Requirement for a Local Oscillator System at the GMRT

The GMRT Analog Receiver, in its simplest form, can be considered as a 2-terminal black box, as given in Figure 23.1.

Figure 23.1: A Two-terminal 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:

1. Thermal noise power kTB.^23.1
2. The astronomical signal, which is usually much weaker than the thermal noise power.
3. 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 base-band frequency range, which is typically from DC to a maximum of 16 MHz. The upper value determines the maximum instantaneous bandwidth of GMRT. The base-band 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 $-100$ dBm, the gain within the black box is about 100 dB.

This large amplification has to be achieved while simultaneously providing the desired band-limiting^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 base-band 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.

... band-limiting^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.