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\chap Trial version of the receiver, design and implementation

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.

\midinsert

\clabel[expected-block-schematic]{Expected system block schematic}

\picw=\pdfpagewidth \setbox0=\hbox{\inspic ./img/Coherent_UHF_SDR_receiver.png }

\par\nobreak \vskip\wd0 \vskip-\ht0

\centerline {\kern\ht0 \pdfsave\pdfrotate{90}\rlap{\box0}\pdfrestore}

\caption/f Expected realisation of signal digitalisation unit.

\endinsert

\sec Required parameters

We require the following technical parameters, to supersede existing digitalization units solutions.

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.

Summary of other additional required parameters follows

\begitems

  * Dynamical range better than 80 dB, see section \ref[dynamic-range-theory] for explanation

  * Phase stability between channels

  * Low noise (all types)

  * Sampling jitter better than 100 metres

  * Support for any number of receivers in the range of 1 to 8

\enditems

Now we analyze several of the parameters in detail.

\sec Sampling frequency

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.

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.

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.

\sec System scalability

For analogue channels' scalability, special parameters of ADC modules are required. Ideally, there should be a separate output for each analogue channel in ADC module. ADC module must also have separate outputs for frames and data output clocks. These parameters allow for conduction at relatively low digital data rates. As a result, the digital signal can be conducted even through long wires. Modular concept allows a separation from central logical unit which supports optimization of number analogue channels.

Clock and data signals will be then handled distinctively in our modular scalable design. Selected ADC chips are guaranteed to have defined clock skew between the sampling and data output clocks. This allows taking data and frame clocks from the first ADC module only. The rest of the data and frame clocks from other ADC modules can be measured for diagnostic purposes (failure detection, jitter measurement etc.), but these redundant signals are not used for data sampling. If more robustness is required in the final application, DCO \glos{DCO}{Data Clock Output} and FR signals may be collected from other modules and routed through an voting logic which will correct possible signal defects.

This system concept allows for scalability, that is technically limited by a number of differential signals on host side and its computational power.  There is another advantage of scalable data acquisition system -- an economic one. Observatories or end users can make a choice of how much money are they willing to spent on radioastronomy receiver system. This freedom of choice is especially useful for science sites without previous experience in radioastronomy observations.

\secc Differential signalling

The above mentioned concept of scalable design requires relatively long circuit traces between ADC and digital unit which captures the data and performs the computations. The long distance between the digital processing unit and the analog-to-digital conversion unit has an advantage in noise retention typically produced by digital circuits. Those digital circuits, such as FPGA \glos{FPGA}{Field-programmable gate array}, Ethernet or other flip-flops blocks and circuit traces, usually work at high frequencies and emit wide-band noise with relatively low power. In such cases any increase in a distance between the noise source and analog signal source increase S/N significantly. However, at the same time, a long distance brings problems with the digital signal transmission between ADC and computational unit. But this obstacle should be resolved more easily in free-space than on board routing. The high-quality differential signalling shielded cables should be used, such as massively produced and cheap SATA \glos{SATA}{Serial ATA} \glos{ATA}{AT Attachment} cables. This technology has two advantages over PCB \glos{PCB}{printed circuit board} signal routing. First, it can use twisted pair of wires for leak inductance suppression in signal path and second, the twisted pair may additionally be shielded by uninterrupted metal foil.

\secc Phase matching

For multiple antenna radioastronomy projects, system phase stability is a mandatory condition. It allows a precise, high resolution imaging of objects, increases signal to noise ratios in several observation methods and allows the use of advanced algorithms for signal processing.

