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The RF tuning frequency,
, can be choosen in either band.
This band is called the signal band. The other band is then called the
image band. The associated RF frequencies are written
and
. We obtain
 |
(55) |
and
 |
(56) |
The sign of the product
is arbitrary. Indeed, it
only defines how the brightnesses are ordered in memory, i.e., whether the
frequency increases or decreases when
increases. It thus is simpler
to assume that this product is always positive. This implies that the
signal frequency increases and the image frequency decreases when
increases. In other words, we adopt the convention that the signal
brightnesses are ordered by increasing frequency when
increases,
i.e., the product
is always positive.
As the absolute value of
is 1, we will greatly simplify the
equations by replacing
with
with the
convention that
in Eqs 55 and 56. We
can then rewrite them as
 |
(57) |
and
 |
(58) |
with
 |
(59) |
Next: Signal and image frequency
Up: Heterodyne receivers and signal/image
Previous: USB and LSB frequency
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2015-03-01