHF range, between 88 and 108 MHz.  The carrier frequencies start at 88.1 MHz and are separated by 200 KHz intervals. The maximum audio bandwidth allowed is 15 KHz and the deviation is limited to +/- 75 KHz. Limiting the deviation to this value leaves a 25 KHz guard band at each end of the channel that limits inter-channel interference.

 

The DR for commercial FM broadcasting is 75/15 = 5.0. This is clearly a wideband FM signal.

   

    TELEVISION AUDIO

 

The audio portion of an analog (NTSC) television signal is sent using FM. The maximum permitted frequency deviation is 25 KHz, and the maximum audio frequency is 15 KHz. The DR for commercial FM broadcasting is 25/15 = 1.67. This is a wideband FM signal. 

 

    POINT-TO-POINT MICROWAVE

 

Telephone companies have made extensive use of microwave systems for medium and long distance trunk circuits, although many of these microwave systems are being replaced by optical fiber. A typical point-to-point  digital microwave system such as the ATT DR6-30  operated with a carrier frequency of 6 GHz and carried 9408 3 KHz voice channels in a bandwidth of 210 MHz. The effective DR is 210 MHz/(3KHz*9408) = 7.44.

   

NARROW BAND FM (NBFM)

 

NBFM is widely used in business and public service communications.   The DR for NBFM is restricted to values between 0.5 and 1.0. By holding the DR to such small values, only the carrier and the first sideband are of significant amplitude. When only one sideband and the carrier are transmitted, the NBFM signal occupies the same bandwidth as an AM signal. This overcomes one of the drawbacks of wideband FM, the large bandwidth required. 

 

The FCC permits the bandwidth of NBFM signals to be from 10 to 30 KHz, depending on the assigned carrier frequency and the type of operation authorized. 

 

   

Previous Page Index Next Page