| 26,29 → 26,29 |
|---|
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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 at 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 radioastronomy network. Scalability and redundancy that are the main requirements of noise reduction algorithms which motivated this diploma project. |
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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 multiantenna 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. |
| 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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| \sec Required receiver parameters |
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| The novel approach of receiver construction described above goes hand-in-hand with new requirements on receiver parameters as well. Currently no additional attempts to improve the signal-to-noise ratio on single antenna are performed. There are however other parameters requested nowadays. |
| The novel approach of the receiver construction described above goes hand-in-hand with 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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| \secc Sensitivity and noise number |
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| Sensitivity and noise number are parameters that are tied together, but multi antenna and multi-receiver arrays force the price of receiver to be kept at minimal value. This implies that the sensitivity and noise number have to be at least so good in the detection (signal $/$ noise $>$ 1 ) of an observed object, that it would be detected on the majority of receivers connected to an observation network. |
| Sensitivity and noise number are parameters that are tied together. However, the requirement for multi-antenna and multi-receiver arrays forces to keep the price of receiver at the minimal value. This implies that the sensitivity and noise number have to be at least so good in the detection (signal $/$ noise $>$ 1 ) of an observed object, that it would be detected on the majority of receivers connected to an observation network. |
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| \secc Dynamic range |
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| \label[dynamic-range-theory] |
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| Dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by everywhere present humans made EMI \glos{EMI}{Electromagnetic interference} radiation on RF \glos{RF}{Radio frequency} 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 \glos{ADC}{analog-to-digital converter} could be estimated from ADC bit depth using a following formula \ref[dynamic-range] |
| The dynamic range represents a huge problem of current radioastronomical receivers. This parameter is enforced by humans present everywhere and creating electromagnetic inference (EMI\glos{EMI}{Electromagnetic interference}) radiation on 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 the analog circuitry in the receiver or by the digitization unit. |
| 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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| \label[dynamic-range] |
| $$ |
| D.R. [dB] = 20 \cdot \log(2^n) \eqmark |
| D.R. \; [dB] = 20 \cdot \log(2^n) \eqmark |
| $$ |
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| The formula \ref[dynamic-range] gives values shown in table below \ref[ADC-dynamic-range]. |
| The formula \ref[dynamic-range] provides values shown in Table~\ref[ADC-dynamic-range] below. |
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| \midinsert \clabel[ADC-dynamic-range]{Dynamic range versus bit depth} |
| \ctable{cc}{ |
| 63,13 → 63,13 |
|---|
| \caption/t Standard bit depths of ADC and its theoretical dynamic range. |
| \endinsert |
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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 |
| easily reaches more than 80 dB above natural noise levels \cite[spectrum-observatory]. 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. But 16 bit range should be optimal as we have spare range for strongest RF signals. Two bytes sample range has in addition a good efficiency in use standard power of 2 data types length. We lock for use 16 bit digital range as optimal for our design. |
| 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 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 comparable dynamical range with received signals. This implies the use of 14 bit ADC at least without any spare of range. However, the 16 bit range should be optimal as we have spare range for the strongest RF signals. Two bytes sample range has in addition a good efficiency in use standard power of 2 data types length. We selected the 16 bit digital range as the optimal one for our design. |
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| \secc Bandwidth |
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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 \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 same at all locations. |
| 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 same at all locations. |
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| \sec State of the art receivers digitalization units |
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