Ratiometric Dicke Radiometer PW East – September 2013

A ratiometric Dicke switch type of radio telescope radiometer has been described by Meech1 that compensates for system gain variations. It periodically switches a calibrating noise source in parallel with the signal path and calculates the ratio of the received power with and without the calibration source so cancelling any receiver gain variation. The output ratio is R, where,

R = (Tsky +Tsys)/(Tsky+Tsys+Tcal)

Clearly, with a knowledge of Tsys and Tcal, the wanted temperature Tsky can be calculated. As with standard Dicke processing, this approach is appropriate for narrow-band total power systems.

For comparison, the standard Dicke switch receiver subtracts the two switch states to give the output S, where,

S = ((Tsky + Tsys) – (Tcal + Tsys))Gsys

The system gain still appears in the calculation but if Tcal is adjusted to equal Tsky, the effect of gain variations is minimised. An alternative is to AGC the output with the AGC control signal taken when the switch is in the Tcal position.

A simpler ratiometric processing scheme2 offered here, is to take the ratio of the standard Dicke switch states, which removes the gain sensitivity completely and is suitable for broadband spectral processing, as the output ratio R, becomes,

R = (Tsky + Tsys)/(Tcal + Tsys)

Knowing Tsys and Tcal, the wanted temperature Tsky and the calibration for R is simply obtained. This scheme has the advantage that it compensates for IF gain variations with frequency when applied to wideband digital FFT systems. The factor 2 loss in sensitivity is still apparent. However the loss can be avoided if the receiver switch is operated manually and the calibration done off-line, averaging for a period considerably longer than when measuring Tsky so that measurement uncertainty is only significant when switched to the antenna. Gain drift between switch states only affects the absolute calibration. IF gain shape and Tsky spectral shape and amplitude is still preserved. In principle, spectral averaging can be extended to achieve very high temperature sensitivity.

References 1. Simulations of a Ratiometric Switching Radiometer for Radio Astronomy. Marcus Leech, VE3MDL. www.sbrac.org/documents/ratiometric_radiometer.doc 2. Cheap Hydrogen Line Radio Telescope for £160 using the RTL SDR. PW East. http://www.y1pwe.co.uk/RAProgs/RadAst.doc Issue 2. 19/11/2013