Introduction to B3
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User's Manual for B3
Basic Information
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Basics
Receiver B3 is a dual-channel heterodyne receiver for the 345 GHz (0.8
mm wavelength) band. Built mainly at the National Research Council's
HIA in Ottawa, in collaboration with SRON/University of Groningen in
Holland and CCLRC/RAL in the UK, it employs two low-noise Niobium SIS
junctions which can be used simultaneously to detect orthogonally
polarised radiation. Since the mixers are pumped by a single local
oscillator they will be tuned to the same frequency. The IF is 4.0
GHz, and thus the signal and image side bands are separated by 8.0
GHz.
Operational Characteristics
B3 is roughly 5 times faster than the single-channel receiver B3i
which it replaced in December 1996, and also offers improved
calibration, stability, and operational flexibility.
- A dual-beam interferometer (referred to as the "DBI") allows
either single- or double-sideband (SSB or DSB) operation.
- SSB operation is usually preferred, and in this mode the
detectors see the sky in the signal sideband, and look into a cold
load at the image sideband frequency, thereby enhancing sensitivity
and improved calibration under most sky conditions.
- DSB mode can be used to observe lines from both sidebands
simultaneously, with some loss of calibration control, or to obtain an
improved signal/noise ratio for continuum observations
(e.g. pointing/focussing).
- The receiver can be automatically tuned in about 10-15 seconds
under normal circumstances under remote computer control. If necessary
full or partial manual tuning can be carried out in the receiver
cabin.
- The available instantaneous bandwidths are limited only by the
autocorrelating spectrometer: up to 920 MHz using both mixers in
parallel, and up to 1.8GHz using one of the mixers.
- B3 usually has excellent baseline stability and all normal
switching schemes.
All of the standard JCMT observing and mapping procedures are
available with this receiver. These include position, beam and (slow
- i.e. at a rate of say, 30 secs) frequency switching. In raster
mapping mode, integration times of at least 3 seconds per point are
recommended to avoid timing problems in the data collection.
The receiver is controlled by the observatory computer system in
communication with a G-64/68302 microprocessor on board the receiver
electronics rack via a GPIB link. The receiver status is continuously
displayed and, if necessary, manipulated by means of a window which
can be opened on a dedicated terminal or the Telescope Operator's
terminal.
Historical Performance
A plot of the SSB receiver temperatures from first light until the
present can be obtained by clicking here.
This plot samples all data for LO frequencies between 330 and
360, and thus includes data for the commonly-observed lines CO 3-2 and
HCN 4-3. The data are color/symbol-coded according to frequency
settings according to the legend at the top right. Unfortunately this
plot, of about 22000 points, is not very helpful in revealing
details. However, it shows several things; for instance
- For the
most part the receiver performance is quite flat with frequency over
this band, as recent plots show, and has mostly remained so throughout
its lifetime at the JCMT. Points outside the main band of Trec values
generally indicates problem or test situations.
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B3 has been a consistent performer; there is only one significant gap,
when it was removed for extensive engineering tests.
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The receiver has become steadily noisier since it arrived; on
average there has been an increase from a mean Trec (SSB) of 110K in
1997 to about 150K now (mid-2002). This, surprisingly, is despite
having repaired a mixer or two and having converted the mixers
to a tunerless variety.
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B3 had a fairly difficult start for the first year as shown by the
excessive scatter at the extreme left of the plot.
A separate plot showing only a narrow range, for CO observed in the
lower sideband (the most common configuration at 345 GHz),
reveals a very similar behavior.
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