1. Time over Threshold Electronics for Neutrino Telescopes
Physics Laboratory
School of Science and Technology
Hellenic Open University
Time over Threshold Electronics for Neutrino Telescopes
George Bourlis
My talk is about the ReadOut Electronics developed for the HELYCON Experiment and the Time Over Threshold Technique adopted for the charge estimation of the photomultipliers. KM3NeT??
VLVnT09, Athens, Greece, October 2009

2. Time over Threshold Analogue signal Comparators’ output t1 t2 t3 t4 t5
The ToT technique is based on time-tagging of the leading and trailing edge of the PMT signal above a certain threshold. The analogue input signals go through comparators which compare them with the desired thresholds.
The outputs of the comparators are then fed to the TDC that performs the time-tagging of the leading and trailing edges.
These values are subsequently used for the reconstruction of the pulse shape and its charge. Applying the scheme to multiple thresholds increases the efficiency of the technique significantly. - t1 t2 t3 t4 t5 t6
HPTDC output
VLVnT09, Athens, Greece, October 2009

3. Readout Electronics 5 PMT Signal Inputs Trigger Output USB Port HPTDC
32 channels (LR) – 8 Channels (HR)
25ps (HR) to 100 ps (LR) accuracy
Self Calibrating
This is a schematic of the ReadOut Electronics. The card bears 5 signal inputs. The signals are amplified and then go through the set of comparators. The outputs of the comparators are driven to the heart of the system, which is a High Precision Time to Digital Converter developed at CERN. The TDC’s outputs are then led along with the GPS signal to the FPGA (a Xilinx one) which includes filter algorithms, trigger logic and performs the event building algorithms. The communication between the FPGA and the hosting computer is handled by a Cypress USB Controller.
The HPTDC has an accuracy of 25ps with 8 channels or 100ps with 32 channels.
The outputs of the HPTDC is the times the input pulses cross the 6 programmable thresholds in either direction (leading and trailing edge) and our aim is to estimate the charge of the pulses using this information.
25ps accuracy TDC
GPS Input
VLVnT09, Athens, Greece, October 2009

4. Readout Electronics VLVnT09, Athens, Greece, 13-15 October 2009
This is a snapshot of the card, where u can see
the HPTDC,
the FPGA
and the USB Controller along with the USB port of the card.
VLVnT09, Athens, Greece, October 2009

5. EZ-USB FX2 HPTDC CONTROL
This is the interface of the program we currently use to communicate with the electronics and perform data acquisition.
We can enable or disable channels and set the thresholds for each channel (there are 6 channels for each input corresponding to the 6 thresholds).
*************
VLVnT09, Athens, Greece, October 2009

6. ToT In Action VLVnT09, Athens, Greece, 13-15 October 2009
These are results of the initial tests performed. You can see a pulse produced by a function generator and the reconstructed pulse using the times of the threshold crossings.
And here both on the same plot
VLVnT09, Athens, Greece, October 2009

7. Board Debugging Configuration
Offset of each input
Scale of each input
Timing among input channels
Minimum threshold that can be applied to the channels of each input
Trigger
Input
splitter
discriminator
The card has gone under extended testing aiming at determining the offset of each of the channels, the exact value of the factor by which each input signal is amplified and determining the timing among the input channels. The initial version of the card bore different multiplication factors for each channel, so that the best choice could be figured out, trying to balance the desire for a higher factor so as to be able to reconstruct large pulses and that for a small factor that would allow the reconstruction of small pulses.
The adopted testing configuration was based on the use of the signal of one HELYCON detector, which is split and driven to the card and a high precision Tektronix oscilloscope. A discriminator is also used for the triggering of the oscilloscope, whose signal is also led to the card to be used for the identification of pulses that have been recorded by both the card and the oscilloscope.
VLVnT09, Athens, Greece, October 2009

8. Multiplication factor vs Thresholds1x
1.5x
2.5x 5x
Measured factor
0.991
1.0035
1.503
2.7735
4.995
Minimum Threshold (mV)
9.78
5.25
1.69
1.57
1.42
Maximum Threshold (mV)
2000
1333
800
400
Pulses exceeding 2V are not distorted but truncated and can still be reconstructed
1.57mV corresponds to around 1/3 of a MIP for the HELYCON detectors
800mV corresponds to MIPs for the HELYCON detectors
Among the results of the testing procedure, the most interesting are those that have to do with the multiplication factors of the inputs. The initial version of the board had inputs with multiplication factors of 1, 1, 1.5, 2.5 and 5. The comparators can go up to 2V which sets the maximum threshold that can be applied to each input in relation with the respective multiplication factor. Of course, pulses higher than 2V are not distorted by the electronics but truncated instead so that they can still be reconstructed. The minimum threshold that can be applied to each channel was determined by the noise level for each input. This noise is due to the signal lines in the board but also to the amplifiers. As you can see the 2.5 and 5 factors allows a minimum threshold of around 1.5mV but for the 2.5 factor the maximum threshold that can be applied is 800mV.
So the chosen factor for the final version of the board is 2.5, which allows a minimum threshold of 1.57mV which is around 1/3 of a MIP for the HELYCON detectors and a maximum threshold of 800mV which corresponds to around MIPs for the HELYCON detectors.
VLVnT09, Athens, Greece, October 2009

