Conversion from 100 Megahertz to Microhertz

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 Microhertz (µHz)

The microhertz is a unit of frequency equal to one millionth of a hertz. Its symbol is µHz. One microhertz is one cycle per million seconds, which is a little over eleven and a half days, so the unit measures things that repeat on the scale of weeks, months or years.

Asteroseismology is where it is most used. Stars oscillate, and the frequencies of those oscillations depend on their internal structure in the same way the pitch of a bell depends on its shape and thickness. Sun-like stars ring at frequencies of a few thousand microhertz, red giants at a few tens, and the largest evolved stars below one. Space telescopes such as Kepler and TESS measured these frequencies for hundreds of thousands of stars by watching their brightness vary by a few parts per million.

The technique gives quantities no other method can supply. From the oscillation spectrum an astronomer can derive a star's mass, radius and age, and the age of a star is otherwise almost impossible to determine. This is how the ages of planet-hosting stars, and therefore of their planetary systems, are now established.

Gravitational-wave astronomy has claimed the band as well. The planned space-based observatory LISA will be sensitive between roughly 100 microhertz and one hertz, a range containing merging supermassive black holes and the many thousands of close binary white dwarfs in our own galaxy. Ground-based detectors cannot reach these frequencies because seismic noise overwhelms them.

Geophysics uses the unit for the slowest tidal constituents, including the fortnightly and monthly lunar tides and the semi-annual and annual solar ones, which together produce the long-period variations that tide predictions must include. Polar motion, the wobble of the Earth's rotation axis, has a fourteen-month period corresponding to about 0.8 microhertz.

Climate and ocean science reaches lower still. The El Nino Southern Oscillation recurs irregularly every two to seven years, well below a hundredth of a microhertz, and long ice-core records resolve cycles slower again.

The Earth itself supplies a textbook example. Its rotation axis wanders in a small circle with a period of about 433 days, the Chandler wobble discovered in 1891, which is a frequency of roughly 0.027 microhertz. Detecting it took decades of positional astronomy, because a signal that slow can only be separated from drift by observing for many cycles. That is the general constraint of the band: a measurement at one microhertz needs a record of at least a few million seconds, and preferably tens of them, so instruments must be stable for years before their data can be read. The unit therefore belongs to patient sciences rather than to fast ones.

One microhertz equals 0.000001 hertz, one cycle per million seconds, or 1000 nanohertz.