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Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer
G Model
ARTICLE IN PRESS
FUSION-8756; No. of Pages 4
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer Praveena Kumari ∗ , Vismaysinh Raulji, Hitesh Mandaliya, Jignesh Patel, Varsha Siju, S.K. Pathak, Rachana Rajpal, R. Jha Institute for Plasma Research, Nr. Indira Bridge, Bhat, Gandhinagar 382428, India
h i g h l i g h t s • • • •
Indigenous development of an electronics and data acquisition system to digitize signals for a desired time and automatization of calibration process. 16 bit DAQ board with form factor of 90 × 89 mm. VHDL Codes written for generating control signals for PC104 Bus, ADC and RAM. Averaging process is done in two ways single point averaging and additive averaging.
a r t i c l e
i n f o
Article history: Received 20 June 2015 Received in revised form 29 April 2016 Accepted 10 May 2016 Available online xxx Keywords: Complex programmable logic device (CPLD) Signal conditioning unit (SCU) Very high speed integrated circuit hardware description logic (VHDL) PC104- family of embedded computer standards define both form factor and computer buses
a b s t r a c t Signal conditioning units (SCU) along with Multichannel Data acquisition system (DAS) are developed and installed for automatization and frequent requirement of absolute calibration of ECE radiometer system. The DAS is an indigenously developed economical system which is based on Single Board Computer (SBC). The onboard RAM memory of 64 K for each channel enables the DAS for simultaneous and continuous acquisition. A Labview based graphical user interface provides commands locally or remotely to acquire, process, plot and finally save the data in binary format. The microscopic signals received from radiometer are strengthened, filtered by SCU and acquired through DAS for the set time and at set sampling frequency. Stored data are processed and analyzed offline with Labview utility. The calibration process has been performed for two hours continuously at different sampling frequency (100 Hz to 1 KHz) at two set of temperature like hot body and the room temperature. The detailed hardware and software design and testing results are explained in the paper. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The electron cyclotron emission radiometer is an effective diagnostic to measure the plasma electron temperature and its thermal distribution. However, to determine the relation between radiometer output and plasma emissivity the system needs to be calibrated. The most common method of calibration is the technique of Hot/cold switch method wherein the output of the radiometer is recorded at two different temperatures i.e. hot and cold. Calibration factors so obtained are used to determine the plasma temperature and its spatial and temporal evolution. The accuracy of calibration thus determines the accuracy of the temperature measurements. Henceforth frequent calibration is done to improve on the method
∗ Corresponding author. E-mail address: [email protected] (P. Kumari).
used. Frequent requirement of independent calibration of radiometer and automatization of its process motivated us to design a Multichannel Data acquisition system (DAS) along with SCU. Our main aim is indigenous development of an electronics and data acquisition system to digitize signals for a desired time. Another aim of this development is to establish an automatization of the calibration process along with the improvement in system sensitivity so that very low signal can also be made detectable. Automatization of calibration process through DAQ will save time, avoids manual error and provides scope for signal processing on acquired data. The DAS is based on Single Board Computer (SBC). The hardware and software design supports the DAS for simultaneous, single shot and continuous acquisition. A Labview based graphical user interface (GUI) provides commands locally or remotely to acquire, process, plot and finally save the data. Offline Signal processing by implementation of averaging technique on the
http://dx.doi.org/10.1016/j.fusengdes.2016.05.013 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
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ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
2
Fig. 1. Block diagram of test set-up of calibration.
Fig. 3. Block diagram of signal conditioning.
2.1. Radiometer digitized data and their analyzing facilities are coded in another Labview GUI.
2. Experimental set-up The process of radiometer calibration is the standard technique of Hot/cold switch method. A silicon carbide based black body source that can be heated upto 600 ◦ C acts as the hot body while the room temperature acts as the cold body. The experimental setup is shown by block diagram in Fig. 1. The horn antenna is made to view the two different sources through a chopper arrangement. The chopper circuit consists of a fan rotating at a speed of 2 Hz that can be varied. Eco-absorber sheets are pasted on the fan wings so that it absorbs the cosmic microwave radiations. A light source and a photo diode arrangement are along with the receiving antenna and black body source. The output of the photo diode so obtained is a square pulse that acts as the reference signal for signal averaging. A thermocouple is used to monitor the temperature from the hot body source. The fan wings are so aligned that they simultaneously cover the receiving antenna and the light source so that the photo diode receives a 0 V reference line as soon as the fan wings move and the antenna is made to view the hot source simultaneously the photodiode records the light source as the high of the square pulse so generated. The 8-channel radiometer output along with the output of the photo diode that acts as the 9th reference channel are acquired in the DAS for the set time and at set sampling frequency. Calibration process has been performed several times for two hours continuously at 1 KHz sampling frequency at hot black body source at constant temperature of 500 ◦ C and RTP. A temperature measurement circuit through thermocouple is also included to monitor the temperature of hot source.
