Conversion from 25 Megahertz to Hertz

Information about the Megahertz (MHz)

The megahertz is a unit of frequency equal to one million hertz. Its symbol is MHz. It covers the part of the radio spectrum that carries most broadcasting and short-range communication, and it was the unit in which computer speed was measured for two decades.

FM radio occupies 87.5 to 108 megahertz almost everywhere, a band chosen after the Second World War because it was high enough to allow the wide channels frequency modulation needs and low enough for transmitters and receivers to be built cheaply. Terrestrial television took the neighbouring bands, and the switch from analogue to digital broadcasting freed large parts of them for mobile telephony.

Aviation communication sits just above FM radio, from 118 to 137 megahertz, using amplitude modulation rather than the clearer frequency modulation. The choice is deliberate: when two aircraft transmit at once, amplitude modulation produces an audible heterodyne squeal that alerts everyone to the collision, whereas frequency modulation would simply capture the stronger signal and silently lose the weaker one.

Computer processors were rated in megahertz from the late 1970s until around 2000. The original IBM PC ran at 4.77 megahertz, and the megahertz race that followed ended when clock speeds stalled near four gigahertz because of heat, after which manufacturers competed on core count instead. Memory bus speeds and display timings are still commonly quoted in megahertz.

Medical ultrasound works between about 2 and 18 megahertz. Higher frequencies give finer resolution but penetrate less deeply, so an abdominal scan uses a lower frequency than an examination of a superficial structure, and the operator chooses the probe accordingly.

Radio-frequency identification, garage door remotes, wireless microphones, marine VHF and amateur radio all live in this range. So do the harmonics that make poorly shielded electronics interfere with each other, which is why regulatory testing concentrates on emissions in the tens and hundreds of megahertz.

Mobile telephony has made the unit a matter of money. Spectrum is auctioned in blocks measured in megahertz, and governments have raised sums in the tens of billions from bands at 700, 800, 900, 1800 and 2600 megahertz. Physics sets the value of each: lower frequencies travel further and penetrate buildings better, so the 700 and 800 bands cover rural areas cheaply, while the higher ones carry more data over shorter distances and suit cities. An operator's coverage map and its data speeds are therefore both consequences of which megahertz it managed to buy, which is why the auctions are followed as closely as any other national asset sale.

One megahertz equals one million hertz, 1000 kilohertz, or 0.001 gigahertz.

Information about the Hertz (Hz)

The hertz is the SI unit of frequency, equal to one cycle per second. Its symbol is Hz. It is a derived unit whose dimension is simply one divided by time, so a quantity in hertz counts how many times something repeats in a second.

It is named after Heinrich Hertz, who between 1886 and 1889 produced and detected radio waves in the laboratory and so confirmed Maxwell's prediction that light and electromagnetic waves are the same phenomenon. The unit was adopted internationally in 1960, replacing the older and more literal cycles per second, which is still occasionally seen on vintage equipment.

Sound occupies the low end of the scale. Human hearing runs from roughly 20 hertz to 20,000 hertz, though the upper limit falls with age and is usually well below 16,000 hertz by middle life. Musical pitch is frequency: the A above middle C is fixed at 440 hertz by convention, and each octave is a doubling, so the same note an octave higher is 880 hertz.

Mains electricity alternates at 50 hertz across most of the world and 60 hertz in North America and parts of Japan and South America. The split is historical rather than technical, dating from competing equipment standards in the late nineteenth century, and it is why appliances and clocks that depend on mains frequency cannot simply be moved between regions.

Screens and computing use it constantly. A display refreshing at 60 hertz redraws sixty times a second, and higher rates reduce visible motion blur. Processor clock speeds, once quoted in megahertz and now in gigahertz, count the same thing: how many times per second the internal clock ticks.

The unit also applies to anything periodic that is not a wave. Heart rate at 60 beats per minute is one hertz, a pendulum with a one-second period swings at one hertz, and the frequency of a rotating shaft in revolutions per second is expressed identically.

One hertz equals one cycle per second, 1000 millihertz, or one thousandth of a kilohertz.