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The Side Band Filter
The Side Band Filter
The Dual Beam Interferometer (DBI)
- RxB3 has a dual beam (Mach-Zehnder) interferometer that
permits single or double sideband observations. The DBI has two movable
arms, though in practice the necessary path differences are achieved by
moving only one arm (number 1) under computer control. The normal operating
position as shown on the PF2 Display screen of arm 2 is 9.0. Unlike
many of the adjustable receiver parameters that are stored in look-up tables
as functions of frequency, the arm position is calculated for each
tuning.
Side Band Rejection
- When tuned in SSB mode, the image side band lines are
typically reduced
in intensity by about 13dB (i.e. a factor of ~ 20). Thus strong lines
in the blocked sideband may still appear weakly in the spectrum. Also, because
the transmission properties of the DBI are not quite flat across the passband,
there may be a small variation in this rejection ratio across the wider
spectral windows.
LO Power
- When tuned in DSB mode the interferometer transmits
both the LO frequency and
the side band frequencies unimpeded. However, in SSb the image band is
blocked and the transmission at the LO frequency is 50%. Consequently, when
the DBI is optimally tuned for DSB operation, the LO power to the mixers is
maximized. In SSB mode at the same frequency it is halved. Generally this is
not of importance but it is useful to keep mind if you're tuning to an extreme
frequency where LO power is at a premium. (But also remember that, especially
at the high frequency end, the atmosphere is relatively opaque and SSB
mode will generally produce significantly lower system temperatures.)
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