Fig. 2. Block diagram of radiometer.
The ECE radiation is collected by a 20 dBi gain horn antenna and passed on to a band pass filters that provide the input RF of 63–75 GHz [2]. This RF is down converted using a SSB mixer to an IF of 1–12 GHz. The signal is then boosted using a pair of cascaded low noise amplifiers with an approximate gain of 58 dB. An 8- way power divider distributes the input IF into 8 channels which are further frequency selected from 1.4 GHz to 10.5 GHz in a step size of 1.3 GHz using narrow band cavity filters. This frequency selective signal is detected using a Schottky detector followed by the electronics video amplifiers for further processing. Fig. 2 depicts the block diagram of the radiometer system. 2.2. Signal conditioning unit The SCU unit contains 8 modules. Design of one module is explained by block diagram in Fig. 3. The microwave components used in radiometer has its own offset and as the stages increases the DC offset gets accumulated. This unwanted offset has to be removed. The signal coming from the hot body source is a slow varying signal of vary low amplitude ranges from 1 V to 200 V. So both the signal and the undesired offset are amplified with same gain by first stage amplifier. A DC feedback circuit is implemented in first stage for offset removal by using the reference terminal of the instrumentation amplifier (INA).The reference terminal provides a direct means of injecting a precise offset to the output.The voltage on the reference terminal can be varied from negative to positive supply as per the requirement. The 2nd stage amplification is applied to the signal with the DC component already removed. Signal passes through an eight order filter module. The cut off frequency can be changed in four steps. As the same SCU set up is used to install during Aditya tokamak experimental shot. So we insert an
Fig. 4. Block diagram of DAS.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
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ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
3
Fig. 5. Picture of stacked board with SBC.
analog optocoupler for isolation with DAQ ground. The differential driver insures the signal amplitude throughout the long cable, laid to the DAQ. 2.3. Data acquisition system The development was started with first prototype design of DAQ [1] on 6Usize PCB with on board FIFO, 12 bit ADC and CPLD. Signal of radiometer is in microscopic range, less than the lowest significant bit (LSB) voltage value (<2.4 mV) of the ADC, so gets buried in the noise band. The DAQ is redesigned to achieve greater resolution and compactness. The PCB size has been reduced drastically to a frame size of 90 × 89 mm by implementation of Low Profile Quad Flat Package (LQFP) package of ADC. The beauty of the DAQ is the 8 channel ADC with 16 bit resolution, can accommodate ±10 V true bipolar input signals with a sampling throughput rate up to 200 Ksps. Block diagram of the DAQ is shown in Fig. 4. Antialiasing filter has been implemented inside the ADC. On board RAM is 512 K. So one shot acquisition of 640 ms at 100 kHz can be taken. There is a provision of optically isolated trigger and clock oscillator. Writing in and Reading out of data from RAM is done through CPLD. The whole system is running on single 5 V supplied by SBC smps. VHDL Codes for generating control signals for PC104 Bus, ADC and RAM, logic for bus interface and data transfer are written in CPLD. The CPLD generates various control signals
Fig. 7. Raw data acquired by DAQ.
for ADC, RAM and coordinate the PC104 bus memory transaction. We have selected Advantech to make single board computer PCM3362 N which is featured with N450 1.66 MHZ processor, on board 4GB Flash display chipset and Ethernet controller. The system clock is 8.25 MHz. Fig. 5 shows the picture of hardware arrangement. Two modules comprising 8 channels each are stacked above one another to get 16 channels. The SBC board is placed at the bottom. The software code written for customized hardware allows maximum 8 modules. Thus modular PCB can be stacked and channel expansion occurs in multiple of eight. 3. Software details The system runs on Windows XP embedded operating system (OS). Some software like Labview run time Engine, Tight BNC is required to be installed on the SBC to make it ready for operation and access. The graphical user interface GUI is the eye of a DAQ. Two GUI applications are programmed in labview language. First application
Fig. 6. Screen shot of averaging utility.
