The previous paper described the current state and future prospects of CMB experiments based on bolometric receivers. Here I intend to do the same for what are often loosely termed heterodyne receivers. The fundamental distinction is between systems that measure the total energy collected by the telescope, and those that convert the electric field of the incoming radiation to voltage which can be directly amplified. (The term `heterodyne' strictly refers to systems in which the signal is down-converted to an intermediate frequency.) The practical consequences of this distinction are that the two classes of experiment have different advantages and disadvantages, and suffer from different systematic effects. Bolometers are intrinsically broad-band and insensitive to polarization; both frequency and polarization response are defined by external filters. Amplifiers are intrinsically polarized, and one struggles to make a single amplifier with more than 50% bandwidth.
Bolometers are `intrinsically' more sensitive, largely due to their higher bandwidths, but in practice amplified systems obtain comparable sensitivity because much longer integration times are available. This is because the relative transparency of the atmosphere at centimetre wavelengths allows ground-based operation (albeit often at extreme sites). There is a further division of `electric field measuring' instruments: the field may simply be squared, to measure the received power; or the fields from two different anntennas may be multiplied, leading to the field of interferometry.