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\chap Testing construction

Whole design of radioastronomy receiver digitalization unit shoud be constructed for the most universal application in signal digitalisation from radioastronomy receivers. Ilustrating problem for its use is signal digitalisation from multiple antenna arrays. This design will be used as part of MLAB Advanced Radio Astronomy System. 

\sec Required parameters

Wide dynamical range and high  3 intercept point are desired. The receiver must accept wide dynamic signals because classic radioastronomy signal in typically weak signal covered by strong man made noise signal.    

\begitems

  * Dynamical range better than 80 dB

  * Phase stability between channels 

  * Noise (all types)

  * Sampling jitter better than 100 metres

\enditems

\sec Sampling frequency

Sampling frequency is limited by technical constrains in testing construction design. This parameter is especially limited by sampling frequencies of analog to digital conversion chips accessible on market. Combination of required parameters -- dynamic range which needs 16bit at least and minimum sampling frequency of 1 MSPS, leads to high end ADC chips. Which does not support such low sampling frequencies at all. Its minimum sampling frequency is 5 MSPS.  

\sec System scalability

For analog channels scalability special parameters of ADC modules were needed. ADC module ideally needs separate output for each I/Q channel. ADC module must have separate inputs for sampling and for data output clocks. This parameters allows conduction of relatively low digital data rates. And digital signal can be conducted on long wires. 

Clock signal will be handled specially in this scalable design. Selected ADC chip guaranteed defined clock skew between sampling and data output clock. This allows taking data and frame  clocks from first ADC module only. Other data and frame clocks from other ADC modules can be measured for diagnostic purposes. (Failure detection, jitter measurement etc.)   

This system concept allows scalability technically  limited by number of differential signals on host side,  and its computational power.  There is another advantage of scalable data acquisition system -- economic reasons. Observatories or end user can pick choice how much money they are able to spent in radioastronomy receiver system. This option is especially useful for science sites without previous experience with radioastronomy observations.     

\secc Differential signalling 

This concept of scalable design requires relatively long traces between ADC and digital unit which captures the data and performs computations.  Distance of digital processing unit and analog to digital conversion unit has advantage in noise retention typically produced by digital circuits. Those digital circuits such as FPGA or other flip-flops block and traces usually works on high frequencies and emits wideband noise with relatively low power.  In such case any distance increase between noise source and analog signal source increase S/N significantly. But this distance also brings problems with digital signal transmission between ADC and computational unit. But this obstruction should be resolved easier in free space than on board routing. The high quality differential signalling shielded cables should be used.  This technology have two advantages on PCB signal routing. It can use two wire twisting for leak inductance suppression of signal path. And this twisted pair may be additionally shielded by uninterrupted metal foil.              

\secc Phase matching

For multiple antenna radioastronomy project, system phase stability is mandatory. It allows precise high resolution imaging of object. 

High phase stability in this scalable design is achieved by centralised frequency generation  and distribution with multi-output LVPECL hubs. These hubs have equiphased outputs for multiple devices. 

This design ensures that all devices have access to defined phase and known frequency.     

\sec System description

In this section testing system will be described.

\secc Frequency synthesis       

Centralised topology was used for frequency synthesis. One precise high frequency and low jitter digital oscillator was used and other working frequencies are delivered by division from it.  This central oscillator has software defined GPS disciplined control loop for frequency stabilisation.\fnote{\url{http://wiki.mlab.cz/doku.php?id=en:gpsdo} SDGPSDO design was developed in parallel to this diploma thesis construction as related project, but it is not explicitly required by specification.}

 This method was used in order to meet modern requirements on radioastronomy equipment, which needs precise frequency and phase stability on wide area for effective radioastronomy imaging. 

Every ADC module will be directly connected to CLKHUB02A module. This module takes sampling clock delevered by FPGA from main local oscillator.  This signal should use high quality differential signaling cable -- SATA cable should be used for this purpose. 

\secc Signal connectors 

Several widely used and commercially easily accessible differential connectors were considered. 

