FREQUENCY MODULATION

FREQUENCY MODULATION

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In telecommunications, frequency modulation (FM) conveys information over a carrier wave by varying its frequency (contrast this with amplitude modulation, in which the amplitude of the carrier is varied while its frequency remains constant). In analog applications, the instantaneous frequency of the carrier is directly proportional to the instantaneous value of the input signal. Digital data can be sent by shifting the carrier's frequency among a set of discrete values, a technique known as frequency-shift keying.

FM signals can be generated using either direct or indirect frequency modulation.

Direct FM modulation can be achieved by directly feeding the message into the input of a VCO.
For indirect FM modulation, the message signal is integrated to generate a phase modulated signal. This is used to modulate a crystal controlled oscillator, and the result is passed through a frequency multiplier to give an FM signal.

A common method for recovering the information signal is through a Foster-Seeley discriminator.

FM is commonly used at VHF radio frequencies for high-fidelity broadcasts of music and speech (see FM broadcasting). Normal (analog) TV sound is also broadcast using FM. A narrow band form is used for voice communications in commercial and amateur radio settings. The type of FM used in broadcast is generally called wide-FM, or W-FM. In two-way radio, narrowband narrow-fm (N-FM) is used to conserve bandwidth. In addition, it is used to send signals into space.

FM is also used at audio frequencies to synthesize sound. This technique, known as FM synthesis, was popularized by early digital synthesizers and became a standard feature for several generations of personal computer sound cards.

FM RADIO

Edwin Armstrong presented his paper: "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation", which first described FM radio, before the New York section of the Institute of Radio Engineers on November 6, 1935. The paper was published in 1936.

As the name implies, wideband FM (W-FM) requires a wider signal bandwidth than amplitude modulation by an equivalent modulating signal, but this also makes the signal more robust against noise and interference. Frequency modulation is also more robust against simple signal amplitude fading phenomena. As a result, FM was chosen as the modulation standard for high frequency, high fidelity radio transmission: hence the term "FM radio" (although for many years the BBC called it "VHF radio", because commercial FM broadcasting uses a well-known part of the VHF band; in certain countries, expressions referencing the more familiar wavelength notion are still used in place of the more abstract modulation technique name).

An example of frequency modulation. This diagram shows the modulating, or message, signal, x_m(t), superimposed on the carrier wave, x_c(t)

FM receivers employ a special detector for FM signals and exhibit a phenomenon called capture effect, where the tuner is able to clearly receive the stronger of two stations being broadcast on the same frequency. Problematically however, frequency drift or lack of selectivity may cause one station or signal to be suddenly overtaken by another on an adjacent channel. Frequency drift typically constituted a problem on very old or inexpensive receivers, while inadequate selectivity may plague any tuner.

The modulated signal, y(t), produced from frequency-modulating x_c(t) with x_m(t).

An FM signal can also be used to carry a stereo signal: see FM stereo. However, this is done by using multiplexing and demultiplexing before and after the FM process. The rest of this article ignores the stereo multiplexing and demultiplexing process used in "stereo FM", and concentrates on the FM modulation and demodulation process, which is identical in stereo and mono processes.

A high-efficiency radio-frequency switching amplifier can be used to transmit FM signals (and other constant-amplitude signals). For a given signal strength (measured at the receiver antenna), switching amplifiers use less battery power and typically cost less than a linear amplifier. This gives FM another advantage over other modulation schemes that require linear amplifiers, such as AM and QAM.

AMATEUR RADIO

TEN METER BAND

The FM Sub-Band

From 29.510 MHz to 29.700, The FM sub-band is usually channelized into repeater and simplex frequencies. The channels are commonly grouped into repeater inputs, simplex, and repeater output frequencies.

Repeater Input Channels: 29.520, 29.540, 29.560, and 29.580 MHz.

Simplex Channel: 29.600 MHz

Repeater Output Channels: 29.620, 29.640, 29.660, and 29.680 MHz.

Repeater Operation on 10 Meters

Common practice for 10 meter repeaters is to use a 100 kHz negative offset for repeater operation. Due to the very few available repeater channels, "odd-splits" (offsets differing from 100 kHz) and non-standard frequencies are not uncommon. Since 10 meters can frequently open up to propagate globally, most 10 meter repeaters use a CTCSS sub-audible access tone. 16 kHz wide signals with 5 kHz deviation is normal in this band. 8 kHz narrow signals with 2.5 kHz deviation can also be found.

Other FM Simplex Channels in use

29.300MHz is a common frequency to find JA hams on. British hams commonly use the 29.400 to 29.500 MHz band for FM as well with 29.400, 29.450, and 29.500 MHz being common. USA hams can be found on FM anywhere above 29.300 MHz, commonly on the above frequencies talking to overseas hams.

