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\chap Introduction
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In beginning of radioastronomy receivers were constructed as simple station with single antenna or multi antenna array with fixed phasing. This approach were used due to limits of previous electronics. Main challenges were noise number and sensitivity due to poor characteristic of active electronic components such transistors and vacuum tubes.
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Many of today radioastronomy equipments were constructed in this manner. They were constructed usually shortly after WWII or during The Cold War as parts of military technology. These systems are slowly modernised and complete new systems are constructed. ALMA, SKA..
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\sec Modern Radio astronomy receiver
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This new radio astronomy receivers have completely different approach to solve the problem of radioastronomy signal reception. They almost exclusive uses multi antenna arrays and mathematical algorithms for signal handling. Radio signal recorded by this metod can be used by many ways. Radio image can be computed (if sufficient cover of u/v plane is achieved), radiation intenzity can be measured, spectrum can be analysed for velocity measurement. etc.
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\secc Observation types
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Today radioastronomy knows several observation types.
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\begitems
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* Spectral observations
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* Intensity observations
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* Velocity observations
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\enditems
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All of these observations ideally needs high frequency resolution and stability. Wide observation bandwidth in hundreds of MHz is usually desirable for easier discrimination of source types.
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\sec Requirements
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This new approach of receiver construction has different requirement on receiver parameters. No signal to noise ratio on single antenna is improved. But other parameters are requested at now.
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\secc Sensitivity and noise number
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These parameters are are tied together, but multi antenna and multi receiver arrays requires to keep price of receiver at minimal values. This implicates that sensitivity and noise number must be least as good to detect (signal /noise > 1 ) observed object on majority of receivers connected to observation network.
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\secc Dynamic range
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Dynamic range is huge problem of current radioastronomy receivers. This parameter is enforced by anywhere present humans made EMI radiation on RF frequencies. The modern radio astronomy receiver must not be saturated by this high levels of signals.
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\secc Bandwidth
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From requirements mentioned above
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The system requires proper handling of huge amount of data.
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\sec Current radioastronomy problems
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Professional radioostoronomers uses uses proprietary digitalisation units \url{http://arxiv.org/abs/1305.3550} or by multichannel sound cadrd on amateur levels \url{http://fringes.org/}
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