Fig. 8. Plot of radiometer’s channel frequency versus V.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
G Model FUSION-8756; No. of Pages 4
ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
4
is responsible for single shot and continuous acquisition. In this Labview, Inport.vi and Outport.vi are used to access I/O port of ISA bus. This vi (virtual instrument) is applicable only for 8, 16- bit port I/O access. In hardware continuous acquisition for long time is not possible due to RAM. So labview application is modified in such a way that only one sample of each channel is read at set interval. The sampling frequency is decided by the time set in the GUI. Maximum sampling rate is limited to1khz. But the acquisition period is unlimited, depending on the remote PC Memory. Of course the plotting will be somewhat difficult. The data stored are in binary format. Another utility is for offline signal processing and averaging of the signal. Fig. 6. shows the screen shot of averaging utility. This GUI calls all nine binary files generated by continuous GUI. The chopper signal is taken as reference as channel 9. The High level and the Low level indicate room temperature (RTP) and hot temperature (HTP) measurements respectively. Indexing is done by detection of High to Low and Low to High transition and accordingly two array containing data corresponding to HTP and RTP are created. Radiometer signals corresponding ch1 to ch8 are averaged according to the index array. Averaging process is done in two ways single point averaging and additive averaging. Finally difference voltage (V) of corresponding mean of HTP and RTP is calculated for each channel and stored in excel file. 3.1. Principle of averaging Averaging is one of the Noise reduction techniques. Signal averaging [4] exploits the fact that if one makes a measurement many times the signal part will tend to accumulate but the noise will be random and tend to cancel it. Assuming a signal voltage Vs (t) which is superimposed with a noise voltage Vn (t) that is comparable or larger than Vs. The voltage actually measured is then the sum of these two parts V(t) = Vs(t) + Vn(t) N records improve the original signal to noise ratio by a factor of N. This improvement is bought at the expense of measurement time. If we record V(t) over some time interval we have a record which contains information about the time variation of Vs. The crucial point is to average many such records of V(t) so that the noise, which is equally likely to be positive or negative, tends to cancel out while the signal builds up. This obviously requires that we start each measurement at the same relative point in the signal, a requirement we will consider when we implement the averaging. This requirement is fulfilled by chopper signal.
4. Results and conclusion Fig. 7 shows the raw data acquired for one of the shots during calibration of the ECE radiometer between hot temperature (HTP, 500 ◦ C) and room temperature (RTP).The square waveform at the top of the graph is the data of the reference channel that indicates whether the radiometer horn is viewing the hot surface or the room temperature. The 0 V line is the readings depict the radiometer horn looking at the hot surface and the 7.5 V line is the radiometer horn looking at the room temperature. However, as can be seen in CH3 and CH4 the output voltage varies sinusoidal with 0 V (RTP) and 7.5 V (HTP) of the reference channel. But the other channels being lossy and less sensitive are unable to detect this change of microvolt level. It is because of this reason that one goes for continuous averaging of the signal that improves the signal to noise ratio and is implemented in our further measurements with the developed software. The Fig. 8 shows the graph between Radiometer’s channel frequency and V, difference voltage between HTP and RTP, calculated from averaging utility for respective channel. The difference voltage as detected by the radiometer CH3 is seen to be highest (25 microvolt) in comparison to other seven channels which indicates that the CH3 is most sensitive while CH7and CH8 are very lossy and lowest in sensitivity. We have done several shots on constant temperature of 500◦ C. We are getting good results except some lossy channels. In further development more statistical analysis will be added in Labview. Emphasis will be given in low noise amplifier design at higher gain and increase in resolution of the DAQ. Acknowledgments I would like to give my sincere gratitude with extreme pleasure to my colleague for their co-operation and whoever guided in designing and its implementation. References [1] R. Rajpal, J. Patel, P. Kumari, V. Panchal, P.K. Chattopadhyay, H. Pujara, Y.C. Saxena, Embedded data acquisition system with MDSPlus, Fusion Eng. Des. 87 (2012) 2166–2169. [2] V. Siju, D. Kumar, P. Shukla, S.K. Pathak, Characterization and calibration of 8-channel E-band heterodyne radiometer system for SST-1 tokamak, Rev. Sci. Instrum. 85 (2014) 053503, http://dx.doi.org/10.1063/1.4873197. [4] PHYS 331, Junior Physics Laboratory.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
ARTICLE IN PRESS
FUSION-8756; No. of Pages 4
Fusion Engineering and Design xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
Fusion Engineering and Design journal homepage: www.elsevier.com/locate/fusengdes
Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer Praveena Kumari ∗ , Vismaysinh Raulji, Hitesh Mandaliya, Jignesh Patel, Varsha Siju, S.K. Pathak, Rachana Rajpal, R. Jha Institute for Plasma Research, Nr. Indira Bridge, Bhat, Gandhinagar 382428, India
h i g h l i g h t s • • • •
Indigenous development of an electronics and data acquisition system to digitize signals for a desired time and automatization of calibration process. 16 bit DAQ board with form factor of 90 × 89 mm. VHDL Codes written for generating control signals for PC104 Bus, ADC and RAM. Averaging process is done in two ways single point averaging and additive averaging.
