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\chap Trial version of the receiver, design and implementation
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\chap Trial version of the receiver, design and implementation
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The whole design of radioastronomy receiver digitalization unit is constructed to be used in a wide range of applications and tasks related to digitalization of signal from radioastronomy receivers. A good illustrating problem for its use is a signal digitalisation from multiple antenna arrays.
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The whole design of the radioastronomic receiver digitization unit is meant to be used in a wide range of applications and tasks related to digitization of a signal. A good illustrating problem for its use is the signal digitization from multiple antenna arrays.
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\midinsert
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\midinsert
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\clabel[expected-block-schematic]{Expected system block schematic}
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\clabel[expected-block-schematic]{Expected system block schematic}
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\picw=\pdfpagewidth \setbox0=\hbox{\inspic ./img/Coherent_UHF_SDR_receiver.png }
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\picw=\pdfpagewidth \setbox0=\hbox{\inspic ./img/Coherent_UHF_SDR_receiver.png }
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\par\nobreak \vskip\wd0 \vskip-\ht0
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\par\nobreak \vskip\wd0 \vskip-\ht0
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\caption/f Expected realisation of signal digitalisation unit.
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\caption/f Expected realisation of signal digitalisation unit.
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\endinsert
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\endinsert
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\sec Required parameters
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\sec Required parameters
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We require the following technical parameters, to supersede existing digitalization units solutions.
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Primarily, we need wide a dynamical range and high IP3. \glos{IP3}{Third-order intercept point} The receiver must accept wide dynamic signals because a typical radioastronomical signal has a form of a weak signal covered by a strong man-made noise or other undesired noises as lighting, Sun emissions etc.
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We require the following technical parameters in order to overcome the existing digitization units solutions. Primarily, we need a wide a dynamical range and a high third-order intercept point (IP3\glos{IP3}{Third-order intercept point}). The receiver must accept signals with the wide dynamics because a typical radioastronomical signal is a weak signal covered by a strong man-made noise or other undesired noises as lighting, Sun emissions, etc.
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Summary of other additional required parameters follows
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\medskip
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\noindent
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The summary of other additional required parameters:
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%
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\begitems
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\begitems
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  * Dynamical range better than 80 dB, see section \ref[dynamic-range-theory] for explanation
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  * Dynamic range better than 80 dB, see section \ref[dynamic-range-theory] for the explanation.
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  * Phase stability between channels
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  * Phase stability between channels.
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  * Low noise (all types)
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  * Low noise (all types).
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  * Sampling jitter better than 100 metres
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  * Sampling jitter better than 100 metres.
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  * Support for any number of receivers in the range of 1 to 8
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  * Support for any number of receivers in the range of 1 to 8.
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\enditems
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\enditems
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\noindent
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Now we analyze several of the parameters in detail.
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We analyze several of the parameters more in detail in the sequel.
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\sec Sampling frequency
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\sec Sampling frequency
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Sampling frequency is not limited by the technical constrains in the trial version. This parameter is especially limited by the sampling frequencies of analog-to-digital conversion chips available on the market and interface bandwidth. Combination of the required parameters -- dynamic range requiring at least 16bit and a minimum sampling frequency of 1$\ $MSPS \glos{MSPS}{Mega-Samples Per Second} leads to the need of high end ADC chips which does not support such low sampling frequencies at all. Their minimum sampling frequency is 5$\ $MSPS.
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The sampling frequency is not limited by the technical constrains in the trial version. This parameter is especially limited by the sampling frequencies of the analog-to-digital conversion chips available on the market and interface bandwidth. Combination of the required parameters -- dynamic range requiring at least 16bit and a minimum sampling frequency of 1$\ $MSPS \glos{MSPS}{Mega-Samples Per Second} leads to the need of high end ADC chips which does not support such low sampling frequencies at all. Their minimum sampling frequency is 5$\ $MSPS.