\begitems

* HDMI % [[http://en.wikipedia.org/wiki/Hdmi|HDMI]]</del>

* SATA  		%{http://en.wikipedia.org/wiki/Serial_attached_SCSI#Connectors|SAS]]/[[http://en.wikipedia.org/wiki/Serial_ATA|SATA]]

* DisplayPort 		%[[http://en.wikipedia.org/wiki/Display_port|DisplayPort]]</del>

* SAS/miniSAS

\enditems

MiniSAS connector was chosen as  the best for use in connection multiple ADC modules.  This miniSAS connector is compatible with existing SATA cabling system and agregates multiple SATA cables to single connector. Translation between SATA and miniSAS is achieved by SAS to SATA adapter cable. This cable is used in servers to connecting SAS controller to multiple SATA hard disc in RAID systems thus is commercially available. 

One drawback is that miniSAS PCB connectors are mainufactured in SMT versions only. But outer metal housing of connector is standard trough hole type. This mechannical design should degrade durability of this connector type. 

\midinsert

\picw=10cm \cinspic ./img/miniSAS_SATA_cable.jpg

\caption/f A type of miniSAS cable similar to used.

\endinsert

\secc Design of ADC modules

This modules have MLAB standard construction with four mounting holes in corner aligned in defined raster. 

Data serial data output of ADC module should be connected directly to FPGA for basic primary signal processing. Used ADC chip has selectable bit width of data output bus thus output SATA connectors has signals arranged to contain a single bit from every ADC channel.  This signal concept enables selection of proper bus bitwidth according to sampling rate. (Higher bus bitwidth downgrades signaling speed and vice versa.)

For connection of this signaling layout, miniSAS to multiple SATA cable should be used.  

For PCB layout KiCAD design suite was used. Used version has the CERN Push \& Shove routing capability integrated but was slightly unstable and sometimes falls on exception during routing. Design must be often saved due to this stability issues. But Open-source KiCAD works well compared to commercial solutions as MentorGraphics PADS or Cadence Orcad.  And much better than widely used Eagle software.

New PCB footprints have been designed for FMC, SATA a and miniSAS connectors. These new footprints were committed to KiCAD github library repository. And they are now publicly accessible from official KiCAD repository at GitHub.  

\secc ADC selection

Several ADC signaling formats currently exist for communication with FPGA. 

\begitems

  * DDR LVDS

  * JEDEC 204B

  * JESD204A

  * Paralel LVDS

  * Serdes

  * serial LVDS

\enditems

Serial LVDS has been selected because uses lowest number of differencial pairs. This parameter is mandatory for construction complexity and reliability. \url{http://www.ti.com/lit/pdf/snaa110}

An ultrasound AFE chips should be ideal for this purpose -- this chips has front-end amplifiers and filters integrated. But theirs drawback is incapability of handling differential input signal and relatively low dynamic range (consists 12bit ADC). This IO has many ADC channels thus scalling are possible in factor of 4 receivers (8 analog channels).

If we require separate output for every analog channel and 16bit deph. Only several ADCs currently exists which meet these requirements.  

\begitems

*[[http://www.linear.com/product/LTC2271|LTC2271]]

*[[http://www.linear.com/product/LTC2191|LTC2190-2195]].

\enditems

All parts in this category are compatible with one board layout. Main differences are in sampling frequency and signal to noise ratio. The slowest one has maximal sampling frequency 20 MHz. But all types have minimal sampling frequency 5 MSPS.  All types were configurable over serial interface (SPI).  SPI seems to be a standard for high-end ADC chips from main manufacturers (Analog Devices, Linear technology, Texas instruments, Maxim integrated..). 

\secc ADC modules interface

All two ADCdual01A modules was connected to FPGA ML605 board trough FMC2DIFF01A adapter board. Construction of this adapter module suppose FMC LPC connector. And this board is not MLAB compatible design. But this board is designed to meet VITA 57 standard specification for boards which uses zone 1 and zone 3. 

This specification guarantee compatibility with others FPGA board which has FMC LPC connector for mezzane cards. Schematic diagram of this adapter board is included in appendix. 

Primary purpose of this PCB is to enable connection of ADC modules from space excluded from PC case.  (In PC box analog circuits cannot be realised without using of massive RFI mitigation techniques). 

Differential signaling connectors should be used for conducting digital signal over relatively long cable. Signalintegrity sensitive links (clocks) are equiped by output driver and translator to LVPECL logic for better signal transmission quality.  

\midinsert

\picw=10cm \cinspic ./img/ML605-board.jpg

\caption/f Used FPGA ML605 development board.

\endinsert

Several SATA connectors and two miniSAS connectors are populated on this board.  This set of connectors allows connection of any number of ADC modules in range of 1 to 8. ADC data outputs should be connected to the miniSAS connectors. Other supporting signal should be routed directly to SATA connectors on adapter. 