a r t i c l e
i n f o
Article history: Received 20 June 2015 Received in revised form 29 April 2016 Accepted 10 May 2016 Available online xxx Keywords: Complex programmable logic device (CPLD) Signal conditioning unit (SCU) Very high speed integrated circuit hardware description logic (VHDL) PC104- family of embedded computer standards define both form factor and computer buses
a b s t r a c t Signal conditioning units (SCU) along with Multichannel Data acquisition system (DAS) are developed and installed for automatization and frequent requirement of absolute calibration of ECE radiometer system. The DAS is an indigenously developed economical system which is based on Single Board Computer (SBC). The onboard RAM memory of 64 K for each channel enables the DAS for simultaneous and continuous acquisition. A Labview based graphical user interface provides commands locally or remotely to acquire, process, plot and finally save the data in binary format. The microscopic signals received from radiometer are strengthened, filtered by SCU and acquired through DAS for the set time and at set sampling frequency. Stored data are processed and analyzed offline with Labview utility. The calibration process has been performed for two hours continuously at different sampling frequency (100 Hz to 1 KHz) at two set of temperature like hot body and the room temperature. The detailed hardware and software design and testing results are explained in the paper. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The electron cyclotron emission radiometer is an effective diagnostic to measure the plasma electron temperature and its thermal distribution. However, to determine the relation between radiometer output and plasma emissivity the system needs to be calibrated. The most common method of calibration is the technique of Hot/cold switch method wherein the output of the radiometer is recorded at two different temperatures i.e. hot and cold. Calibration factors so obtained are used to determine the plasma temperature and its spatial and temporal evolution. The accuracy of calibration thus determines the accuracy of the temperature measurements. Henceforth frequent calibration is done to improve on the method
∗ Corresponding author. E-mail address: [email protected] (P. Kumari).
used. Frequent requirement of independent calibration of radiometer and automatization of its process motivated us to design a Multichannel Data acquisition system (DAS) along with SCU. Our main aim is indigenous development of an electronics and data acquisition system to digitize signals for a desired time. Another aim of this development is to establish an automatization of the calibration process along with the improvement in system sensitivity so that very low signal can also be made detectable. Automatization of calibration process through DAQ will save time, avoids manual error and provides scope for signal processing on acquired data. The DAS is based on Single Board Computer (SBC). The hardware and software design supports the DAS for simultaneous, single shot and continuous acquisition. A Labview based graphical user interface (GUI) provides commands locally or remotely to acquire, process, plot and finally save the data. Offline Signal processing by implementation of averaging technique on the
http://dx.doi.org/10.1016/j.fusengdes.2016.05.013 0920-3796/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
G Model FUSION-8756; No. of Pages 4
ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
2
Fig. 1. Block diagram of test set-up of calibration.
Fig. 3. Block diagram of signal conditioning.
2.1. Radiometer digitized data and their analyzing facilities are coded in another Labview GUI.
2. Experimental set-up The process of radiometer calibration is the standard technique of Hot/cold switch method. A silicon carbide based black body source that can be heated upto 600 ◦ C acts as the hot body while the room temperature acts as the cold body. The experimental setup is shown by block diagram in Fig. 1. The horn antenna is made to view the two different sources through a chopper arrangement. The chopper circuit consists of a fan rotating at a speed of 2 Hz that can be varied. Eco-absorber sheets are pasted on the fan wings so that it absorbs the cosmic microwave radiations. A light source and a photo diode arrangement are along with the receiving antenna and black body source. The output of the photo diode so obtained is a square pulse that acts as the reference signal for signal averaging. A thermocouple is used to monitor the temperature from the hot body source. The fan wings are so aligned that they simultaneously cover the receiving antenna and the light source so that the photo diode receives a 0 V reference line as soon as the fan wings move and the antenna is made to view the hot source simultaneously the photodiode records the light source as the high of the square pulse so generated. The 8-channel radiometer output along with the output of the photo diode that acts as the 9th reference channel are acquired in the DAS for the set time and at set sampling frequency. Calibration process has been performed several times for two hours continuously at 1 KHz sampling frequency at hot black body source at constant temperature of 500 ◦ C and RTP. A temperature measurement circuit through thermocouple is also included to monitor the temperature of hot source.