High phase stability in our scalable design is achieved through centralized frequency generation  and distribution with multi-output LVPECL \glos{LVPECL}{Low Voltage Emitter-coupled logic} hubs (CLKHUB02A), that have equiphased outputs for multiple devices. LVPECL logic is used on every system critical clock signal distribution hub. LVPECL logic has an advantage over LVDS \glos{LVDS}{Low-voltage differential signaling} in signal integrity robustness. LVPECL uses higher logical levels and higher signalling currents. Power consumption of LVPECL logic is nearly constant over the operating frequency range due to the use of bipolar transistors. This minimizes voltage glitches which are typical for CMOS \glos{CMOS}{Complementary metal–oxide–semiconductor } logic. One drawback of its parameters is a high power consumption of LVPECL logic which easily reaches tens of milliamperes per device.

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

\sec System description

This section deals with the description of the trial version based on Xilinx ML605 development board \ref[ML605-development-board]. The board had been used in a previous project and has not been used since then, but the FPGA parameters are more than sufficient of what we need for fast data acquisition system.

\secc Frequency synthesis

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 \glos{GPS}{Global Positioning System}  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 thesis itself and thus it is described in a separate document}

We have used new methods of software frequency monitoring and compensation in order to meet modern requirements on radioastronomy equipment which need precise frequency and phase stability over a wide baseline scales for effective radioastronomy imaging.

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] ).

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.

Timestamps should be seen in image \ref[meteor-reflection] (above and below the meteor reflection).

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.

\midinsert \clabel[LO-noise]{Phase noise of the local oscillator}

\ctable{lcc}{

	&	 \multispan2 \hfil Phase Noise [dBc/Hz] \hfil 		\cr

Offset Frequency	&	$F_{out}$ 156.25 MHz	& $F_{out}$ 622.08 MHz \cr

100 [Hz]	&	–105	&	–97 \cr

1 [kHz]	&	–122	&	–107 \cr

10 [kHz]	&	–128	&	–116 \cr

100 [kHz]	&	–135	&	–121 \cr

1 [MHz]	&	–144	&	–134 \cr

10 [MHz]	&	–147	&	–146 \cr

100 [MHz]	&	n/a	&	–148 \cr

}

\caption/t Phase noise of the used Silicon Laboratories Si570 chip. Offset frequency is measured from carrier frequency. Values shown in the table are given for two different carrier frequencies.

\endinsert

Every ADC module will be directly connected to CLKHUB02A module which takes sampling clock signal delivered by FPGA from the main local oscillator.  This signal should use high quality differential signalling cable -- we should use SATA cable for this purpose. FPGA may slightly affect the clock signal quality by adding a noise, but it has a negligible effect on the application where developed system will be used.

\label[signal-cables] \secc Signal cable connectors

Several widely used and commercially easily accessible differential connectors were considered to be use in our design.

\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

At the end, MiniSAS connector was chosen as the best option to be used in connecting together multiple ADC modules. A transition between SATA and miniSAS is achieved by SAS to SATA adapter cable which is commonly used in servers to connect SAS controller to multiple SATA hard disc in RAID systems and thus is commercially easily available. It is compatible with existing SATA cabling systems and aggregates multiple SATA cables to a single connector. It also has SPI configuration lines which can be seen in the following picture \ref[img-miniSAS-cable] as standard pinheader connector.

The main drawback of miniSAS PCB connectors lies in the fact, that they are manufactured in SMT versions only. SMT design may eventually decrease the durability of the connector even if outer metal housing of connector is designed to be mounted using a standard through-hole mounting method.

\midinsert

\clabel[img-miniSAS-cable]{Used miniSAS cable}

\picw=5cm \cinspic ./img/miniSAS_SATA_cable.jpg

\caption/f An example of miniSAS cable similar to used.

\endinsert

\secc Signal integrity requirements

\label[diff-signaling]

We use ADC devices that have DATA clock frequency eight times higher than sampling frequency in a single line output mode, implying a 40 MHz output bit rate. This implies a $ 1/4 \cdot 10^7 = 25\ $ns time length of data bit, which is equivalent to 7.5m light path in free space. If we use copper PCB with FR4 substrate layer or coaxial/twinax cable, we could obtain velocity factor of 0.66 at worst condition. Then the light path for the same bit rate $t_s$ will be 4.95 m. Although we do not have any cables in the system with comparable lengths, the worst data bit skew described by data sheets of the used components is $0.3 \cdot t_s$, which is 1.485 m. Therefore the length matching is not critical in our current design operating on lowest sampling speed. Length matching may become critical in future versions with higher sampling rates, where the cable length must be matched. However SATA cabling technology is already prepared for that case and matched SATA cables are a standard merchandise.