Signal configuration used in testing construction is described in tables. 

\secc Output data format

\midinsert

\ctable {cccccccccc}{

\hfil

 & \multispan9 \hfil 160bit packet \hfil \crl \tskip4pt

Data name &  FRAME  & \multispan2 \hfil ADC1 CH1 \hfil & \multispan2 \hfil ADC1 CH2 \hfil & \multispan2  \hfil ADC2 CH1 \hfil & \multispan2 \hfil ADC2 CH2 \hfil  \cr

Data type & uint32 & int16 & int16 & int16 & int16 & int16 & int16 & int16 & int16 \cr

Content & saw signal & $t1$ &  $t_{1+1}$ &  $t1$ &  $t_{1+1}$ &  $t1$ &  $t_{1+1}$ &  $t1$ &  $t_{1+1}$ \cr

}

\caption/t System device "/dev/xillybus_data2_r" data format

\endinsert

\sec Achieved parameters

\secc Data reading and recording 

For reading data stream from ADC driver Gnuradio software was used. Gnuradio suite consist gnuradio-companion which is a graphical tool for creating signal flow graphs and generating flow-graph source code. This tool was used to create basic RAW data grabber to record and interactive wiev data stream output from ADC modules. 

\midinsert

\picw=15cm \cinspic ./img/screenshots/Grabber.grc.png

\caption/f ADC recorder flow graph created in gnuradio-companion.

\endinsert

\midinsert

\picw=15cm \cinspic ./img/screenshots/Grabber_running.png

\caption/f User interface window of running ADC grabber.

\endinsert

Interactive graber wiewer user interface shows live osciloscope-like time-value display for all data channels and live time-frequency scrolling display (waterfall wiev) for displaying frequency components of grabbed signal. 

\secc ADC module parameters

Two pieces of ADC module design were realised and tested first piece denoted as ADC1 has LTC21190

ADC chip populated with LT660015 front-end operational apmlifier. This ADC1 module has 1kOhm resistors populated on inputs which gives to module internal attenuation of input signal. Value of this attenuation is described by formula 

\midinsert

\picw=15cm \cinspic ./img/screenshots/ADC1_CH2_FFT.png

\caption/f Sine signal from ADC1 module with LTC21190 and LT6600-5 devices.

\endinsert

T

ADC1 CH1  maximal input 705.7 mV

\midinsert

\picw=15cm \cinspic ./img/screenshots/ADC1_CH2_FFT.png

\caption/f Sine signal from ADC1 module with LTC21190 and LT6600-5 devices.

\endinsert

LTC2271

6600125

1k

ADC2 CH1 maximal input 380 mV

%\chap Example of usage

%\sec Simple polarimeter station

%\sec Basic interferometer station

%\sec Simple passive Doppler radar

\chap Proposed final system

Construction of final system which should be used for real radioastronomy observations will be described. This chapter is mainly theoretical analysis of systems which should be used for data handling. Realisation of these ideas are planed for future development after full evaluation and testing of actual functional example design. 

\sec Custom design of FPGA board

In beginning of the project coustom design of FPGA interface board was supposed. This FPGA board should include PCI express interface and should have lower price than functional example construction. This board should have MLAB compatible design which is backward compatible with existing or improved design of ADC modules. For connection of this board an another adapter board with PCIe host interface was supposed. 

Thunderbolt technology standard was supposed for use in this PC to PCIe -> FPGA module. Thunderbolt chips are currently available on the market for reasonable prices. But specification for these devices are accessible for licensed users only and Intel has mass market oriented licensing policy,   which makes this technology inaccessible for low quantity product design.  In consequence of this external PCI Express cabling and expansion slots should be better solution. 

But this systems and cables are still very expensive. For example (http://www.opalkelly.com/products/xem6110/) has price tag 995 USD at time of writing this thesis.

Therefore better approach must be found.

\sec Parralella board computer

Parallella is gon

\sec GPU based computational system 

\chap Conclusion 

Special design of scalable data-aquisition system was proposed. This system has parameters 

\sec Possible future improvements

Several ADC module imperfections such as useless separation of FRAME and DCO signal to two connectors should be mitigated. And this two signals should be merged to one SATA connector. This modification removes one redundant SATA cable between analog to digital converter nest and between computational unit nest.