Fig. 2. Block diagram of radiometer.
The ECE radiation is collected by a 20 dBi gain horn antenna and passed on to a band pass filters that provide the input RF of 63–75 GHz [2]. This RF is down converted using a SSB mixer to an IF of 1–12 GHz. The signal is then boosted using a pair of cascaded low noise amplifiers with an approximate gain of 58 dB. An 8- way power divider distributes the input IF into 8 channels which are further frequency selected from 1.4 GHz to 10.5 GHz in a step size of 1.3 GHz using narrow band cavity filters. This frequency selective signal is detected using a Schottky detector followed by the electronics video amplifiers for further processing. Fig. 2 depicts the block diagram of the radiometer system. 2.2. Signal conditioning unit The SCU unit contains 8 modules. Design of one module is explained by block diagram in Fig. 3. The microwave components used in radiometer has its own offset and as the stages increases the DC offset gets accumulated. This unwanted offset has to be removed. The signal coming from the hot body source is a slow varying signal of vary low amplitude ranges from 1 V to 200 V. So both the signal and the undesired offset are amplified with same gain by first stage amplifier. A DC feedback circuit is implemented in first stage for offset removal by using the reference terminal of the instrumentation amplifier (INA).The reference terminal provides a direct means of injecting a precise offset to the output.The voltage on the reference terminal can be varied from negative to positive supply as per the requirement. The 2nd stage amplification is applied to the signal with the DC component already removed. Signal passes through an eight order filter module. The cut off frequency can be changed in four steps. As the same SCU set up is used to install during Aditya tokamak experimental shot. So we insert an
Fig. 4. Block diagram of DAS.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
G Model FUSION-8756; No. of Pages 4
ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
3
Fig. 5. Picture of stacked board with SBC.
analog optocoupler for isolation with DAQ ground. The differential driver insures the signal amplitude throughout the long cable, laid to the DAQ. 2.3. Data acquisition system The development was started with first prototype design of DAQ [1] on 6Usize PCB with on board FIFO, 12 bit ADC and CPLD. Signal of radiometer is in microscopic range, less than the lowest significant bit (LSB) voltage value (<2.4 mV) of the ADC, so gets buried in the noise band. The DAQ is redesigned to achieve greater resolution and compactness. The PCB size has been reduced drastically to a frame size of 90 × 89 mm by implementation of Low Profile Quad Flat Package (LQFP) package of ADC. The beauty of the DAQ is the 8 channel ADC with 16 bit resolution, can accommodate ±10 V true bipolar input signals with a sampling throughput rate up to 200 Ksps. Block diagram of the DAQ is shown in Fig. 4. Antialiasing filter has been implemented inside the ADC. On board RAM is 512 K. So one shot acquisition of 640 ms at 100 kHz can be taken. There is a provision of optically isolated trigger and clock oscillator. Writing in and Reading out of data from RAM is done through CPLD. The whole system is running on single 5 V supplied by SBC smps. VHDL Codes for generating control signals for PC104 Bus, ADC and RAM, logic for bus interface and data transfer are written in CPLD. The CPLD generates various control signals
Fig. 7. Raw data acquired by DAQ.
for ADC, RAM and coordinate the PC104 bus memory transaction. We have selected Advantech to make single board computer PCM3362 N which is featured with N450 1.66 MHZ processor, on board 4GB Flash display chipset and Ethernet controller. The system clock is 8.25 MHz. Fig. 5 shows the picture of hardware arrangement. Two modules comprising 8 channels each are stacked above one another to get 16 channels. The SBC board is placed at the bottom. The software code written for customized hardware allows maximum 8 modules. Thus modular PCB can be stacked and channel expansion occurs in multiple of eight. 3. Software details The system runs on Windows XP embedded operating system (OS). Some software like Labview run time Engine, Tight BNC is required to be installed on the SBC to make it ready for operation and access. The graphical user interface GUI is the eye of a DAQ. Two GUI applications are programmed in labview language. First application
Fig. 6. Screen shot of averaging utility.
