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Most of the present-day operating radioastronomy equipment has been constructed in a similar manner. It was produced usually shortly after the WWII \glos{WWII}{Second World War} or during the Cold War as a part of the military technology.
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Most of the present-day operating radioastronomy equipment has been constructed in a similar manner. It was produced usually shortly after the WWII \glos{WWII}{Second World War} or during the Cold War as a part of the military technology.
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Today, we have an access to components with much higher quality, repeatability and a lower price as compared to the components accessible to previous generation of radioastronomers. That is why we can develop a better radioastronomical equipment, powerful enough to make new astronomical discoveries possible.\fnote{Most of astronomy-related discoveries in the last fifty years came from the field of radioastronomy.}
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Today, we have an access to components with much higher quality, repeatability and a lower price as compared to the components accessible to previous generation of radioastronomers. That is why we can develop a better radioastronomical equipment, powerful enough to make new astronomical discoveries possible.\fnote{Most of astronomy-related discoveries in the last fifty years came from the field of radioastronomy.}
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We have the capacities necessary to develop a receiver which will have a wide bandwidth, a high third-order intercept point and preferably an option for phase and frequency locking to other receivers located at another radioastronomical site on the Earth. Currently there exist several receivers with the above-mentioned parameters, for example USRP2, USRP B210 \glos{USRP}{Universal Software Radio Peripheral} or HackRF which are commercially available. However all of them lack scalability and have higher prices unaffordable to our amateur radioastronomical network. Scalability and redundancy are the main requirements of noise reduction algorithms which acted as a motivation for this diploma thesis.
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We have the capacities necessary to develop a receiver which will have a wide bandwidth, a high third-order intercept point and preferably an option for phase and frequency locking to other receivers located at another radioastronomical site on the Earth. Currently there exist several receivers with the above-mentioned parameters, for example USRP2, USRP B210 \cite[USRP-sdr] \glos{USRP}{Universal Software Radio Peripheral} or HackRF \cite[hackrf-sdr] which are commercially available. However all of them lack scalability and have higher prices unaffordable to our amateur radioastronomical network. Scalability and redundancy are the main requirements of noise reduction algorithms which acted as a motivation for this diploma thesis.
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New radio astronomy systems such LOFAR\glos{LOFAR}{Low-Frequency Array} are explicit examples of the scalability and redundancy approach. LOFAR has a completely different and novel structure developed to solve the problems of radioastronomy signal reception. It uses exclusively multi-antenna arrays and mathematical algorithms for signal handling. Radio signals recorded by LOFAR can be used in multiple ways: radio images can be computed (if sufficient cover of $u/v$ plane is achieved), the radiation intensity can be measured, the spectrum can be analysed for velocity measurement, etc.
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New radio astronomy systems such as LOFAR\glos{LOFAR}{Low-Frequency Array} are explicit examples of the scalability and redundancy approach. LOFAR has a completely different and novel structure developed to solve the problems of radioastronomy signal reception. It uses exclusively multi-antenna arrays and mathematical algorithms for signal handling. Radio signals recorded by LOFAR can be used in multiple ways: radio images can be computed (if sufficient cover of $u/v$ plane is achieved), the radiation intensity can be measured, the spectrum can be analysed for velocity measurement, etc.
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\sec Required receiver parameters
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\sec Required receiver parameters
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The novel approach of the receiver construction described above goes hand-in-hand with the new requirements on receiver parameters as well. No additional attempts to improve the signal-to-noise ratio on single antenna have been performed currently. There are however other parameters requested nowadays.
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The novel approach of the receiver construction described above goes hand-in-hand with the new requirements on receiver parameters as well. No additional attempts to improve the signal-to-noise ratio on single antenna have been performed currently. There are however other parameters requested nowadays.
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Sensitivity and noise number are parameters that are tied together. However, the requirement for multi-antenna and multi-receiver arrays forces to keep the prices of receivers at the minimal value. This implies that the sensitivity and noise number have to be at least as good in the detection (signal $/$ noise $>$ 1 ) of an observed object, that it would be detected by the majority of receivers connected to an observational network.
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Sensitivity and noise number are parameters that are tied together. However, the requirement for multi-antenna and multi-receiver arrays forces to keep the prices of receivers at the minimal value. This implies that the sensitivity and noise number have to be at least as good in the detection (signal $/$ noise $>$ 1 ) of an observed object, that it would be detected by the majority of receivers connected to an observational network.
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\label[dynamic-range-theory] \secc Dynamic range
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\label[dynamic-range-theory] \secc Dynamic range
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The dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by humans which are present everywhere and create electromagnetic inference (EMI\glos{EMI}{Electromagnetic interference}) radiation in the radio frequency (RF\glos{RF}{Radio frequency}) band. The modern radioastronomy receiver must not be saturated by these high levels of signals but still needs to have enough sensitivity to see faint signals from natural sources. The dynamic range is limited either by the construction of analog circuitry in the receiver or by the digitization unit.
