| 53,10 → 53,8 |
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| Dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by everywhere present humans made EMI radiation on RF frequencies. The modern radio astronomy receiver must not be saturated by this high levels of signals but still needs to have enough sensitivity to see faint signals from natural sources. Dynamic range is limited either by the construction of analogue circuitry in receiver or by the digitalisation unit. |
| The maximal theoretical dynamic range of ADC could be estimated from ADC bit depth using a following formula \ref[dynamic-range] |
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| %\clabel[dynamic-range] |
| $$ |
| D.R. (dB) = 20 * log(2^n) |
| $$ |
| \label[dynamic-range] |
| $$ D.R. (dB) = 20 * log(2^n) $$ |
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| The formula \ref[dynamic-range] gives values shown in table below \ref[ADC-dynamic-range]. |
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| 63,12 → 61,12 |
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| \midinsert \clabel[ADC-dynamic-range]{Dynamic range versus bit depth} |
| \ctable{cc}{ |
| \hfil ADC Bits & Dynamic range [dB] \cr |
| 8 & 48 \cr |
| 10 & 60 \cr |
| 12 & 72 \cr |
| 14 & 84 \cr |
| 16 & 96 \cr |
| 24 & 144 \cr |
| 8 & 48 \cr |
| 10 & 60 \cr |
| 12 & 72 \cr |
| 14 & 84 \cr |
| 16 & 96 \cr |
| 24 & 144 \cr |
| } |
| \caption/t Standard bit depths of ADC and its theoretical dynamic range. |
| \endinsert |
| 77,7 → 75,7 |
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| If we look at actual spectrum occupancy in Europe (measured in power spectral density) we see that signal dynamic range in spectra |
| \url{http://observatory.microsoftspectrum.com} |
| easily reaches 80+ dB above natural noise levels. If we don't want to deal with receiver saturation or poor sensitivity we need a receiver and digitalization unit which has comparable dynamical range of with received signals. This imply use of least 14 bit ADC without any spare of range. |
| easily reaches more than 80 dB above natural noise levels. If we don't want to deal with receiver saturation or poor sensitivity we need a receiver and digitalization unit which has comparable dynamical range of with received signals. This imply use of least 14 bit ADC without any spare of range. |
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| \secc Bandwidth |
| 84,52 → 82,25 |
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| Historically, the parameter of bandwidth in radioastronomical receiver used to be within the kilohertz range. Small bandwidth was acceptable because observations were processed directly by listening or by paper chart intensity recorder. Chart recorder integrated energy of signal over defined small bandwidth which was suitable for detecting the intensity variance of microwave background. No wide-band transmitters existed in that era (except for TV 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 same at all locations. |
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| \sec Current status of receivers digitalization units |
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| % dopsat navaznost na aktualne pouzivane digitalizacni jednotky |
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| Only few digitalization systems dedicated for radioastronomy currently exists. Currently existing systems uses either custom design of whole receiver or they are constructed from commercially available components. Open-source principle attempts are very rare in radioastronomy field. |
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| \secc Custom digitalization system |
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| Custom designs usually uses non-recurring engineering for development specific solution for observation project thus costs of this instruments are very high if developed instrument are not reproduced many times. Typical instrument developed in one piece with leads to enormous founding resources draws is Arecibo ALFA survey multi beam feed Array. |
| Custom designs usually uses non-recurring engineering for development specific solution for observation project thus costs of this instruments are very high if developed instrument are not reproduced many times. Typical example of instrument developed and manufactured in one piece with enormous founding resources draws is Arecibo ALFA survey multi beam feed Array. |
| Another opposite example for custom receiver and digitalization unit design is LOFAR system developed by Astron in Netherlands. \url{http://arxiv.org/abs/1305.3550} |
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| LOFAR is innovative radioastronomy system which uses the phased antenna array approach in enormous scale and thousands of antennas are manufactured an deployed on field. The center of LOFAR system is situated in Netherlands and periferal antennas and network are extended to other European countries. |
| LOFAR is innovative radioastronomy system which uses the phased antenna array approach in enormous scale and thousands (around $2*10^4$) of antennas are manufactured an deployed on field. The centrer of LOFAR system is situated in Netherlands and peripheral antennas and connection network are extended to other European countries. |
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| LOFAR project must use low cost hardware due to systems scale. Special construction techniques are used to keep overall project budget at acceptable levels (specially designed polystyrene supporting blocks for HBA antennas for example). Many of used components are manufactured in mass scale for other than scientific use LBA antennas masts are made from standard PVC plastic waste pipes and LOFAR uses low cost direct sampling receiver. Whole project has been designed by Netherlands Institute for Radio Astronomy, which produces many similarly sophisticated devices. \url{http://www.astron.nl/other/desp/competences_DesApp.htm} |
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| LOFAR project must use low cost hardware due to project scale to keep overall project budget at acceptable levels. LOFAR in order of thisuses direct sampling receiver technology |
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| ASTRON ADCs |
| http://www.astron.nl/other/desp/competences_DesApp.htm |
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| \secc Modular digitalization systems |
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| Due to cost restrictions in science and astronomy instruments development, an reuse of engineering work should be useful. One modular digitalization and data processing system currently exit. It is being developed at Berkley \url{https://casper.berkeley.edu/wiki/Main_Page}. CASPER is in development from around 2005. CASPER's designers an engineers remarkably noticed a lack of such hardware in radioastronomy science, theirs ideas are summarised in paper \url{https://casper.berkeley.edu/papers/200509URSI.pdf}. Unfortunately they use proprietary connector standard and technology and develops modular system based purely on Tyco Z-DOK+ connectors family. Z-DOK connectors have relatively high pricing (around 40 USD) \url{http://www.digikey.com/product-detail/en/6367550-5/6367550-5-ND/2259130}. Z-DOK connectors are high quality differential pairs connectors, but price of these connectors is comparable with value of one ADC channel in our design described in following part of document. |
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| One modular digitalization system currently exit. It is beaing developed at Berkley |
| In opposite to professional astronomers which uses proprietary digitalization units, amateur radioastronomers currently uses multichannel sound cards \url{http://fringes.org/} or self designed digitalisation units. Devices constructed by amateurs are usually non reproducible \url{http://wwwhome.cs.utwente.nl/~ptdeboer/ham/sdr/} . It is evident that current radioastronomy lacks of proper hardware which could be used on both communities - professionals and amateurs. Optimal solution for this situation should be open-source hardware. |
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| \url{https://casper.berkeley.edu/wiki/Main_Page} |
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| Z-DOK connectors have relatively high pricing around 40 USD http://www.digikey.com/product-detail/en/6367550-5/6367550-5-ND/2259130 |
| This price is comparable with value of one ADC channel in our design described in following part of document. |
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| Mudular radioastronomy hardware: https://casper.berkeley.edu/papers/200509URSI.pdf |
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| In opposite of professional astronomers which uses proprietary digitalization units, amateur radioastronomers currently uses multichannel sound cards \url{http://fringes.org/} |
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| It is evident that current radioastronomy lacks of proper hardware which could be used on both communities professionals and amateurs. In addition open-source hardware |
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