Pole STAR Performance Measurements We received the repaired 810 GHz local oscillator system from U. Mass last Friday (October 6). The LO works great and puts out 43 uW. Unlike most other 810 LO's, the multiplier is fixed tuned at our observing frequency and self-biased, making it ideal for polar operations. When coupled through our quasi-optical LO power divider and Martin-Puplett diplexer, each of the four Cologne mixers receives sufficient LO power to conduct performance tests. Indeed, the LO power to 2 of the mixers must be attenuated to keep them from going into saturation. Click here for a COOL image of a laser beam being diplexed by our quasi-optical LO power splitter. At optical wavelengths, the beams emerge with different intensities because of optical attenuation in the quartz "beamsplitters".

Figure P1: Repaired LO system installed on Pole STAR. Click on thumbnail for full-size image!

Below are figures showing the LO pumped IV curves of each of the 4 mixers together with IF power sweeps. The red curve shows the IF power output when a HOT (290K) load is placed in front of the receiver. The blue curve shows the response with a COLD (77K) load . For receivers channels 1, 2, and 3 we obtain Y-factors of ~1.2. Channel 4 currently has a Y-factor of 1.12. It is most likely that we will need to direct more LO power to mixer 4 to get better performance. The LO beam for mixer 4 is the one most sensitive to alignment in our LO power splitter box. With a bit more adjusting of the optics, we feel confident we can get this Y-factor up to the level obtained with the other mixers. A Y-factor of 1.2 corresponds to a receiver noise temp of ~970K and includes all loss (optics, windows, filters, IF amp, etc.) in the system.

Screen shots were made of the receiver passbands on a spectrum analyzer under HOT and COLD load conditions. These are shown for receiver #2 in the "Total Power Stability" figures below. As with all the mixers, the receiver response is found to be constant across the 1 GHz IF passband.

Receiver stability was measured by monitoring the IF output power of 2 of the receivers over ~1000 sec. The IF power variation was well less than 1 percent over this period. Strip charts of these measurements are shown below.

It should be noted that all the above measurements were made with the SIS junctions biased in `resistive-mode'. Typically, junctions are biased to maintain a constant bias voltage. Our bias system has this ability and we are now making the small modifications needed to permit this mode of operation with the Cologne mixers. Even in resistive mode, the system is stable enough for observing on AST/RO.

Beam Pattern Measurements A computer controlled antenna test range (Figure P2, right) was constructed to measure the 4 Pole STAR beams. We used a liguid nitrogen load with a 3/8" aperture as the signal source. The signal was chopped and synchronously detected using a lock-in amplifier connected to the receivers' total power output. Maps of the 4-beams and their relative placement are shown below. To within the measurement errors the beams have the right size and shape. One beam (from Mixer 3) appears to have a squint angle compared to the others. This squint is probably due to a small mechanical misalignment of the mixer. Once the dewar is warm, we will try adjusting the mixer mount. Even with the squint, the Mixer 3 beam will clear all apertures on AST/RO and make it onto the sky.

Figure P2: Daedal X-Y stages used for beam measurements. The location of the X-Y stages is approximately at the distance of the azimuth bearing for AST/RO. Click on thumbnail for full-size image!