The simplified block diagram of frequency translation scheme used to convert the RF signals from each antenna of the GMRT to the base-band signals required by the sampler is given in Figure 23.2. A schematic of the typicaly used mixing scheme at the GMRT is shown in Figure 23.3.
The pair of RF signals from the Front End (FE), which are typically in the range of 30 to 1660 MHz are initially converted to a first IF signal centered at 70 MHz. The choice of the first IF frequency has been decided by the availability of commercial sharp cut-off band-pass filters with a wide choice of bandwidth at this frequency23.4. This translation from RF to the first IF needs a first LO signal, which should be tunable at least over the range of 100 to 1590 MHz. The GMRT is designed to simultaneously process two RF signals (RF1 and RF2, also called the 130 signal and the 175 signal respectively). These signals could be either (a) two polarisation signals at the same RF band, or (b) one polarisation signal in each of two different RF bands. To cater to case (b), we need two independent first LO sources.
The pair of first IF signals are brought from each antenna to the central electronics building (CEB) by a single optical fibre for further processing. Hence, we need to separate the two first IF signals (centered at 70 MHz) in the frequency domain before they can be combined and fed to the optical transmitter (OTx) unit. For this, one of the first IF signals is translated to a second IF signal centered at 130 MHz and the other, to another second IF centered at 175 MHz. The choice of these centre frequencies has been decided by (a) The maximum bandwidth of the first IF signal and (b) the need to keep the overall band occupancy on the fibre to within an octave. The value for the two second LO signals chosen for the GMRT are hence 200 and 105 MHz. At the CEB the reverse translation is done, after the optical receiver (ORx) unit, to produce a pair of third IF signals, centered at 70 MHz. This requires two third LO signals, at 200 and 105 MHz respectively.
The last stage of frequency translation is to base-band, using a fourth LO signal, which can be set to any frequency from 50 to 90 MHz in 100 Hz steps. The step size is determined by the need to incorporate online doppler tracking, so that a spectral line under observation can be confined to a specified channel in the correlator throughout the entire observation.