The local oscillator chain consists of a Gunn oscillator, a directional coupler, a harmonic mixer, a waveguide attenautor, and two multiplers; a doubler and a tripler. The Gunn was built by John Carlstrom and the multipliers by Radiometer-physics (Peter Zimmermann's company). These components are illustrated in Figure 3. The LO is on a vertical strip bolted to the front of the dewar. The Gunn has the usual two backshorts. The positions of the backshorts are controled by two micrometers. The micrometer perpendicular to the waveguide axis controls the size of the Gunn diode cavity, and thus controls the oscillator frequency. It is often referred to as the tuning backshort. 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. Attached to the Gunn is a directional coupler. The directional coupler has two output ports. One port carries away about 10% of the Gunn's output power. This power is used to drive a harmonic mixer. The harmonic mixer provides an error signal for the phaselock box. The waveguide attenuator is attached to the 2nd output port of the directional coupler. The multipliers are attached to the output of the attenuator. Each multiplier is biased individually and has two backshorts. This complexity is necessary for the multiplier to operate over a wide frequency range (425-500 GHz). The output of the tripler is launched into the optical system by way of a small Potter horn. From the Potter horn the LO beam expands until it reaches a polyethylene lens. The lens roughly parallizes the beam. The diameter of the LO beam is about the same as the telescope beam. The LO beam is then injected into the signal path by using a 0.25 mil Mylar beamsplitter. With the beamsplitter only about 1 to 2% of the LO power is injected into the signal path. The rest of the LO power is terminated in an absorbing load.