A scalable multi-channel analogue signal generator is presented. It uses a commercial low-cost graphics card with multiple outputs in a standard PC as signal source. Each color signal serves as independent channel to generate an analogue signal. A custom-built external PCB was developed to adjust the graphics card output voltage levels for a specific task, which needed differential signals. The system furthermore comprises a software package to program the signal shape. The implementation of the signal generator is presented as well as an application where it was successfully utilized

Müller, F

Schmitt, K

Shen, W

Stamen, R

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ATL-DAQ-PROC-2009-03219October2009A Low-cost Multi-channel Analogue Signal GeneratorF. Mu¨llera, K. Schmitta, W. Shena, R. Stamenaa Kirchhoff Institute for Physics, University of Heidelberg, Germanyfmueller@kip.uni-heidelberg.deAbstractA scalable multi-channel analogue signal generator is pre-sented. It uses a commercial low-cost graphics card with multi-ple outputs in a standard PC as signal source. Each color signalserves as independent channel to generate an analogue signal. Acustom-built external PCB was developed to adjust the graphicscard output voltage levels for a specific task, which needed dif-ferential signals. The system furthermore comprises a softwarepackage to program the signal shape.The implementation of the signal generator is presented aswell as an application where it was successfully utilized.I. INTRODUCTIONThe presented signal generator provides up to 12 indepen-dent analogue signals on a low-cost basis. It consists of a stan-dard PC hosting a commercial multi-monitor graphics card thatacts as source for the analogue signals. The graphics card iscontrolled by a dedicated software package running on the samemachine. An external device was developed as part of the signalgenerator, being an example of how to condition the signal.A possible application, the emulation of analogue signals ofthe ATLAS calorimeter trigger inputs for the Level-1 PrePro-cessor test rig, is described in section VI..II. CONCEPTThe signal generator consists of three building blocks. Ina first step, the signal is programmed either from basic pulseshapes or from pulses recorded with an oscilloscope. These sig-nals are mapped to a 8-bit digital signal, as shown in figure 1(left). At this point, the signal is strictly positive, featuring anartificial, non-zero baseline. The generation of negative signals,i.e. the application of an offset, is performed at a later step.The analogue signal is generated in a second step, using a com-mercial graphics card as signal source. The basic idea is to usethe DAC of the graphics card and the already existing periph-ery of the card (bus, memory, control unit) to generate analoguesignals. Each color channel of the graphics card thereby servesas an independent signal source, with the native properties fromthe graphics card specification, as given below. These can beconsidered sufficient for many applications, like e.g. analoguecomponents of the LHC experiments at CERN. The signal isunipolar, as illustrated in figure 1 (middle).Figure 1: The desired pulse shape is created and mapped to the 8-bit output of the graphics card (left), which generates a single-endedunipolar signal (middle). To condition the signal for a specific task,gain and offset are adjusted, including the conversion to e.g. differen-tial signals (right) [2].Finally, the signal is conditioned to a specific application bya third building block, which is a dedicated external device. Thisdevice has to perform a calibration of the graphics card signal.In addition, the artificially introduced baseline is taken into ac-count by shifting the signals with a global offset. This offset isapplied using a dedicated channel of the graphics card. The lastoperation is to adjust the voltage levels to the desired range ofthe application.Only this last operation is patricular for the specific application.In the following, the case of a differential output and an addi-tional fan-out of the signal is presented, which corresponds tothe application given in section VI.. Figure 1 (right) shows thesignal after conditioning.III. GRAPHICS CARD AS SIGNAL SOURCEEach color channel of the graphics card serves as an inde-pendent signal source. It is an unipolar signal with an 8-bitresolution of the output voltage and a time resolution (”pixelclock”) of up to 5 ns. This can be considered sufficient to rep-resent an analogue signal for systems operated at a lower speed,like e.g. many 40 MHz systems at the LHC. The signal is rep-resented by a fixed image consisting of three signals at a time(red, green, blue). The