Parkes RFI Studies - Digital TVReference Fault Report - Communicated by email:50cm RFI
The RFI, resulitng from licensed transmissions of new digital TV, from Mt Canonolas (100km in distance), Mt Ulandra (200km in distance), and potentially, from Mt Sugarloaf (near Newcastle, more than 300km away), manifests itself in the 50cm portion of the new 10cm/50cm receiver. The new 50cm receiver is designed to cover 640-704MHz, bandwidth 64MHz, and replaces the older 50cm receiver, centred at 660MHz, with a bandwidth of 36MHz. A snapshot of the analogue TV and digital TV is shown below. Note from the display, that there are a number of strong analogue and digital TV transmissions, just outside the band of interest for the 50cm receiver. The levels of these just out-of-band transmitters, are potentially allowing 3rd order intermodulation products falling within the 50cm RF spectrum of interest. Using the information available from the Australian Communications Authority (ACA) database for licensed transmitters, the spectrum allocation for Digital TV, and Analogue TV, which may potentially cause interference to the new 50cm receiver are given in the link below.
The 50cm receiver utilizes room temperature LNAs, external to the cryogenic receiver package, where the 10cm LNAs are housed. The 50cm amplifiers are mounted at the output of the feed system, from which, the signals are taken to the 50cm radio frequency (RF) module. In the RF module, at the moment, image rejection/pre-selection filters are used prior to a 2 stage amplifier system, for eventual transmission of the signals to the ATNF Parkes Conversion system. There are plans to replace the 1st stage of amplifiers in the RF module, with higher 1dB compression point amplifiers, and to move the image rejection/pre-selection filters post the 1st stage amplifiers. This procedure will, hopefully, alleviate the potential for spurious 3rd order intermodulation products from manifesting themselves in the RF spectrum of interest. Below, is a snapshot of the analogue and digital TV, as measured with the Caltech-Parkes-Swinburne Recorder Mk2 (CPSR2), showing possible intermodulation effects.