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\secc Frequency synthesis
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\secc Frequency synthesis
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We have used a centralized topology as a basis for frequency synthesis. One precise high-frequency and low-jitter digital oscillator has been used \cite[MLAB-GPSDO], while other working frequencies have been derived from it by the division of its signal. This central oscillator has a software defined GPS disciplined control loop for frequency stabilization.\fnote{SDGPSDO design has been developed in parallel to this diploma thesis as a related project, but it is not explicitly required by the diploma thesis.}
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We have used a centralized topology as a basis for frequency synthesis. One precise high-frequency and low-jitter digital oscillator has been used \cite[MLAB-GPSDO], while other working frequencies have been derived from it by the division of its signal. This central oscillator has a software defined GPS disciplined control loop for frequency stabilization.\fnote{SDGPSDO design has been developed in parallel to this diploma thesis as a related project, but it is not explicitly required by the diploma thesis.}
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We have used methods of frequency monitoring compensation in order to meet modern requirements on radioastronomy equipment which needs precise frequency and phase stability over a wide scale for effective radioastronomy imaging.
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We have used methods of frequency monitoring compensation in order to meet modern requirements on radioastronomy equipment which needs precise frequency and phase stability over a wide scale for effective radioastronomy imaging.
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GPSDO device consists the Si570 chip with LVPECL output. Phase jitter of GPSDO is determined mainly by Si570 phase noise. Parameters of used Si570 from source ... are summarised in table.
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GPSDO device consists the Si570 chip with LVPECL output. Phase jitter of GPSDO is determined mainly by Si570 phase noise. Parameters of used Si570 from source ... are summarised in table \ref[LO-noise].
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\midinsert \clabel[LO-noise]{Available ADC types}
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dBc/Hz
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\ctable{lcc}{
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& \multispan2 Phase Noise [dBc/Hz] \cr
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Offset Frequency 156.25 MHz & 622.08 MHz
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Offset Frequency & $F_out$ 156.25 MHz & $F_out$ 622.08 MHz \cr
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100 Hz & –105 & –97
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100 [Hz] & –105 & –97 \cr
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1 kHz & –122 & –107
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1 [kHz] & –122 & –107 \cr
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10 kHz & –128 & –116
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10 [kHz] & –128 & –116 \cr
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100 kHz & –135 & –121
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100 [kHz] & –135 & –121 \cr
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1 MHz & –144 & –134
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1 [MHz] & –144 & –134 \cr
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10 MHz & –147 & –146
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10 [MHz] & –147 & –146 \cr
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100 MHz & n/a & –148
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100 [MHz] & n/a & –148 \cr
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}
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\caption/t The summary of available ADC types and theirs characteristics.
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\endinsert
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Every ADC module will be directly connected to CLKHUB02A module which takes sampling clock signal delivered by FPGA from main local oscillator. This signal should use high quality differential signaling cable -- we should use SATA cable for this purpose.
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Every ADC module will be directly connected to CLKHUB02A module which takes sampling clock signal delivered by FPGA from main local oscillator. This signal should use high quality differential signaling cable -- we should use SATA cable for this purpose. FPGA may slightly affect clock signal quality by additive noise, but has negligible effect in application where developed system will be used.
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GPSDO design included in data acquisition system has special feature -- generates time marks for precise time-stamping of received signal. Timestamps are created by disabling of local oscillator for 100 us as result rectangle click in input signal is created which appears as horizontal line in spectrogram.
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GPSDO design included in data acquisition system has special feature -- generates time marks for precise time-stamping of received signal. Timestamps are created by disabling of local oscillator for 100 us as result rectangle click in input signal is created which appears as horizontal line in spectrogram.
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Timestamps should be seen in image \ref[meteor-reflection] (above and below meteor reflection).
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Timestamps should be seen in image \ref[meteor-reflection] (above and below meteor reflection).
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Time-marking should be improved in future by digitalisation GPS signal directly with dedicated ADC channel. Datafile then consists samples from channels of radio-astronomy receivers along with GPS signal containing precise time information.
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Time-marking should be improved in future by digitalisation GPS signal directly with dedicated ADC channel. Datafile then consists samples from channels of radio-astronomy receivers along with GPS signal containing precise time information.
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