longest possible continuous signal thatcan be encoded into the image is in the order of 10µs, whichcorresponds to one line on the screen. This restriction is due tothe need for horizontal synchronisation of analogue CRT moni-tors and emerges as a blanking space at the end of each line andeach screen, where the electrical output is zero. This typicallytakes 20% of the total time. The total signal length neverthelessis up to 10 ms, the minimal frequency about 100 Hz (”monitorfrequency”).In order to maximize the number of channels, graphics cardswith multiple monitor outputs were tested. The Matrox QIDPro [1] was chosen as the model with the best electrical proper-ties.Figure 2: Noise measurement of the Matrox Millenium G400 withDAC set to zero. All outputs feature a constant, non-zero offset within±20 mV [2].Figure 2 shows the measurements of the noise of the MatroxMillenium G400, which has very similar properties as the usedmodel. The noise was determined by measuring the output withDAC set to zero, i.e. by displaying a black image, resulting ina gaussian noise with a RMS of 3.4 mV. The linearity was alsomeasured and found to be within a 1% deviation over the out-put voltage range. Furthermore, figure 2 shows a deviation ofthe signal from zero in spite of the DAC set to zero. This offsetwas found on all color channels of the graphics card to be con-stant within 20 mV, which has to be corrected on the subsequentcalibration stage of the external conditioning device.IV. SOFTWARE PACKAGEFigure 3: Graphical user interface of the software package to create,modify and save signal shapes. The main view shows a 40 MHz clock(green), a constant signal (blue) and a typical pulses for the ATLASLAr type calorimeters (red). These signals were used in the applicationdescribed in section VI. [2].A software package was developed to program and createthe signals. It consists of two parts. The first is a graphical toolthat offers basic pulse shapes, modification tools and the pos-sibility to import external data. The prepared pulse shapes arestored in a generic file format. Three of the signals are mergedinto a fixed image which correspond to the desired signal shapeat the output of the graphics card. The second tool is a con-sole application for linux that connects to a dedicated X windowserver running on the pc that hosts the graphics card. Thus itdrives the graphics card by displaying the saved signals as fixedimages at full screen, resulting in a repeating signal as long asthe application is running.V. SIGNAL CONDITIONINGFigure 4: The Fan-out and Application Board (FAB) performs calibra-tion, fan-out and conditioning of the signals [2].An external device was developed to condition the outputsignal to the voltage levels for a specific task. It is a PCB thatconsists of several buffer stages to calibrate for gain and offset.Up to six monitor outputs can serve as inputs. One channel isexplicitly used to apply a global offset on all other signals inorder generate negative, as well as positive, signals. The out-put are 16 differential signals, which can be configured by anupstream fan-out stage.  offsetcalibrationdiff. outputFigure 5: Scheme of the signal conditioning: Calibration for gain andbaseline, application of a global offset and preparing of the output sig-nal (in this case: differential signal).Figure 5 shows a scheme of the signal conditioning for onechannel. At the first stage, baseline and gain of the input sig-nal from the graphics card are calibrated. This calibration isoffsetcalibration  diff. outputoverlay:original signalbaselinegainFigure 6: Signal chain on the Fan-out and Amplification Board for a typical pulse of a the ATLAS LAr type calorimeter. Left picture: First, theoriginal signal (red) is calibrated for gain and baseline (blue). Middle picture: Then a global offset (blue) is applied, resulting in a continuousbaseline (green). Right picture: Finally the single-ended signal is converted to a differential signal (green). Also shown are the two branches ofthe differential signal (blue, red) and the original signal (red overlay) [2].implemented by variable resistors of an operational amplifierin inverted circuit and has to be performed once. At the secondstage, a global, negative offset is applied on all channels in orderto make negative signals possible, using a dedicated, invertedchannel of the graphics card. Hence, the offset is programmableby software, taking into account the artificially introduced base-line at the creation of the signal, as described in section II.. Thepresented version of the device for the signal conditioning wasdeveloped for a task that required multiple differential signals.Therefore, the signals are then