\secc ADC modules design

\midinsert

\clabel[adcdual-preview]{Preview of designed ADCdual PCB}

\picw=10cm \cinspic ./img/ADCdual01A_Top_Big.JPG

\picw=10cm \cinspic ./img/ADCdual01A_Bottom_Big.JPG

\caption/f Realised PCB of ADCdual01A modules. Differential pairs routings are clearly visible.

\endinsert

\secc ADC selection

There exist several standard ADC signalling formats currently used in communication with FPGA.

\begitems

  * DDR LVDS

  * JEDEC 204B

  * JESD204A

  * Paralel LVDS

  * Serdes

  * serial LVDS

\enditems

As a result of our need to use the smallest number of cables possible, the choice fell on the serial LVDS format. Small number of differential pairs is an important parameter determining the construction complexity and reliability\cite[serial-lvds]. No many currently existing ADC devices have this kind of digital interface. An ultrasound AFE device chips seem to be ideal for this purpose -- the chip has integrated both front-end amplifiers and filters. It has a drawback though - it is incapable of handling differential input signal and has a relatively low dynamic range (as it consists only of 12bit ADC) and has many single ended ADC channels. Consequently, the scaling is possible only by a factor of 4 receivers (making 8 analogue single ended channels).

If we add a requirement of separate output for every analogue channel and a 16bit depth we find that there are only a few 2-Channel simultaneous sampling ADCs currently existing which meet these criteria. We have summarized those ADCs in the following table \ref[ADC-types]

\midinsert

\typosize[9/11] \def\tabiteml{ }\let\tabitemr=\tabiteml

\clabel[ADC-types]{Available ADC types}

\ctable{lccccccc}{

\hfil ADC Type & LTC2271 & LTC2190 & LTC2191 & LTC2192 & LTC2193 & LTC2194 & LTC2195 \cr

SNR [dB] & 84.1 & 77 & 77 & 77 & 76.8 & 76.8 & 76.8  \cr

SFDR [dB] & 99 & 90 & 90 & 90 & 90 & 90 & 90  \cr

S/H Bandwidth [MHz] & 200 & \multispan6 550 \strut \cr

Sampling rate [MSPS] & 20 & 25 & 40 & 65 & 80 &  105 & 125  \cr

Configuration & \multispan7 SPI \strut \cr

Package & \multispan7 \hfil 52-Lead (7mm $\times$ 8mm) QFN \hfil \strut \cr

}

\caption/t The summary of the currently available ADC types and theirs characteristics.

\endinsert

All parts in this category are compatible with one board layout. The main differences lay in the sampling frequency and signal to noise ratio, with the slowest having a maximum sampling frequency of 20 MHz. However, all of them have a minimal sampling frequency of 5 MSPS and all are configurable over a serial interface (SPI). SPI seems to be a standard interface used in high-end ADC chips made by the largest manufacturers (Analog Devices, Linear technology, Texas instruments, Maxim integrated..).  We have selected two slowest types for our evaluation design. Following that, a PCB for this part have been designed.

We have decided that ADCdual01A modules will have a standard MLAB construction layout with four mounting holes in corners aligned in defined raster of 400 mils.

Data serial data outputs of ADC modules should be connected directly by LVDS signalling levels conducted by SATA cables to FPGAs for the basic primary signal processing. The ADC chips used in the modules have a selectable bit width of data output bus and thus the output SATA connectors have signals arranged to contain a single bit from every ADC channel.  This creates a signal concept enabling a selection of proper bus bit-width according to the sampling rate (higher bus bit-width downgrades signalling speed and vice versa.)