Fig. 8. Plot of radiometer’s channel frequency versus V.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013
G Model FUSION-8756; No. of Pages 4
ARTICLE IN PRESS P. Kumari et al. / Fusion Engineering and Design xxx (2016) xxx–xxx
4
is responsible for single shot and continuous acquisition. In this Labview, Inport.vi and Outport.vi are used to access I/O port of ISA bus. This vi (virtual instrument) is applicable only for 8, 16- bit port I/O access. In hardware continuous acquisition for long time is not possible due to RAM. So labview application is modified in such a way that only one sample of each channel is read at set interval. The sampling frequency is decided by the time set in the GUI. Maximum sampling rate is limited to1khz. But the acquisition period is unlimited, depending on the remote PC Memory. Of course the plotting will be somewhat difficult. The data stored are in binary format. Another utility is for offline signal processing and averaging of the signal. Fig. 6. shows the screen shot of averaging utility. This GUI calls all nine binary files generated by continuous GUI. The chopper signal is taken as reference as channel 9. The High level and the Low level indicate room temperature (RTP) and hot temperature (HTP) measurements respectively. Indexing is done by detection of High to Low and Low to High transition and accordingly two array containing data corresponding to HTP and RTP are created. Radiometer signals corresponding ch1 to ch8 are averaged according to the index array. Averaging process is done in two ways single point averaging and additive averaging. Finally difference voltage (V) of corresponding mean of HTP and RTP is calculated for each channel and stored in excel file. 3.1. Principle of averaging Averaging is one of the Noise reduction techniques. Signal averaging [4] exploits the fact that if one makes a measurement many times the signal part will tend to accumulate but the noise will be random and tend to cancel it. Assuming a signal voltage Vs (t) which is superimposed with a noise voltage Vn (t) that is comparable or larger than Vs. The voltage actually measured is then the sum of these two parts V(t) = Vs(t) + Vn(t) N records improve the original signal to noise ratio by a factor of N. This improvement is bought at the expense of measurement time. If we record V(t) over some time interval we have a record which contains information about the time variation of Vs. The crucial point is to average many such records of V(t) so that the noise, which is equally likely to be positive or negative, tends to cancel out while the signal builds up. This obviously requires that we start each measurement at the same relative point in the signal, a requirement we will consider when we implement the averaging. This requirement is fulfilled by chopper signal.
4. Results and conclusion Fig. 7 shows the raw data acquired for one of the shots during calibration of the ECE radiometer between hot temperature (HTP, 500 ◦ C) and room temperature (RTP).The square waveform at the top of the graph is the data of the reference channel that indicates whether the radiometer horn is viewing the hot surface or the room temperature. The 0 V line is the readings depict the radiometer horn looking at the hot surface and the 7.5 V line is the radiometer horn looking at the room temperature. However, as can be seen in CH3 and CH4 the output voltage varies sinusoidal with 0 V (RTP) and 7.5 V (HTP) of the reference channel. But the other channels being lossy and less sensitive are unable to detect this change of microvolt level. It is because of this reason that one goes for continuous averaging of the signal that improves the signal to noise ratio and is implemented in our further measurements with the developed software. The Fig. 8 shows the graph between Radiometer’s channel frequency and V, difference voltage between HTP and RTP, calculated from averaging utility for respective channel. The difference voltage as detected by the radiometer CH3 is seen to be highest (25 microvolt) in comparison to other seven channels which indicates that the CH3 is most sensitive while CH7and CH8 are very lossy and lowest in sensitivity. We have done several shots on constant temperature of 500◦ C. We are getting good results except some lossy channels. In further development more statistical analysis will be added in Labview. Emphasis will be given in low noise amplifier design at higher gain and increase in resolution of the DAQ. Acknowledgments I would like to give my sincere gratitude with extreme pleasure to my colleague for their co-operation and whoever guided in designing and its implementation. References [1] R. Rajpal, J. Patel, P. Kumari, V. Panchal, P.K. Chattopadhyay, H. Pujara, Y.C. Saxena, Embedded data acquisition system with MDSPlus, Fusion Eng. Des. 87 (2012) 2166–2169. [2] V. Siju, D. Kumar, P. Shukla, S.K. Pathak, Characterization and calibration of 8-channel E-band heterodyne radiometer system for SST-1 tokamak, Rev. Sci. Instrum. 85 (2014) 053503, http://dx.doi.org/10.1063/1.4873197. [4] PHYS 331, Junior Physics Laboratory.
Please cite this article in press as: P. Kumari, et al., Development of electronics and data acquisition system for independent calibration of electron cyclotron emission radiometer, Fusion Eng. Des. (2016), http://dx.doi.org/10.1016/j.fusengdes.2016.05.013