Propagation

One can expect up to 50 miles on ground wave propagation and when sky wave "skip" is present hundreds to thousands of miles are possible. Chicago via the British Virgin Islands repeater and back to Detroit is a common daily occurance when the 10-Meter FM band is hot.

The Satellite Sub-Band

From 29.300 MHz to 29.510 MHz the satellite sub band allows amateur radio operaters to communicate with orbiting OSCARs.

Satellite Operation on 10 Meters

Many OSCARs have either an uplink or a downlink in the 29 MHz range. Information about particular satellites and operational modes is available from AMSAT.

As of the current writing, most if not all of the satellites actually using the sub band are non functional

VHF

VHF (Very high frequency) is the radio frequency range from 30 MHz to 300 MHz. Frequencies immediately below VHF are denoted High frequency (HF), and the next higher frequencies are known as Ultra high frequency (UHF).

VHF propagation characteristics are ideal for short-distance terrestrial communication, with a range generally somewhat farther than line-of-sight from the transmitter. Unlike high frequencies (HF), the ionosphere does not usually reflect VHF radio and thus transmissions are restricted to the local area (and don't interfere with transmissions thousands of kilometres away). VHF is also less affected by atmospheric noise and interference from electrical equipment than lower frequencies. Whilst it is more easily blocked by land features than HF and lower frequencies, it is less affected by buildings and other less substantial objects than UHF frequencies.

Two unusual propagation conditions can allow much farther range than normal. The first, tropospheric ducting, can occur in front of and parallel to an advancing cold weather front, especially if there is a marked difference in humidities between the cold and warm air masses. A duct can form approximately 250 km (155 mi) in advance of the cold front, much like a ventilation duct in a building, and VHF radio frequencies can travel along inside the duct, bending or refracting, for hundreds of kilometers. For example, a 50 watt Amateur FM transmitter at 146 MHz can talk from Chicago, to Joplin, Missouri, directly, and to Austin, Texas, through a repeater. In a July 2006 incident, a NOAA Weather Radio transmitter in north central Wisconsin was blocking out local transmitters in west central Michigan, quite far out of its normal range.

The second type, much more rare, is called Sporadic E, referring to the E-layer of the ionosphere. A sunspot eruption can pelt the Earth's upper atmosphere with charged particles, which may allow the formation of an ionized "patch" dense enough to reflect back VHF frequencies the same way HF frequencies are usually reflected (skywave). For example, KMID (TV Channel 2; 54–60 MHz) from Midland, Texas was seen around Chicago, pushing out Chicago's WBBM-TV. These patches may last for seconds, or extend into hours. FM stations from Miami, Florida; New Orleans, Louisiana; Houston, Texas and even Mexico were heard for hours in central Illinois during one such event. Mid summer 2006 central Iowa stations were heard in Columbus, NE and blocking out Omaha radio and TV stations for several days. Similar propagation effects can affect land-mobile stations in this band, rarely causing intereference well beyond the usual coverage area.

Frequency assignments between US and Canadian users are closely coordinated since much of the Canadian population is within VHF radio range of the US border. Certain discrete frequencies are reserved for radio astronomy. The general services in the VHF band are:
30–46 MHz: Licensed 2-way land mobile communication.
30–88 MHz: Military VHF-FM, including SINCGARS
43–50 MHz: Cordless telephones, 49 MHz FM walkie-talkies and radio controlled toys, and mixed 2-way mobile communication. The FM broadcast band originally operated here (42-50 MHz) before moving to 88-108 MHz.
50–54 MHz: Amateur radio 6 meter band; 50 MHz is an amateur radio band used for a variety of uses including DXing, FM repeaters and radio control, which usually takes place on a "set-aside" band between 50.8 and 51 MHz.
55-72 and 77-88 MHz TV channels 2 through 6 (VHF-Lo), known as "Band I" internationally; a tiny number of DTV stations will appear here. See North American broadcast television frequencies
72–76 MHz: Radio controlled models, industrial remote control, and other devices. Model aircraft operate on 72 MHz while surface models operate on 75 MHz in the USA and Canada, air navigation beacons 74.8-75.2 MHz.
88–108 MHz: FM radio broadcasting (88–92 non-commercial, 92–108 commercial in the United States) (Known as "Band II" internationally)
108–118 MHz: Air navigation beacons VOR
118–137 MHz: Airband for air traffic control, AM, 121.5 MHz is emergency frequency
137-138 Space research, space operations, meteorological satellite
138–144 MHz: Land mobile, auxiliary civil services, satellite, space research, and other miscellaneous services
144–148 MHz: Amateur radio band 2 Meters
148-150 Land mobile, fixed, satellite
150–156 MHz: "VHF Business band," the unlicensed Multi-Use Radio Service (MURS), and other 2-way land mobile, FM
156–158 MHz VHF Marine Radio; narrow band FM, 156.8 MHz (Channel 16) is the maritime emergency and contact frequency.
160-161 MHz Railways
162.40–162.55: NOAA Weather Stations, narrowband FM
175-216 MHz television channels 7 - 13 (VHF-Hi), known as "Band III" internationally. A minority of DTV channels may appear here.
174–216 MHz: professional wireless microphones (low power, certain exact frequencies only)
216–222 MHz: land mobile, fixed, maritime mobile,
222–225 MHz: 1.25 meters (US) (Canada 219-220, 222-225 MHz) Amateur radio
225 MHz and above: Military aircraft radio (225–400 MHz) AM, including HAVE QUICK, dGPS RTCM-104
The large technically and commercially valuable slice of the VHF spectrum taken up by television broadcasting has attracted the attention of many companies and governments recently, with the development of more efficient digital television broadcasting standards. In some countries much of this spectrum will likely become available (probably for sale) in the next decade or so (June 12, 2009, in the United States).

