Effective Dynamic Range of Digitizers in the OVRO-LWA Telescope

Abstract

In radio astronomy, very coarse signal quantization is acceptable because of the noise-like nature of the signals [1]. Thus, 2-level (1 bit) quantization was used in early spectrometers and VLBI, 3-level quantization was used in the original VLA, 4-level (2 bit) is still used in VLBI, and 8-level (3 bit) quantization is used in wide-bandwidth modes of the current JVLA and ALMA. This limits quantization noise to a small fraction of the system noise, provided that the signal level to the digitizer is optimally set [1]. However, additional resolution is sometimes needed, for three reasons. First, a wide-bandwidth receiving system may have gain and noise temperature that vary considerably with frequency, making the spectrum at the digitizer non-flat. This requires a digitizer whose voltage range accommodates a higher total power than if the spectrum were flat, while still providing enough resolution at the weakest-signal frequency. Consequently, more quantization levels (bits) are needed. Second, the desired signal (system noise, including instrumental and astronomical noise) may be accompanied by undesired signals ("RFI"). If the total power in RFI is comparable to or greater than the total power in the desired signal, then the digitizer voltage range must be even larger to avoid saturation when the RFI is present. Third, the system noise power and receiver gain may vary with time, making it difficult to maintain the optimum level at the digitizer input. In this memo, the implications for the OVRO LWA telescope are investigated. The telescope has a desired frequency range of 20 to 85 MHz, and the over that range the signal from one antenna has a significant spectral dynamic range because of both the sky brightness variation and the antenna gain variation. Outside that range, there is substantial RFI due to FM radio (88-108 MHz) and HF communication signals reflected from the ionosphere (5-20 MHz, mostly). The out-of-band RFI can be suppressed by filtering, so this study is partly for the purpose of determining how much suppression is needed.

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