(TSS notes here)

Introduction

Recent changes

For those with some familiarity with B3, here are some additional details:

• With the installation of the second mixer in channel A another moving part, the mixer backshort, was removed. The backshort drive mechanisms are still both in place, but without connection to the mixer block, and are set to a fixed value for all frequencies. The control software calling for tuning of the backshort positions has been disabled. Leaving the external backshort drives in place allows the spare tuned mixer to replace a mixer should one fail, since there is presently no spare.
• In mid-1999 the mixer in channel B was replaced with the new tunerless version, and the tuned mixer in channel A swapped for the one originally in channel B. The mixer block taken from channel A was sent back to HIA where it was been converted to a tunerless version. The original channel A had developed a nasty fault: an increase in noise by about 50% about 100 MHz wide in the center of the IF band. A postmortem on the mixer (see picture provided by Charles Cunningham) shows that the entire structure had been shifted off-center under pressure from the contact bellows.
• B3 has had a record of operational problems, which had considerably worsened in the early part of 1999. The main focus of the work during B3's protracted absence from the cabin in mid-1999 was an overhaul of the fairly complex control electronics, mechanical components and related software. This work took much longer than expected, and some revisions of the on-board microcomputer code were necessary.

Performance expectations for B3

No dedicated commissioning tests have been carried out so far, and although it appears that users can expect the modified B3 to behave almost identically to the previous versions of the receiver, all observers would be well advised to obtain, at the least, spectra toward standard sources for comparison with existing data. It would be useful also to make observations especially of the planets Mars and Uranus if possible in order to obtain antenna and beam efficiencies. Beam maps would be really nice to have, but need to be obtained only under stable and dry conditions to be useful.

B3 has been found to tune reasonably reliably between LO frequencies of 322 and 369 GHz; that is, sky frequencies from 318 to 373 GHz should be accessible with both mixers. In the absence of any faults B3 now tunes from one frequency to another within about 15 seconds. Some operational idiosyncracies still exist, but in general there are workarounds known to the telescope operators for those oddities that remain. Double-sideband receiver temperatures are about 110-120K for both mixers across the band with the exception of the upper reaches of channel A. We will provide plots showing the final numbers once we have had the opportunity to carry out this work.

Both channels of B3 are capable of observing a window 920 MHz wide. Channel B should be used for wideband work up to the spectrometer maximum of 1.8 GHz instantaneous bandwidth, since the band is somewhat limited by the HEMT amplifier in channel A.

The future