The Base-band System

The base-band system of GMRT processes the IF signals received from the antennas and makes them compatible for the correlator.

The maximum bandwidth of the IF signal is 32 MHz. Considering the fact that the correlator system can run at a maximum clock speed of around 40 MHz, a Single Side-Band (SSB) conversion with image rejection approach is used in the base-band. This results in two base-band signals, (the Upper Side Band, (USB) and the Lower Side Band (LSB)) each with a maximum bandwidth of 16 MHz, for each of the third IF signal. There are hence 120 base-band outputs resulting from 60 third IF signals (typically one from each of two polarizations) from 30 antennas of the GMRT. A simplified block diagram for the system to handle one of the third IF signals is given in Figure 23.4

Figure 23.4: A simplified block diagram of the base-band processing at the GMRT.

The input third IF signal at 70 MHz is converted to two base-band signals, the Upper Side Band (USB) corresponding to $70 + 16$ MHz and Lower Side Band (LSB), to $70 - 16$ MHz. This single side-band mixers are based on in-phase and quadrature-phase power dividers for the third IF and fourth LO as well as a broadband quadrature network in the base-band. The typical image rejection which has been achieved is 25 dB.

The base-band system has facility for a wide choice for bandwidth, from 62.5 kHz to 16 MHz, in octave steps. This is achieved in the variable low pass filter block. The power output from the system is kept constant by automatically increasing, the gain as the bandwidth is decreased, to keep the product constant. In addition the ALC stage ensures that the sampler is supplied with a constant power. For some applications (eg. Pulsar observations) however, the finite time constant of the ALC produces undersireable distortions of the astronomical signals. For these observations, the ALC can be switched off.