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The Antenna Base Receiver

From the Common Box, the signal is brought down via a coaxial cable21.8 to the Antenna Based Receiver (ABR), which is housed in a shielded room inside the antenna shell21.9 Figure 21.4 shows a schematic block diagram (and also gives the expected signal levels for a 32 MHz bandwidth) at different stages of the ABR, and the nominal values to be set for the pre and post attenuators (see below).

Figure 21.4: Schematic block diagram of the antenna base receiver. The nominal values that the attenuators should be set to, as well as the expected power levels at different stages are also shown. See the text for more information.

The High pass filter (HPF) at the input of the ABR has a rejection of about 40 dB at 70 MHz and provides the IF rejection21.10. After mixing the signal power level can be adjusted using a variable attenuator21.11 (which can be set from 0 to 30 dB in steps of 2 dB). After this the the signal passes through a SAW filter where one of three bandwidths (32 MHz, 16 MHz and 5.5 MHz) can be chosen. The net gain of the filter is independent of the chosen bandwidth due to the incorporation of bandwidth compensating gain circuitry. The signal is then up-converted to either 130 or 175 MHz (depending on which polarization it is), passed through further gain and an attenuation21.12 and then an Automatic Level Controller, (ALC). The ALC ensures that the even if the RF signal level varies (for eg. because you point at a bright source) the signal level at the IF remains at the optimum level for transmission through the optical fiber21.13The IF powers are continuously monitored and the monitoring data is sent to the CEB. The recommended IF power level is $-20$ dBm per polarization.

At the final stage of the ABR, the LO round trip carriers, the monitoring data as well as the astronomical signal are combined in a 5 way combiner and sent to the CEB via the optical fiber link.


... cable21.8
The loss in this cable is a strong function of frequency. This fact can be used to advantage in the bypass mode for image rejection. In the bypass mode if one places the I LO above the RF of interest, the image frequency is suppressed due to the greater attenuation at higher frequencies.
... shell21.9
The concrete structure on which the dish rests is called in local parlance the ``antenna shell''.
... rejection21.10
i.e. prevents passage of a 70 MHz signal from the RF directly through to the 70 MHz IF stages.
... attenuator21.11
Which in local parlance is called the ``pre-attenuator''.
... attenuation21.12
Also settable from 0 to 30 dB in steps of 2 dB, and called the ``post-attenuator''.
... fiber21.13
The ALC has a time constant of the order of 0.1 seconds. This can produce artifacts in signals (eg. pulsars) whose short timescale structure is of interest. For such observations there is a provision to switch the ALC off.

next up previous contents
Next: The Fiber-Optic Link Up: GMRT Receivers Previous: The Multi Frequency Front   Contents