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Requirement for a Local Oscillator System at the GMRT
The GMRT Analog Receiver, in its simplest form, can be considered
as a 2terminal black box, as given in Figure 23.1.
Figure 23.1:
A Twoterminal representation of the GMRT analog receiver system.

The receiving element (i.e. any of the dual linearly polarised
feed systems of the GMRT) is connected at the input of the black box
and provides a signal consisting of:
 Thermal noise power kTB.^{23.1}
 The astronomical signal, which is usually much weaker than the
thermal noise power.
 Unwanted Radio Frequency Interference (RFI), which could occur
anywhere in the frequency spectrum and is often much stronger
than the thermal noise power.
The output of the black box is in baseband frequency range, which
is typically from DC to a maximum of 16 MHz. The upper value determines the
maximum instantaneous bandwidth of GMRT. The baseband signals are then
digitized by a sampler. The nominal power level needed for the sampler
is 0 dBm. Since the typical input power level is dBm, the gain
within the black box is about 100 dB.
This large amplification has to be achieved while simultaneously
providing the desired bandlimiting^{23.2} and spurious free dynamic range^{23.3} in the presence of strong RFI. For this, the electronics
system within the black box has been implemented as a heterodyne receiver,
where the RF signal from the receiving element is converted to the
baseband signal via different stages of frequency translation (see also
Chapter 3). This frequency translation requires
multiple Local Oscillator signals.
Footnotes
 ... kTB.^{23.1}
 where k is the Boltzman
constant, T is the system temperature and B is the bandwidth
of the signal. At the GMRT for a 32 MHz bandwidth this power
is typically of the order of 100 dBm.
 ... bandlimiting^{23.2}
 i.e. one needs to filter
the signal so that only frequencies within the band of astronomical
interest are accepted.
 ... range^{23.3}
 i.e.
one needs to ensure that the entire system is sufficiently linear
so that RFI at one frequency does not produce spurious spikes at other
frequencies.
Next: The Frequency Translation Scheme
Up: Local Oscillator and Baseband
Previous: Local Oscillator and Baseband
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NCRATIFR