In order to connect the above mentioned signalling layout, miniSAS to multiple SATA cable should be used as described in section \ref[signal-cables].

A KiCAD design suite had been chosen for PCB layout. However, the version, despite having integrated CERN Push \& Shove routing capability, is slightly unstable as it sometimes crushes due to an exception during routing. On the basis of these stability issues, the design had to be saved quite often. On the other hand, compared to commercially available solutions, such as MentorGraphics PADS or Cadence Orcad,  the Open-source KiCAD provides an acceptable option and it easily surpasses a widely used Eagle software.

As a part of work on the thesis, new PCB footprints for FMC, SATA, ADCs a and miniSAS connectors have been designed and were committed to KiCAD github library repository. They are now publicly available on the official KiCAD repository at GitHub.

ADCdual01A module has several digital data output formats. Difference between these modes lays in the number of differential pairs used.

\begitems

    * 1-lane mode

    * 2-lane mode

    * 4-lane mode

\enditems

All of the above-mentioned modes are supported by the module design. For the discussed data acquisition system, the 1-lane mode was selected. 1-lane mode allows a minimal number of differential pairs between ADCdual01A and FPGA. Digital signalling scheme used in 1-lane mode is shown in the following image \ref[1-line-out].

\midinsert

\clabel[1-line-out]{Single line ADC output signals}

\picw=15cm \cinspic ./img/ADC_single_line_output.png

\caption/f Digital signalling schema for 1-line ADC digital output mode.

\endinsert

ADCdual01A parameters can be set either by jumper setup (referred to as a parallel programming  in the device's data sheet) or by SPI interface. SPI interface has been chosen for our system, because of the parallel programming's lack of options (test pattern output setup for example).

Complete schematic diagram of ADCdual01A module board is included in the appendix.

\secc ADC modules interface

\midinsert

\picw=10cm \cinspic ./img/FMC2DIFF_Top_Big.JPG

\picw=10cm \cinspic ./img/FMC2DIFF_Bottom_Big.JPG

\caption/f Realised PCB of FMC2DIFF01A module.

\endinsert

Both of the ADCdual01A modules were connected to FPGA ML605 board trough FMC2DIFF01A adapter board. The design of this adapter expects the presence of FMC LPC connector on host side and the board is, at the same time, not compatible with MLAB. It is, on the other hand, designed to meet the VITA 57 standard specifications for boards which support region 1 and region 3. VITA 57 regions are explained in the picture \ref[VITA57-regions].

This industry standard guarantees the compatibility with other FPGA boards that have FMC LPC connectors for Mezzanine Card. Schematic diagram of designed adapter board is included in the appendix.

The primary purpose of the PCB is to enable the connection of ADC modules located outside the PC case with ML605 development board. (In PC box analog circuits cannot be realized without the use of massive RFI mitigation techniques).

Differential signalling connectors should be used for conducting digital signal over relatively long cables. The signal integrity sensitive links (clocks) are equipped with output driver and translator to LVPECL logic for better signal transmission quality.

LVPECL level signal connectors on FMC2DIFF01A board are dedicated to transmit the clock signals. We have selected  the SY55855V and SY55857L dual translators. Dual configuration in useful due to fact, that SATA cable contains two differential pairs.

The SY55855V is a fully differential, CML/PECL/LVPECL-to-LVDS translator. It achieves LVDS signalling up to 1.5Gbps, depending on the distance and the characteristics of the media and noise coupling sources.

LVDS is intended to drive 50 $\Omega$ impedance transmission line media such as PCB traces, backplanes, or cables. SY55855V inputs can be terminated with a single resistor between the true and the complement pins of a given input \cite[SY55855V-chip].

The SY55857L is a fully differential, high-speed dual translator optimized to accept any logic standard from single-ended TTL/CMOS to differential LVDS, HSTL, or CML and translate it to LVPECL. Translation is guaranteed for speeds up to 2.5Gbps (2.5GHz toggle frequency). The SY55857L does not internally terminate its inputs, as different interfacing standards have different termination requirements\cite[SY55857L-chip].

