dBm – A measure of microwave power on a logarithmic scale. Power in dBms equals 10×log10 of the power in milliwatts.
DSB – Double sideband. Seesideband.
excitation temperature – A measure of the ratio of the population densities of two quantum states in a gas. It is the temperature for which the Boltzmann factor would predict the actual ratio of states. This unit is convenient because it is used to calculate line intensities. In local thermodynamic equilibrium, the excitation tem- perature is equal to the kinetic temperature. This is often not the case with carbon monoxide in molecular clouds. In some cases, especially with the CO J=1-0 line at 115 GHz, the excitation temperature can be much larger than the kinetic tempera- ture, and can even take on negative values.
GaAs – Gallium Arsenide. A semiconductor used for making high-speed microwave circuits. See MMIC.
HEMT – High Electron Mobility Transistor. A type of field-effect transistor (FET) that can provide gain up to very high frequencies (∼100 GHz) with low noise. See Duh et al. (1988).
heterodyne mixer– An electronic device that reduces a signal to a lower frequency by mixing it with a sine wave. Typically, the original signal is called the radio fre- quency, or RF, the sine wave is called the local oscillator, or LO, and the output is called the intermediate frequency, or IF. The frequencies are related by IF=|RF-LO|.
IF – Intermediate Frequency. See heterodyne mixer.
instance – A variable, as opposed to a class or data type. For example, r1, r2, andr3could be instances of class resistor. Seeobject oriented programming.
eral radio telescopes to synthesize a single large telescope with very high resolution.
IV curve – A plot of current versus voltage in an electronic device. If the IV curve is not linear, e.g., for a diode, then the device can mix frequencies. Seeheterodyne mixer andSIS mixer.
kinetic temperature– The usual meaning of temperature, related to the random kinetic energy of the particles that make up a substance.
LNA – Low-Noise Amplifier. Typically refers to transistor amplifiers at frequen- cies above about a gigahertz.
LO – Local Oscillator. See heterodyne mixer.
LSB – Lower sideband. Seesideband.
LVG – Large Velocity Gradient. An approximation that takes advantage of the large Doppler shifts in molecular clouds to simplify calculation of the radiation field.
In a nutshell, radiative transfer calculations become a local problem when spectral lines in different parts of a system are Doppler shifted to sufficiently different frequen- cies. See Appendix E.
MMIC – Monolithic Microwave Integrated Circuit. Typically, MMICs are small analog integrated circuits a few millimeters square, fabricated on gallium-arsenide substrates, and often used for microwave amplifiers. See GaAs.
NbTiN– Niobium titanium nitride. A high-temperature superconductor (Tc= 15 K, compared to niobium at Tc= 9.5 K) that is used to make low-noise SIS mixers from about 700 GHz to 1.2 THz because it has a larger energy gap than niobium. See Bumble et al. (2001).
noise temperature– A measure of the noise power generated in a high-frequency circuit. This measure is primarily useful when the Rayleigh-Jeans limit (Tn hν/kB) applies. A circuit with noise temperature Tn is equivalent to a noiseless circuit plus a noise power spectral density ofkBTn at its input.
object oriented programming – An approach to computer programming that combines data with functions into user-defined data types called classes, and pro- vides a way for subclasses to inherit functionality from parent classes. For example, SuperMix represents resistors with a class, since a resistor has both data (such as a resistance and a temperature) and functions (such as to calculate its scattering matrix), and inherits properties common to all circuit elements from a device parent class. See Booch (1994).
parsec – A measure of distance equal to 3.26 light-years, or 3.086× 1016 meters.
Abbreviated pc.
PDR – Photodissociation region, sometimes called photon dominated region. Re- gions where hot, bright stars illuminate opaque molecular clouds with far-ultraviolet radiation. The radiation creates layers of ionized atoms and dissociated molecules, and heats the gas in the region to temperatures in the range of ∼100 to ∼1,000 K.
See Hollenbach and Tielens (1997) and Hollenbach and Tielens (1999).
QMMIC – Quasi-MMIC. An analog microwave circuit with all components inte- grated except the transistors, which are fabricated separately and bump-bonded to the QMMIC substrate. SeeMMIC.
RF– Radio Frequency. See heterodyne mixer.
sideband – In a heterodyne receiver, the signal frequency can be higher or lower than the local oscillator frequency. Upper sideband (USB) refers to the case when RF>LO, and lower sideband (LSB) when RF<LO. A single sideband (SSB) receiver only detects one of the sidebands, whereas a double sideband (DSB) receiver detects both sidebands simultaneously. See heterodyne mixer.
SIS mixer – Superconductor-Insulator-Superconductor mixer. A heterodyne mixer based on an SIS superconducting tunnel junction. The mixing process involves quan- tum tunneling through the insulating layer. SIS mixers are currently the most sensi- tive coherent detectors in the range from around 100 GHz to over 1 THz. See Tucker and Feldman (1985), Zmuidzinas and LeDuc (1992), and Bin (1997).
SSB– Single sideband. See sideband.
USB – Upper sideband. Seesideband.
velocity – Since the shape of spectral lines in molecular clouds is typically deter- mined by Doppler shifts, spectra are often plotted as functions of Doppler velocity rather than frequency. Spectral lines in galaxies are typically hundreds of kilometers per second wide.
WASP– Wideband Analog SPectrometer. A lag correlation spectrometer being de- veloped at the University of Maryland that has a moderate number of channels (128) and can be configured for large frequency coverage, normally 4 GHz. See Harris and Zmuidzinas (2001).