9. HELYCON PMT & Detector Photomultiplier Tube PH: XP1912
At “nominal” High Voltage
Gain: ~ 4 105
<charge>/p.e. ~ 0.07pCb
<pulse height>/p.e. ~ 1.05mV
Rise Time: 1.2 ns
Photomultiplier Tube PH: XP1912
Charge (in units of mean p.e. charge)
At the Detector Center
Data
- Monte Carlo Prediction
Charge (pCb)
Single pe
Before going on a few words about the HELYCON photomultipliers and detectors. The PMT is the Photonis XP1912, these are its characteristics, here is the slope for a number of PMTs and this is the charge distribution for different light intensities at the single pe level.
This is charge distribution in units of mean photoelectron charge at the detector center for MIPs
and this one is the charge distribution in units of MIP response during showers collecting.
VLVnT09, Athens, Greece, October 2009

10. Sample Pulses Input 1 VLVnT09, Athens, Greece, 13-15 October 2009
These are some pulses as recorded by the oscilloscope (the line) and the red dots correspond to the data acquired by the electronics.
VLVnT09, Athens, Greece, October 2009

11. Charge vs Time over Threshold
3 thresholds
Charge (pC)
Time Over Threshold (s)
1st Threshold only
1st & 2nd Threshold
1st, 2nd & 3rd Threshold
4mV
50mV
15mV
In order to evaluate the performance of the Time Over Threshold technique for charge estimation, we have used data from a HELYCON detector. The pulses’ times over 3 thresholds (4, 15 and 50mV) were calculated
and this is the distribution of the charge versus the sums of the times over threshold for pulses that crossed only the 1st threshold, the 1st and the 2nd threshold and finally all 3 thresholds.
VLVnT09, Athens, Greece, October 2009

12. Charge versus Time over Threshold
6 thresholds
The chosen thresholds allow the reconstruction of higher pulses with good accuracy, while smaller pulses (fewer thresholds crossed) can still be reconstructed with acceptable accuracy
100mV
And this is the same distribution for 6 thresholds where all 6 areas are plotted together.
70mV
40mV
20mV
10mV
3mV
VLVnT09, Athens, Greece, October 2009

13. Charge Parameterization
Then, the charge was parameterized using a polynomial function of the times over threshold. These are the results for pulses that crossed the 1st & 2nd threshold and for pulses that crossed the first 3 thresholds.
Parameterization of the charge vs the sum of the times over threshold for each of the areas were 1, 2, 3, 4, 5 or 6 thresholds were crossed
VLVnT09, Athens, Greece, October 2009

14. Measured Charge – Estimated Charge Measured Charge – Estimated Charge
Charge Estimation
4 thresholds
Measured Charge – Estimated Charge
Measured Charge – Estimated Charge
Measured Charge
Estimated Error
-Then we estimated the charge using the parameterizations and compared it to the measured charge. Here you can see the distribution of the measured charge minus the estimated charge divided with the measured charge for pulses that crossed 4 thresholds.
-The resolution turns out to be around 4.3%.
-The respective pull distribution shows that the estimation is unbiased.
Estimated Resolution 4.3%
Pull distribution sigma of 1.04 ± 0.07
VLVnT09, Athens, Greece, October 2009

15. Charge Estimation Resolution
Charge estimation resolution vs number of thresholds crossed
Thresholds crossed 1 2 3 4 5 6
Charge estimation resolution (%)
13.6
6.9
5.6
4.3
3.4
2.9
And this is the charge estimation resolution versus the number of crossed thresholds. Of course it gets better if more thresholds are crossed.
VLVnT09, Athens, Greece, October 2009

16. Conclusions
The time over threshold technique allows for charge estimation with very good resolution for higher pulses (even if the thresholds do not go very far up the pulse height). This resolution goes down to ~3% with 6 thresholds which is sufficient both for the HELYCON and the KM3NeT telescopes.
Lower pulses can still be reconstructed with acceptable accuracy if the lower thresholds are more closely spaced near the baseline.
Charge estimation resolution depends on the number of thresholds crossed and on their specific values. The choice of the applied thresholds depends on the pulses recorded.
The above
VLVnT09, Athens, Greece, October 2009

17. Sample Pulses
Input 1
VLVnT09, Athens, Greece, October 2009

18. Sample Pulses
Input 1
VLVnT09, Athens, Greece, October 2009