Inputs of both used chips are terminated accordingly to used logic. The LVDS input is terminated differentially by 100 $\Omega$ resistor between positive and negative inputs. PECL input is terminated by Thevenin resistor network. Thevenin termination method was selected as optimal one, due to the absence of a proper power voltage (1,3 V) for direct termination by 50 $\Omega$ resistors. Termination on FPGA side is realized directly by settings the proper digital logic type on input pins.

\midinsert

\clabel[ML605-development-board]{ML605 development board}

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

\caption/f FPGA ML605 development board.

\endinsert

\midinsert

\clabel[VITA57-regions]{VITA57 board geometry}

\picw=10cm \cinspic ./img/VITA57_regions.png

\caption/f Definition of VITA57 regions.

\endinsert

Three differential logic input/output, one PECL input and one PECL output SATA connectors and two miniSAS connectors are populated on this board.  This set of connectors allows a connection of any number of ADC modules within the range of 1 to 8. ADC data outputs should be connected to the miniSAS connectors, while other supporting signals should be routed directly to SATA connectors on adapter.

Lengths of differential pairs routed on PCB of the module are not matched between the pairs. Length variation of differential pairs is not critical in our design according to facts discussed in paragraph \ref[diff-signaling]. Nevertheless, signals within differential pairs themselves are matched for length. Internal signal trace length matching of differential pairs is mandatory in order to minimize jitter and avoid a dynamic logic hazard conditions on digital signals, that represents the worst scenario. Thus the clocks' signals are routed in the most precise way on all designed boards.

Signal configuration used in our trial design is described in the following tables \ref[minisas-interface], \ref[SPI-system] and \ref[clock-interconnections].

\midinsert \clabel[minisas-interface]{miniSAS differential pairs connections}

\ctable {cccc}

{

miniSAS	&	SATA pair	&	FMC signal	&	Used as	\cr

P0	&	1	&	LA03	&	 not used 	\cr

P0	&	2	&	LA04	&	 not used 	\cr

P1	&	1	&	LA08	&	 not used 	\cr

P1	&	2	&	LA07	&	 not used 	\cr

P2	&	1	&	LA16	&	ADC1  CH1 (LTC2190)	\cr

P2	&	2	&	LA11	&	ADC1  CH2 (LTC2190) 	\cr

P3	&	1	&	LA17	&	ADC2 CH1 (LTC2271)	\cr

P3	&	2	&	LA15	&	ADC2 CH2 (LTC2271)	\cr

}

\caption/t miniSAS (FMC2DIFF01A J7) signal connections between modules.

\endinsert

\midinsert \clabel[SPI-system]{SPI configuration interface connections}

\ctable {ccc}

{

SPI connection J7	&	FMC signal	&	Connected to	\cr

SAS-AUX1	 &	LA14\_N	&	SPI DOUT	\cr

SAS-AUX2	 &	LA14\_P	&	SPI CLK	\cr

SAS-AUX3	 &	LA12\_N	&	CE ADC1	\cr

SAS-AUX4	 &	LA12\_P	&	CE ADC2	\cr

SAS-AUX5	 &	LA13\_N	&	soldered to GND	\cr

SAS-AUX6	 &	LA13\_P	&	not used	\cr

SAS-AUX7	 &	LA09\_N	&	not used	\cr

SAS-AUX8	 &	LA09\_P	&	soldered to GND	\cr

}

\caption/t SPI system interconnections

\endinsert

SPI interface is used in an unusual way in this design. SPI Data outputs from ADCs are not connected anywhere and read back is not possible, thus the configuration written to registers in ADC module cannot be validated. We have not observed any problems with this system, but it may be a possible source of failures.

