By Flavio Falcinelli
A radiometer is a receiver which measures the average power of the radiation picked up by the antenna within its receiving "window", showing how the power of the received signal varies over time. As can be seen from the following figure, the quadratic detection and subsequent integration process does not preserve the spectral characteristics of the signal: it provides a single value that represents its average power within the receiver passband.
If you are using stable broadband receivers (the amplification factor of the system and the detector characteristics should not change during the measurement), you can reach very high sensitivities, especially thanks to the possibility of integrate the detected signal (moving average operation using many signal samples) with long time constants, admitted that the phenomenon to be studied is sufficiently stationary in time.
To study the signal in the frequency domain, highlighting its spectrum “signature” and identifying the various components of different frequency distributed within the bandwidth of the receiver, it is necessary to adopt a different structure.
Basic architectures of receivers used in radio astronomy: radiometers and radio-spectrometers.
Neglecting the complex and expensive architecture of the radio-spectrometer with banks of narrow-band filters, outdated and of little interest, the two types that can be useful are frequency scanning radio-spectrometer and FFT radio-spectrometer.
The first is a frequency conversion receiver (heterodyne principle) where the "receiving window " is periodically scanned by a local variable frequency oscillator which moves, inside of the intermediate frequency channel, a narrow portion of the frequencies received. At each step of the scan it is calculated signal power as in a radiometer. After the scan, we will have a complete representation of the received signal power spectrum.
This representation will be much more accurate (and slower), the narrower will be the bandwidth of the intermediate frequency channel filter with respect to the reception band. Moreover, the greater will be the constant of integration which performs the average of the channel power, the lower the amplitude of noise visible in the track and still slower the scan. These parameters are optimized as a function of the stationarity characteristics in time in the measured signal and of the required sensitivity for the receiving system.
To also obtain the phase information of various spectral components, as happens in laboratory vector analyzers, additional circuits are necessary.
A frequency scannig radio-spectrometer does not scan in real time the entire reception band: during each scanning period is measured only the small portion of frequencies selected from the local oscillator, as wide as the bandwidth of the intermediate frequency channel filter . To capture any rapid spectral changes is necessary to foresee a sufficiently "agile" scan, to the disadvantage of sensitivity related to the integration of the detected signal.
Many amateur radio telescopes that analyze the profile of the hydrogen line at 1420 MHz, for example, operate according to this principle, using radio amateur receivers such as spectrum analyzers in frequency scanning.
No particularly action is required in data processing: the output already contains the intensity information of each spectral component. You just have to "digitize" the detected signal at each scanning step with an analog-digital converter (ADC) not particularly fast, managing the measurement process via dedicated software on PC.
An efficient radio-spectrometer uses numerical techniques to calculate instantly spectrum of the signal present within the receiver passband. This is possible thanks to the evolution of modern electronic devices and to the computing power of Personal Computers (PC).
As can be seen from the block diagram, the structure of the FFT radio-spectrometer is conceptually very simple, although technologically advanced: the whole receiver passband (directly or after a translation in frequency) is acquired via a fast analog-to-digital converter and the relative samples in the time domain are converted into spectral samples (in the frequency domain) by a processor (it is almost always a PC managed by dedicated software) that uses the mathematical algorithm DFT (Discrete Fourier Transform), a numerical version of the Fourier Fast Transform (FFT).
It is thus obtained the instantaneous spectrum of the signals within the bandwidth of the instrument. It is obvious that, in case of the presence of a conversion stage (heterodyne) of the reception frequency, the local oscillator (fixed) should be characterized by high spectral purity and stability not to degrade the performance of the instrument.
The obvious advantage of this system is the possibility to analyze, in real time (latency calculation in part), high bandwidths in the frequency domain. Of course, the actual performances of the system are functions of the technological capability of the devices used (fast ADC), and the computing power of the processor that performs the DFT.
The instrument is widely used in the monitoring of disturbances in the frequency bands of radio astronomy interest, in the monitoring of space debris and meteorological events (tracking techniques with bi-static radar), in the study of molecular lines, in the SETI researches and in many other applications.
In the following chapters we will describe a radio-spectrometer made by us (RALSpectrum prototype) for the study of neutral hydrogen spectral line at 21 centimeters and a similar receiver built for Meteor Scatter experiments in VHF band.
Thanks to the spread of SDR architectures (Software Defined Radio), to the increase in PC computing power and the ability to find excellent free programs for spectral analysis on the web, you can now build economical radio-spectrometers usable in amateur radio astronomy applications.
Again, surfing the web you will find many examples and imaginative applications, ranging from the receipt of sporadic Jupiter's radiation and solar radio-storms in the frequency band from 20 MHz to over 40 MHz, to radio-echoes reception of meteors in the VHF band, to the study of the neutral hydrogen line at 1420 MHz and the SETI researches.
This is an industry in constant and rapid evolution: also in our pages we will present projects, experiments and products dedicated to the study in the domain of radio astronomical signals frequency.
A well-known example (and documented on the web) of "minimal" and economical radio-spectrometer usable for amateur radio astronomy experiments: it is a USB stick originally produced as a USB 2.0 DIGITAL TV TUNER RECEIVER, modified by us to improve the heat dissipation and immunity to external disturbances. By combining with this device antennas, low noise RF amplifiers (LNA) and, if appropriate, bandwidth filters to improve the reception dynamic and optimize the immunity to external interference, you may tune to frequencies from about 20 MHz to about 1500 MHz with an instantaneous bandwidth up to 10 MHz. On the web you can find free software and drivers available for download to turn these devices in "radio-scanner".