DC Processors for Radiotelescopes

Once the radio frequency energy has been converted to a DC signal by the detector, we need to transform it in other ways that make it easier to record.  Even though we have tried very hard to not introduce much additional noise from our receiver into the signal, there will usually be much more of this unwanted noise at this point than the actual noise we are trying to measure. In other words the output of the detector will consist of a lot of receiver noise added to a small amount of noise from the cosmic source. Lets say our recording system could measure from 0 to 5 volts. If we were to amplify our DC signal to fill most of this range, only a small change in the recorded output would be due to the source.  What we need to do is remove most of the noise contributed by the receiver before we amplify the DC signal by a large amount. This function is provided by an offset circuit which simply subtracts a steady DC voltage from the signal voltage.  This is easily accomplished with a operational amplifier connected as a voltage adder.

Even though we refer to the detected signal as a DC (direct current) signal, it still varies rapidly in intensity because it retains much of its noise character. The smoothing out of these rapid fluctuations is accomplished by an integrator. The integrator function is accomplished by using a capacitor as a holding tank for the incoming signal. Imagine that the signal is water coming through a hose and that the water pressure fluctuates in this hose. If we empty the hose in a large water tank and take the outflow of water from a small tap in the bottom of the tank, it is easy to imagine that the fluctuations in water pressure will be largely absent from our outflow tap. The integrator performs an additional service in that by averaging the signal over time it greatly increases the sensitivity of the measurement.

Lastly out DC processor amplifies the detected signal to a level where it matches the range of our recording device. The amplification function as well as the other functions of the DC processor is usually accomplished by use of integrated circuits called operational amplifiers. It is very important to use high quality "op amps" and other components in the DC processor.