One might choose to drop the phase rotation on the second LO and let the fringes drift at their natural fringe rates. These rates are opposed in sign for the USB and LSB, and they might be separated electronically. However the natural fringe rate sometimes goes to zero (when the angular distance between source and baseline direction is minimum or maximum), and at least in these cases the method would fail.
It would be more practical to offset the LO1 and LO2 phase rates
and
from their nominal values by the
same amount
. If the offsets have the same sign, they
will compensate for the USB and offset the fringe rate by
in the LSB. If
is large enough, the LSB
signal will be cancelled.
Note that offsetting
by a fixed amount is
equivalent to offsetting the LO1 frequency.
This is a simple method to reject the unwanted sideband. Note that the associated noise is not rejected.
Assume a variable phase offset is added to the LO1 phase command appropriate for compensating the geometrical delay variation:
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(7.14) |
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(7.15) |
One may also switch the phase by , in which case the sign of all
the correlated voltages is reversed. This has the advantage of
suppressing any offsets in the system. Actually both switching
cycles are combined in a 4-phase cycle:
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Signal |
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(7.16) |
In a antenna system one needs to switch the relative phases of all
antenna pairs. This could be done by applying the above square-wave
switching on antenna 2, then on antenna 3 at twice the switching
frequency, and so on. In practice the switching waveforms are
orthogonal Walsh functions.