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The dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by humans which are present everywhere and create electromagnetic inference (EMI\glos{EMI}{Electromagnetic interference}) radiation in the radio frequency (RF\glos{RF}{Radio frequency}) band. The modern radioastronomy receiver must not be saturated by these high levels of signals but still needs to have enough sensitivity to see faint signals from sources being observed. The dynamic range is limited either by the construction of analog circuitry in the receiver or by the digitization unit.
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The maximal theoretical dynamic range of analog-to-digital converter (ADC \glos{ADC}{analog-to-digital converter}) could be estimated from ADC bit depth using a following formula~\ref[dynamic-range]
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The maximal theoretical dynamic range of analog-to-digital converter (ADC)\glos{ADC}{analog-to-digital converter} could be estimated from ADC bit depth as
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\label[dynamic-range]
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\label[dynamic-range]
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$$
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$$
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D.R. \; [dB] = 20 \cdot \log(2^n) \eqmark
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dynamic range \; [dB] = 20 \cdot \log(2^n) \eqmark
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$$
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$$.
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The formula \ref[dynamic-range] provides values shown in Table~\ref[ADC-dynamic-range] below.
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The maximal theoretical dynamic range for standard bit depths of ADC is shown in Table \ref[ADC-dynamic-range].
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\midinsert \clabel[ADC-dynamic-range]{Dynamic range versus bit depth}
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\midinsert \clabel[ADC-dynamic-range]{Dynamic range versus bit depth}
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\ctable{cc}{
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\ctable{cc}{
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\hfil ADC Bits & Dynamic range [dB] \cr
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\hfil ADC Bits & Dynamic range [dB] \cr
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8 & 48 \cr
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8 & 48 \cr
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If we look at the actual spectrum occupancy in Europe (measured in the power spectral density), we see that the signal dynamic range in spectra easily reaches more than 80 dB above the natural noise levels~\cite[spectrum-observatory]. If we do not want to deal with the receiver saturation or the poor sensitivity, we need a receiver and digitization unit which has a dynamical range comparable with received signals. This implies the use of at least 14 bit ADC without any spare range. However, the 16 bit range should be optimal as it offers some spare range for the strongest RF signals. The two bytes of sample range have in addition a good efficiency in the use of standard power of 2 data types length. We have selected the 16 bit digital range as the optimal one for our design.
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If we look at the actual spectrum occupancy in Europe (measured in the power spectral density), we see that the signal dynamic range in spectra easily reaches more than 80 dB above the natural noise levels~\cite[spectrum-observatory]. If we do not want to deal with the receiver saturation or the poor sensitivity, we need a receiver and digitization unit which has a dynamical range comparable with received signals. This implies the use of at least 14 bit ADC without any spare range. However, the 16 bit range should be optimal as it offers some spare range for the strongest RF signals. The two bytes of sample range have in addition a good efficiency in the use of standard power of 2 data types length. We have selected the 16 bit digital range as the optimal one for our design.
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\secc Bandwidth
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\secc Bandwidth
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Historically, the bandwidth parameter in radioastronomical receivers used to be within the kilohertz range. Such a narrow bandwidth was acceptable because observations were processed directly by listening or by a paper chart intensity recorder. The chart recorder integrated the energy of a signal over a defined narrow bandwidth which was suitable for detecting the intensity variance of a microwave background. No wide-band transmitters existed in that era (except for TV\glos{TV}{Television} transmitters) and tuning to other neighbouring frequency was easy as they were mostly vacant. Parallel observations from several places were unnecessary as well because the electromagnetic conditions were nearly the same at all locations.
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Historically, the bandwidth parameter in radioastronomical receivers used to be within the kilohertz range. Such a narrow bandwidth was acceptable because observations were processed directly by listening or by a paper chart intensity recorder. The chart recorder integrated the energy of a signal over a defined narrow bandwidth which was suitable for detecting the intensity variance of a microwave background. No wide-band transmitters existed in that era (except for TV\glos{TV}{Television} transmitters) and tuning to other neighbouring frequency was easy as they were mostly vacant. Parallel observations from several places were unnecessary as well because the electromagnetic conditions were nearly the same at all locations.
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Currently we are not able to use an clear part of radio-frequency spectrum for radioastronomical observations.
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%%
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\sec State of the art in receivers' digitization units
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\sec State of the art in receivers' digitization units
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Only a few digitization systems dedicated to radioastronomy exist currently. Today's systems use either a custom design of the whole receiver or they are constructed from commercially available components. Open-source principle attempts are very rare in the radioastronomy field.
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Only a few digitization systems dedicated to radioastronomy exist currently. Today's systems use either a custom design of the whole receiver or they are constructed from commercially available components. Open-source principle attempts are very rare in the radioastronomy field.
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\secc Custom digitization system
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\secc Custom digitization system
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Custom designs usually use non-recurring engineering for the development of a specific solution for an observational project. Consequently, such instruments are very costly if the developed instrument is not reproduced many times. A typical example of the instrument developed and manufactured in a single piece with enormous funding requirements was the Arecibo ALFA\glos{ALFA}{Arecibo L-Band Feed Array}.
