Efficiency of Asymmetric Switching with a Spectrally Selective Radiometer

N. E. Kol’tsov

Transactions of IAA RAS, issue 55, 10–15 (2020)

DOI: 10.32876/ApplAstron.55.10-15

Keywords: the spectrum-selective radiometer, asymmetry switching, accuracy, precision and sensitivity of the radiometer

Abstract

The problem. The RT-32 radio telescopes′ traditional radiometers with square-law detectors of broadband noise signals are replaced by spectrally selective radiometers which provide high accuracy of radiometric measurements of the energy parameters of radio emission under the influence of radio interference. These radiometers are used for regular observations of cosmic emission sources with variable intensity of radio emission and for monitoring changes in signal levels from standard (reference) space radio sources. For such studies, the sensitivity of the radiometer and the accuracy of measuring the power and noise temperatures of the received radio signal are of paramount importance. In radiometric mea¬surements, the receivers of radio telescopes operate in the square wave modulation mode of the gain and the noise genera¬tor, thus simulating the antenna noise. Time for signal receiving (τs) and time for noise registration (τn) equal half of the switching period tmod; time for signal integration (ts) equals time for noise integration (tn). The radiometer must be accurate enough for investigating radio emission from remote cosmic source and to measuring a weak change of the signal power. But it is not always possible to increase the sensitivity of the radiometer and the accuracy of measurements by increasing the signal integration time. The time for signal integration ts is limited because it denominates result updating when the source is scanning. The aim. The purpose of the paper is to clarify the possibilities of increasing the radiometer sensitivity and the accuracy of measuring the power of the radio signal received due to the asymmetric switching on the noise generator and the receiving device. Method. The efficiency coefficient is calculated as a ratio of the radiometer sensitivity with asymmetric switching to the sensitivity of the radiometer with the usual symmetric square wave switching. Three operating modes of the radiometer are considered: modulation by a sqare wave when the noise accumulation time exceeds the signal accumulation interval (τs = τn); debalanced modulation (τs > τn) with the same signal and noise accumulation intervals (tn = ts); asymmetric switching (τs > τn) with asymmetric accumulation (tn > ts). Result. The radiometer with symmetry switching and not symmetry integration (τs = τn, tn > ts) makes it possible to increase the sensitivity of a spectrally selective radiometer (by 21–29 %) compared to a radiometer operating in the usual mode of symmetric (at equal time intervals) accumulation of signal and noise. This method is easy to implement on RT-32 radio telescopes. In the asymmetric modulation mode with asymmetric accumulation it is possible to additionally (by 3–6 %) increase the sensitivity of the radiometer but this requires the improvement of the modulation units of the receiving device. The asymmetry switching with symmetry integration (τs > τn, tn = ts) is unacceptable as the sensitivity of the radiometer is only reduced.

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