UHF

Ultra high frequency (UHF) designates a range of electromagnetic waves with frequencies between 300 MHz and 3 GHz (3,000 MHz). Also known as the decimetre band or decimetre wave as the wavelengths range from one to ten decimetres (10 cm to 1 metre). Radio waves with frequencies above the UHF band fall into the SHF (Super high frequency) and EHF (Extremely high frequency) bands, all of which fall into the Microwave frequency range.

UHF transmission and reception will be enhanced or degraded by tropospheric ducting as the atmosphere warms and cools throughout the day. The main advantage of UHF transmission is the physically short wave that is produced by the high frequency. The size of transmission and reception antennas, is related to the size of the radio wave. The UHF antenna is stubby and short. Smaller and less conspicuous antennas can be used with higher frequency bands. The major disadvantage of UHF is its limited broadcast range and reception, often referred to as line-of-sight between the station's transmission antenna and the reception antenna, as opposed to VHF's very long broadcast range and reception which is less restricted by line-of-sight.

UHF is widely used in two-way radio systems and cordless telephones whose transmission and reception antennas are closely spaced. UHF signals travel over line-of-sight distances. Transmissions generated by two-way radios and cordless telephones do not travel far enough to interfere with local transmissions. A number of public safety and business communications are handled on UHF. Civilian applications such as GMRS, PMR446, UHF CB, and 802.11b ("WiFi") are popular uses of UHF frequencies. A repeater is used to propagate UHF signals when a distance that is greater than the line-of-sight is required.

A brief summary of some UHF frequency use:

300–420 MHz: Government use, including meteorology, military aviation, and federal two-way use
420–450 MHz: Government radiolocation and amateur radio (ham - 70 cm band)
433 MHz: Short range consumer devices including automotive, alarm systems, home automation, temperature sensors
450–470 MHz: UHF business band, General Mobile Radio Service, and Family Radio Service 2-way "walkie-talkies", public safety
470–512 MHz: TV channels 14–20
512–698 MHz: TV channels 21–51 (channel 37 used for radio astronomy)
698–806 MHz: Was auctioned in March 2008; bidders got full use after the transition to digital TV was completed on June 12, 2009 (formerly TV channels 52–69)
806–824 MHz: Public safety and commercial 2-way (formerly TV channels 70–72)
824–851 MHz: Cellular A & B franchises, terminal (mobile phone) (formerly TV channels 73–77)
851–869 MHz: Public safety and commercial 2-way (formerly TV channels 77–80)
869–896 MHz: Cellular A & B franchises, base station (formerly TV channels 80–83)
902–928 MHz: ISM band: cordless phones and stereo, radio-frequency identification, datalinks, amateur radio (33 cm band)
929–930 MHz: Pagers
931–932 MHz: Pagers
935–941 MHz: Commercial 2-way radio
941–960 MHz: Mixed studio-transmitter links, SCADA, other.
960–1215 MHz: Aeronautical Radionavigation
1240–1300 MHz: Amateur radio (ham - 23 cm band)
1452–1492 MHz: Military use (therefore not available for Digital Audio Broadcasting, unlike Canada/Europe)
1710–1755 MHz: AWS mobile phone uplink (UL) Operating Band
1850–1910 MHz: PCS mobile phone—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz
1920–1930 MHz: DECT Cordless telephone
1930–1990 MHz: PCS base stations—order is A, D, B, E, F, C blocks. A, B, C = 15 MHz; D, E, F = 5 MHz
2110–2155 MHz: AWS mobile phone downlink (UL) Operating Band
2300–2310 MHz: Amateur radio (ham - 13 cm band, lower segment)
2310–2360 MHz: Satellite radio (Sirius and XM)
2390–2450 MHz: Amateur radio (ham - 13 cm band, upper segment)
2400–2483.5 MHz: ISM, IEEE 802.11, 802.11b, 802.11g, 802.11n Wireless LAN, IEEE 802.15.4-2006, Bluetooth, ZigBee, Microwave oven

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