The local oscillator chain consists of a Gunn oscillator, an isolator, a harmonic mixer, a waveguide attenuator, a tripler, and a dielectric lens. The whole assembly is bolted to the dewar. These components are drawn and identified in Figure 3. The Gunn has the usual two backshorts. The positions of the backshorts are controlled by two micrometers. The micrometer perpendicular to the waveguide axis controls the the size of the Gunn diode cavity, and thus controls the oscillator frequency. The other micrometer driven backshort controls the match of the diode to the waveguide and is often referred to as the power backshort. There is of course some interaction between the two backshorts. A tuning sheet for the Gunn is attached. A waveguide isolator is next in line after the Gunn. The isolator allows power to travel in only one direction and thereby eliminates reflected power from entering the Gunn's output port. A directional coupler is attached to the output of the isolator. The directional coupler has two output ports. One port carries away about 10% of the Gunn's power. This power is used to drive the harmonic mixer. The harmonic mixer provides an error signal for the phaselock box. A waveguide attenuator is attached to the second port of the directional coupler. It is used to vary the amount of L.O. power that strikes the tripler. The last component in the L.O. chain is the tripler itself. The tripler was built by Millitech and utilizes a self-biasing, tunerless design. The hardest part of tuning almost any millimeter/submillimeter receiver is adjusting the multiplier. Because the 230 GHz multiplier (tripler) is tunerless, no adjustments are necessary; making the 230 GHz receiver relatively simple to tune.