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Custom designs usually use non-recurring engineering for the development of a specific solution for an observational project. Consequently, such instruments are very costly if the developed instrument is not reproduced many times. A typical example of the instrument developed and manufactured in a single piece with enormous funding requirements was the Arecibo ALFA\glos{ALFA}{Arecibo L-Band Feed Array} \cite[alfa].
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Another example, this time a custom-designed receiver and digitization unit design but duplicated many times is LOFAR system developed by Astron in the Netherlands~\cite[lofar]
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Another example, this time a custom-designed receiver and digitization unit design but duplicated many times is LOFAR system developed by Astron in the Netherlands~\cite[lofar]
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LOFAR is an innovative radioastronomy system which uses a phased antenna array approach at an enormous scale. Thousands (around $2 \cdot 10^4$) of antennas are manufactured an deployed in the field. The centrer of LOFAR system is situated in the Netherlands and peripheral antennas and a connection network are extended to other European countries.
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LOFAR is an innovative radioastronomy system which uses a phased antenna array approach at an enormous scale. Thousands (around $2 \cdot 10^4$) of antennas are manufactured an deployed in the field. The centrer of LOFAR system is situated in the Netherlands and peripheral antennas and a connection network are extended to other European countries.
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\midinsert
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\midinsert
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\clabel[lofar-antenna]{Lofar antenna configuration}
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\clabel[lofar-antenna]{Lofar antenna configuration}
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\picw=10cm \cinspic ./img/lofar_antenna.jpg
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\picw=10cm \cinspic ./img/lofar_antenna.jpg
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\caption/f One LOFAR LBA\glos{LBA}{Low Band Antenna} antenna element.
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\caption/f One LOFAR LBA\glos{LBA}{Low Band Antenna} antenna element.
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\endinsert
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\endinsert
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LOFAR project must use a low cost hardware due to the system scale. A special construction techniques were employed to keep the overall project budget at acceptable levels (specially designed polystyrene supporting blocks for High Band Antenna (HBA\glos{HBA}{High Band Antenna}) for example). Many of used components are manufactured on a massive scale for other than scientific use. LBA antennas' masts are made from a standard Polyvinyl chloride (PVC\glos{PVC}{Polyvinyl chloride}) plastic waste pipes. LOFAR uses low cost direct sampling receiver. The entire project was designed by the Netherlands Institute for Radio Astronomy which produces many similarly sophisticated devices~\cite[astron-devices].
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LOFAR project must use a low cost hardware due to the system scale. A special construction techniques were employed to keep the overall project budget at acceptable levels (specially designed polystyrene supporting blocks for High Band Antenna (HBA\glos{HBA}{High Band Antenna}) for example). Many of used components are manufactured on a massive scale for other than scientific use. LBA antennas' masts are made from a standard Polyvinyl chloride (PVC\glos{PVC}{Polyvinyl chloride}) plastic waste pipes \ref[lofar-antenna]. LOFAR uses low cost direct sampling receiver. The entire project was designed by the Netherlands Institute for Radio Astronomy which produces many similarly sophisticated devices~\cite[astron-devices].
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\secc Modular digitization systems
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\secc Modular digitization systems
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Due to cost restrictions in science and astronomy instruments development, a reuse of engineering work is preferable. There is one example of a modular digitization and data processing system. The system Collaboration for Astronomy Signal Processing and Electronics Research (CASPER\glos{CASPER}{Collaboration for Astronomy Signal Processing and Electronics Research}) has been in development at the University of Berkeley~\cite[casper-project] since around 2005. CASPER designers and engineers noticed a remarkable lack of such a hardware in radioastronomy science. Their ideas are summarised in the paper~\cite[casper-paper]. Unfortunately they use a proprietary connector standard and technology. They have developed a modular system based purely on Tyco Z-DOK+ connectors family. CASPER data processing board with Z-DOK connectors is shown in Figure~\ref[casper-roach]. Z-DOK connectors have a relatively high pricing (around 40 USD)~\cite[Z-DOK-connectors], but are high quality differential pairs connectors. However, the price of these connectors is comparable with the price of one ADC channel in the design described in this diploma thesis.
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Due to cost restrictions in science and astronomy instruments development, a reuse of engineering work is preferable. There is one example of a modular digitization and data processing system. The system Collaboration for Astronomy Signal Processing and Electronics Research (CASPER\glos{CASPER}{Collaboration for Astronomy Signal Processing and Electronics Research}) has been in development at the University of Berkeley~\cite[casper-project] since around 2005. CASPER designers and engineers noticed a remarkable lack of such a hardware in radioastronomy science. Their ideas are summarised in the paper~\cite[casper-paper]. Unfortunately they use a proprietary connector standard and technology. They have developed a modular system based purely on Tyco Z-DOK+ connectors family. CASPER data processing board with Z-DOK connectors is shown in Figure~\ref[casper-roach]. Z-DOK connectors have a relatively high pricing (around 40 USD)~\cite[Z-DOK-connectors], but are high quality differential pairs connectors. However, the price of these connectors is comparable with the price of one ADC channel in the design described in this diploma thesis.
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