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Radiometric Receivers

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A radiometric receiver is used in passive sensing, such as radio astronomy. A key characteristic of a radiometric receiver is that it usually has incredibly wide bandwidth.

One classic radiometric receiver is the Dicke radiometer. Robert Dicke developed it during WWII when he worked at the Rad Lab. He appears in our Microwave Hall of Fame!  The Dicke radiometer is characterized by a switchable resistor (the Dicke switch) on the input that is used to calibrate noise temperature. Here is a web site that provides a block diagram of the Dicke radiometer.

http://www.astro.ucla.edu/~wright/COBE-artists-conceptions.html

Here some advice from Terry that started on our discussion board:

A radiometer is pretty much what the title says – it’s a device that measures incoming radiation. It consists of a device to collect the signal and a device to measure the power of the collected signal. This applies to any type of radiation – light, thermal, microwave.

For a microwave radiometer, the collector is an antenna and the detector is usually a square-law detector. The directionality of the antenna allows you to spatially select the incoming radiation. The antenna pattern determines a surface area which defines the area over which the signal is collected. The output is the sum total of the power incident on that area at all frequencies within the bandwidth of the antenna and detector. That’s it, pretty simple concept and you have within your reach a perfect example of a radiometer: your cell phone! It has an antenna that collects microwave signals and it has a detector that measures the power of the collected signal and displays it on your phone as a series of vertical bars. The more bars, the more power.

In your lab you probably have a power meter. Hook that up to an antenna and you have a radiometer. If your lab has a spectrum analyzer, hook that up to antenna and you have a radiometer that displays the power at each frequency instead of the sum total that the power meter reads. All the rest is about how to separate the radiation of interest from all the other radiation collected. If the radiation you wish to measure is coherent and not very noisy, it is possible to integrate the radiometer signal over time. The desired signal will continue to accumulate as time goes on and the undesired noise will tend to average out if it is truly random noise.

A Dicke radiometer has the same components as above except it has a switch and a reference signal. The reference signal can be another antenna pointed away from the desired radiation or a simple fixed load. The switch is placed between the antenna and detector and alternately selects between the desired radiation antenna signal and the reference signal (the load or other antenna). In this case you subtract the detected reference signal from the detected collection of signals from the radiation source you wish to measure. Atmospheric variations and detector gain fluctuations below the frequency of the switching speed of the Dicke switch tend to be suppressed. Unwanted signals like RFI (Radio Frequency Interference) that are collected by your system are always a problem and a Dicke switch is just one of the many ways that are used to mitigate unwanted signals. Similarly, any known signal that clutters up your output can be subtracted or calibrated out of the measurement. A superheterodyne receiver allows you to use an LO to increase the frequency selectivity and decrease the out of band noise. Mostly you go to a mountain top in a very isolated place away from civilization and its electronic signals. For more information you might read this National Radio Astronomy Observatory publication:www.cv.nrao.edu/course/astr534/Radiometers.html. I would also refer you to a unique, excellent introductory book, Microwave Radiometer System: Design and Analysis, Neils Skou, Artech, 1989, 0-89006-368-0. And to the new website: www.caltechmicrowave.org.

 

Author: The Other Editor
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