fanned out and converted fromsingle ended signals to differential ones at the last stage. Thislast stage, of course, varies for the specific task.Figure 6 shows the development of the signals after the severalstages of the signal conditioning.VI. APPLICATIONThe signal generator was successfully applied in a test bedfor the PreProcessor Module (PPM) of the ATLAS Level-1Calorimeter Trigger. One of the main tasks of the PreProces-sor is the digitisation of the analogue pulses from the ATLAScalorimeters at a rate of 40 MHz. These pulses are transmitteddifferentially with a voltage amplitude of up to 2.5 Volts. Thekey characteristics are a rise time of 50 ns and an undershoot ofup to −0.5 Volts for signals from calorimeters based on LiquidArgon technonolgy. The typical shape can be seen in the fig-ure 1. Considering the sampling rate of 200 MHz, the presentedsignal generator can be considered highly sufficient to emulatethe analogue ATLAS calorimeter pulses.A. Test Bed for the PreProcessor ModuleThe test bed for the analogue parts of the ATLAS Level-1Calorimeter Trigger Pre-Processor is shown in figure 7. Sincethe connectivity of the PPM is 4 connectors with 16 channelseach, the signal generator was set up with 8 independent sig-nals of the graphics card that are fanned out and converted to 16differential signals.  1Fan-out and Amplification BoardATLAS Level-1 Calorimeter Trigger PreProcessor ModuleSoftwareGraphics CardPCClock Board168+2differentialsingle-endeddedicatedchannel40 MHzclockFigure 7: The setup of the PreProcessor test bed consists of the sig-nal generator, which delivers 16 differential channels. Furthermore, anexternal device (clock board) uses a dedicated channel to provide a 40MHz clock.In order to achieve a synchronous sampling of the pulses onthe PPM with respect to the signal generator, the test setup alsohas to provide the bunch crossing frequency of 40 MHz to thePPM. This requires an additional device, since the signal gener-ator suffers from the blanking space that prevents the generationof such continuous signals.B. Clock Synchronisation BoardThe Clock Synchronisation Board uses another dedicatedchannel of the graphics card to provide a clock synchronous tothe 16 signal channels. The device features a CPLD for ba-sic logic function and routing, and a voltage controlled oscilla-tor in a phase-locked loop (PLL). An incoming 40 MHz signalfrom the graphics card serves as reference clock, while the in-hibit function of the PLL is used to bridge the intrinsic blankingspace of the graphics card signal. Therefore, the reference clockis analysed to detect the beginning of the blanking space. This isachieved by a monoflop that is charged by the reference signal.Once the reference clock stops, the monoflop turns to zero. Thisactivates the inhibit of the voltage controlled oscillator, wherebyit sustains the 40 MHz clock. After the blanking space, the PLLensures that the voltage controlled oscillator synchronises withthe reference clock again. See figure 8 for illustration.  ~ 10 μs ~ 40 μs ~ 2.5 μs40 MHz clock vsync blanking spacegraphics card (dedicated channel)40 MHz clock signalclock board(PLL)40MHz clock signaldedicated channelFigure 8: Scheme of the 40 MHz clock, provided by a dedicated chan-nel for reference. A PLL with a voltage controlled oscillator is usedto both synchronise and bridge the intrinsic blanking space, using theinhibit function of the phase detector of the PLL.Figure 9: Digitised signal (yellow) and original signal (red overlay) [2].C. MeasurementFor the measurement, the signal generator was providingpulses from a test beam pulse library [3] as well as the referenceclock. The PPM was configured to digitise the analogue signalswithout further processing. The result is shown in figure 9, withthe digitised signal in black, and the original signal as an overlayin red. Both are in very good agreement. The similar digitisa-tion levels of two consecutive pulses furthermore demonstratethe synchronisation of the generated signals and the PPM sam-pling frequency, provided by the Clock Synchronisation Board.VII. SUMMARYThe presented signal generator is applicable in all fields withneed for multiple analogue signals where a blanking space is nodrawback, or can be compensated as described. The advantagesare multiple, easily programmable signals with acceptable qual-ity at very low expense.REFERENCES[1] Matrox Graphics Inc., User Guide matrox QID Series,10876-301-0221, 2008.01.11[2] F. Mu¨ller, Rate Metering for the ATLAS Experiment, HD-KIP 08-22, 2008[3] H. Stenzel, Trigger Tower Analogue Signal Library,November 2001.

A Low-cost Multi-channel Analogue Signal Generator

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