Radio Astronomy

The Radio Telescope Pre-Amplifier

Cosmic radio signals are generally very weak. To measure them we have to amplify them by factors of millions of times. The electronic components in our radio telescope produce random electrical noise which also gets amplified by this huge amount. It is easy to see that the noise from the early parts of the chain of amplifiers gets multiplied by more than the later stages. If we are not careful, this noise can totally hide the weak noise we are trying to measure from the cosmic radio source. The role of the preamplifier is to boost this incoming signal from the antenna many times while adding as little noise as possible. The preamplifier is often called an LNA or low noise amplifier. Special transistors are used in this stage to accomplish this. Professional observatories also use cooling of the amplifiers to very low temperatures, just a few degrees above absolute zero to minimize the amount of noise contributed by the components. You will sometimes hear amateur radio astronomers and other people interested in receiving weak signals refer to the noise figure of an amplifier. We won't get into the specifics of what this means but a noise figure of less than .5 decibels (dB) is considered very good.

The Radio Telescope Local Oscillator 

The local oscillator produces a signal which is injected into the mixer along with the signal from the antenna in order to effectively change the antenna signal to a frequency which can be handled by the IF amplifier. Many radio telescopes use a quartz crystal derived local oscillator signal. Quartz crystal oscillators are quite stable and drift little in frequency. Because most radio telescopes are quite broadband in nature, a small amount of frequency drift in the local oscillator may be tolerable. One must be careful, however, that the drift is not so great that received signal frequency begins to infringe upon the bandpass of the antenna or any front end filters that precede the mixer. Also, it is possible to drift into an interfering signals frequency. When either of these things happen, it can result in a change in the output of the telescope which could be mis-interpreted as a real change in received cosmic noise

Radio Telescope IF Amplifiers

The intermediate frequency, ( IF ), amplifier is a radio frequency amplifier which processes the output of the mixer. In addition to amplifying the signal, the IF amp usually has some form of bandpass filtering so that only a selected range of frequencies is allowed through. These filters are often of the SAW, crystal lattice, or ceramic varieties. Common, IF frequencies are 70, 45, 21.4, and 10.7 MHz, however there is no restriction to these frequencies. One difference between most radio telescope IF amps and those found in communications receivers is that communications receivers usually employ some form of automatic gain control, (AGC). AGC circuits need to be disabled in communications receivers modified for radio astronomy use as they tend to mask the subtle changes in signal strength we are trying to detect.

The amount of gain needed for the IF amplifier is determined by the signal level exiting from the mixer, the amount lost in the IF amp filter(s) and the appropriate level required for the square law detector which follows. It is sometimes difficult to determine all of these values ahead of time and so a variable attenuator is sometimes introduced at the IF amp output prior to the detector

Radio Telescope Square Law Detectors

If you become a dedicated amateur radio astronomer, you will no doubt hear much talk about "square law detectors". It is interesting that one of the simplest electronic configurations in the radio telescope show draw so much attention, but all of this attention is due to its' very important role. The radio frequency energy exiting from the earlier portions of the receiver alternates in polarity around some central voltage. If we could just hook up a DC (direct current) meter to this signal it would read zero volts because the positive and negative swings of the voltage would cancel each other out. In order to measure the intensity of the signal we thus must throw half of it away! We need a door which only allows passage of the signal in one direction and this is the semiconductor diode. Even the symbol we use for the diode suggests this quality, an arrowhead pointing towards a line.

There are several types of diodes. The types most commonly used by amateur radio telescope makers are the germanium diode and the schottky diode. If we pass just the right range of current through these diodes, the voltage we measure coming out of them will be the square of the input, and thus will be proportional to the power which is fed to them from the receiver. It is the power received by the radio telescope antenna that we want to measure and we owe to detector the ability to measure it. If this is a bit confusing, fear not. You will no doubt have time to sort it out in one of those discussions I refer to above.

DC Processors for Radio Telescopes

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.

Radio Telescope Recording Devices

To be of any value, the output of a radio telescope must be recorded. For the simple radio telescope described here, what we want is a record of how strong the signal is over time. If we are using drift scan observations, we can relate the time a particular value of signal strength was recorded to where in the sky our antenna was pointed. The result is often called a strip chart. Below is a great example of a strip chart from the web pages of the radio observatory at the University of Indianapolis in Indiana. This is a chart of the radio source, Taurus A, taken as the rotation of the Earth moved the beam of their antenna across this region in the constellation Taurus

Mapping

Drift scans are nice, but what if you want to make a map of the skys radio emissions ? Well, all you have to do is plot drift scans of the sky at a series of elevations separated by somewhat less than the angular beamwidth of your antenna. If you live in the northern hemisphere, you could begin by pointing at an elevation close to the north celestial pole near Polaris, and running your telescope for 24 hours. If you had a beamwidth of say 10 degrees, you would then lower the elevation by about five to seven degrees and making a strip chart for that elevation. You would continue the process until the beam was point a bit above your horizon and then combine the data files to make a 2 dimensional map of the sky. In reality, there is quite a bit more to it than this. Calibrations half to be maintained and other factors like the interference and radio noise from the ground considered. Still, you get the idea.

If you intend to record sporadic events such as Jupiter's noise storms or meteor reflections with your radio telescope, you can use PC's sound card as a recording resource. Sound cards are great for recording sounds in these specialized cases, but remember, the output of the generalized radio telescope in our example is not sound, but a slowly varying voltage which corresponds to the amount of energy or antenna is picking up from the region of sky towards which it is pointed.

For more information Visit:    Jim Sky's website.  Radio-Sky Publishing,