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Heterodyne receivers mix a local oscillator frequency,
, to the
astronomical signal to transfer the frequency axis from high radio
frequencies,
, to low intermediate frequencies,
, which
are then suitable for easy electronic processing. A consequence is that two
bands of radio frequencies, namely the lower side band (LSB) and the upper
side band (USB) are mixed in the observed spectrum, i.e., the set of
brightnesses,
, are associated to one IF frequency axis and two RF
frequency axes linked through
![\begin{displaymath}
\ensuremath{f_\ensuremath{\mathrm{IF}}^{\ensuremath{\mathrm...
...ensuremath{\mathrm{LO}}^{\ensuremath{\mathrm{obs}}}} \right] }
\end{displaymath}](img110.png) |
(51) |
with
for the RF frequency axis belonging to the lower side
band (LSB) tuning and
for the RF frequency axis belonging to
the upper side band (USB). The measured spectra is thus in the IF axis and
Eq. 1 must be modified as
 |
(52) |
with
 |
(53) |
The constancy of
is a property of the heterodyne receivers.
Combining the last 3 equations, we yield
 |
(54) |
Next: Signal and image frequency
Up: Heterodyne receivers and signal/image
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Gildas manager
2015-03-01