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We calculated a minimum data bandwidth data rate for eight receivers, 2 bytes per sample and 5$\ $MSPS as $8 \cdot 2 \cdot 5\cdot 10^6 = 80\ $MB/s. Such data rate is at the limit of the actual writing speed of classical HDD \glos{HDD}{Hard disk drive} and it is almost double the real bandwidth of USB 2.0 \glos{USB 2.0}{Universal Serial Bus version 2.0}  interface. As a result of these facts we must use faster interface. Faster interface is especially needed in cases where we require faster sampling rates than ADC's minimal 5$\ $MSPS sample rate.
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We calculated a minimum data bandwidth data rate for eight receivers, 2 bytes per sample and 5$\ $MSPS as $8 \cdot 2 \cdot 5\cdot 10^6 = 80\ $MB/s. Such data rate is at the limit of the actual writing speed of classical HDD \glos{HDD}{Hard disk drive} and it is almost double the real bandwidth of USB 2.0 \glos{USB 2.0}{Universal Serial Bus version 2.0}  interface. As a result of these facts we must use faster interface. Faster interface is especially needed in cases where we require faster sampling rates than ADC's minimal 5$\ $MSPS sample rate.
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The most perspective interface for use in our type of application is USB 3.0 or PCI Express interface. However, USB 3.0 is a relatively new technology without good development tools currently available. We have used PCI Express \glos{PCI Express}{Peripheral Component Interconnect Express}  interface as the simplest and the most reliable solution.
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The most perspective interface for use in our type of application is USB 3.0 or PCI Express interface. However, USB 3.0 is a relatively new technology without good development tools currently available. We have used PCI Express \glos{PCI Express}{Peripheral Component Interconnect Express}  interface as the simplest and the most reliable solution.
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\sec System scalability
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\sec System scalability
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The GPSDO device consists of Si570 chip with LVPECL output. Phase jitter of GPSDO \glos{GPSDO}{GPS disciplined oscillator} is determined mainly by Si570 phase noise. Parameters of the Si570 are summarized in the following table \ref[LO-noise] (source \cite[si570-chip] ).
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The GPSDO device consists of Si570 chip with LVPECL output. Phase jitter of GPSDO \glos{GPSDO}{GPS disciplined oscillator} is determined mainly by Si570 phase noise. Parameters of the Si570 are summarized in the following table \ref[LO-noise] (source \cite[si570-chip] ).
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The GPSDO design, that is included in data acquisition system, has special feature -- it generates time marks for a precise time-stamping of the received signal. Timestamps are created by disabling the local oscillator's outputs, connected to SDRX01B receivers, for 100 us.  As result, a rectangular click in the ADC input signal is created which appears as a horizontal line in spectrogram.
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The GPSDO design, that is included in data acquisition system, has special feature -- it generates time marks for a precise time-stamping of the received signal. Timestamps are created by disabling the local oscillator's outputs, connected to SDRX01B receivers, for 100 us.  As result, a rectangular click in the ADC input signal is created which appears as a horizontal line in spectrogram.
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Timestamps should be seen in image \ref[meteor-reflection] (above and below the meteor reflection).
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Timestamps should be seen in image \ref[meteor-reflection] (above and below the meteor reflection).
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Time-marking should be improved in future by digitalization of GPS signal received by antenna on observational station. Following that, the GPS signal can be directly sampled by a dedicated receiver and one separate ADC module. Datafile then consists of samples from channels of radio-astronomy receivers along with the GPS signal containing precise time information.
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Time-marking should be improved in future by digitization of GPS signal received by antenna on observational station. Following that, the GPS signal can be directly sampled by a dedicated receiver and one separate ADC module. Datafile then consists of samples from channels of radio-astronomy receivers along with the GPS signal containing precise time information.
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\midinsert \clabel[LO-noise]{Phase noise of the local oscillator}
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\midinsert \clabel[LO-noise]{Phase noise of the local oscillator}
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\ctable{lcc}{
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\ctable{lcc}{
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	&	 \multispan2 \hfil Phase Noise [dBc/Hz] \hfil 		\cr
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	&	 \multispan2 \hfil Phase Noise [dBc/Hz] \hfil 		\cr