\midinsert \clabel[clock-interconnections]{System clock interconnections}

\ctable {lccc}

{

Signal	&	FMC signal	&	FMC2DIFF01A	&	ADCdual01A	\cr

DCO	&	CLK1\_M2C	&	J5-1	&	J13-1	\cr

FR	&	LA18\_CC	&	J10-1	&	J12-1	\cr

ENC	&	LA01\_CC	&	J2-1(PECL OUT)	&	J3-1	\cr

SDGPSDO01A LO	&	CLK0\_M2C	&	J3-1 (PECL IN)	&	N/A	\cr

}

\caption/t Clock system interconnections

\endinsert

\secc FPGA function

Several tasks in separate FPGA blocks are performed by FPGA. In the first block the FPGA prepares a sampling clock for ADCdual01A modules by dividing the signal from the main local oscillator. This task represents a separate block in FPGA and runs asynchronously to other logical circuits. Second block is a SPI configuration module, which sends configuration words to ADC modules and it is activated by opening of Xillybus interface file. Third block represents the main module, which resolves ADC - PC communication itself and it communicates via PCIe, collect data from ADC hardware and creates data packet \ref[xillybus-interface]. Last block is activated after the ADC is configurated via SPI.

Communication over PCIe is managed by proprietary IP Core and Xillybus driver, which transfers data from FPGA registers to host PC. Data appear in a system device file named  "/dev/xillybus_data2_r" on the host computer. Binary data which appear in this file after its opening are shown in the table below \ref[xillybus-interface].

\midinsert

\def\tabiteml{ }\let\tabitemr=\tabiteml

\clabel[xillybus-interface]{Grabber binary output format}

\ctable {lccccccccc}{

\hfil & \multispan9 \hfil 160bit packet \hfil \strut \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 \strut  \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

Data packet block which is carried on PCI Express isa  described in the table \ref[xillybus-interface]. The data packet consist of several 32bit words. The first word contains FRAME number and it is filled with saw signal for now, with incremental step taking place every data packet transmission. The following data words contain samples from ADCs' first and second channel. Samples from every channel are transmitted in pairs of two samples. Number of ADC channels is expandable according to the number of physically connected channels. An CRC word may possibly be added in the future to the end of the transmission packet for data integrity validation.

FRAME word at the beginning of data packet, now filled with incrementing and overflowing saw signal, is used to ensure that no data samples ale lost during the data transfers from FPGA. FRAME signal may be used in the future for pairing the ADC samples' data packet with another data packet. This new additional data packet should carry meta-data information about the sample time jitter, current accuracy of the local oscillator frequency etc.

Detailed description of the currently implemented FPGA functions can be found in a separate paper \cite[fpga-middleware]. HDL source codes for FPGA at a state in which it was used are included on the enclosed CD. More recent development versions are publicly available from MLAB sources repository.

Detailed description of currently implemented FPGA functions can be found in separate paper \cite[fpga-middleware]. HDL \glos{HDL}{Hardware description language} source codes for FPGA at state which was used are included on enclosed CD. Future development versions are publicly available from MLAB sources repository \cite[mlab-sdrx].

\secc Data reading and recording

In order to read the data stream from the ADC drive, we use Gnuradio software. Gnuradio suite consists of gnuradio-companion package which is a graphical tool for creating signal-flow graphs and generating Python flow-graph source code. This tool has been used to create a basic RAW data grabber to record and interactively view waterfall plots using the data streams output from ADC modules.

\midinsert

\clabel[grabber-flow-graph]{Gnuradio flow graph for signal grabbing}

\picw=\pdfpagewidth \setbox0=\hbox{\inspic ./img/screenshots/Grabber.grc.png }

\par\nobreak \vskip\wd0 \vskip-\ht0

\centerline {\kern\ht0 \pdfsave\pdfrotate{90}\rlap{\box0}\pdfrestore}

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

\endinsert

\midinsert

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

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

\endinsert

The interactive grabber-viewer user interface shows live oscilloscope-like time-value display for all data channels and live time-frequency scrolling display (a waterfall view) for displaying the frequency components of the grabbed signal. Signal is grabbed to file with exactly the same format, as described in the table \ref[xillybus-interface].