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The Constant Fraction Discriminators CFD8c, CFD7x, CFD4c, CFD1c and CFD1x (11.0.1701.1)  

 

Figure 3.7b: Oscilloscope traces of in- and output signals of the CFD, traces 1-4 from top: input signal, analogue 

monitor output signal, digital monitor output signal, CFD timing output signal (which triggers all traces). The red 

arrows indicate the 

timing transition

 which appears slightly shifted in the different traces due to internal delays. 

This and the following images of signal traces have been obtained with a 300 MHz analogue oscilloscope (a large 

number of signals with varying pulse heights are superimposed). Similar views can be obtained from digital 

oscilloscopes with adequate “persistence” setting 

The 

Threshold level

 (see below) was set so that even the smaller signals from the pulse height distribution are registered. This can 

be verified both on the first trace (input signal) and on the analogue monitor output (trace 2). The input signal cannot always 

be displayed together with the CFD outputs unless the signal source (e.g. amplifier) has two independent outputs sockets. 

Therefore the trace of the signal input was omitted on the next figures. The analogue walk monitor may serve as well for 

verifying the threshold level but it is to note that the analogue walk monitor signal height is damped by a factor of 10. 

 

For achieving the ideal settings it is first of all important to note that the CFD operates best (in all respects) and yields the 

optimal temporal resolution if the input signals are as high as possible but only up to -2 V. The threshold should not be set 

smaller than 50 mV for the multi-channel versions because then there is a risk of inter-channel cross talk (see Chapter 3b.5.3)

 

Please note that it is preferable to obtain such high signals rather by a low amplifier gain but high “raw” signal from the given 

detector (e.g. high gain on the physical electron multiplier MCP, PMT, Channeltron …) if one has this choice. This can for 

example be achieved by increasing the bias voltage on the physical electron multiplier. 

 

For a given electronic noise level, signal-to-noise ratio can only be improved by increasing the gain of the primary charge input 

from the detector source to the electronics. Only then signals have optimal signal-to-noise ratio which will finally determine 

achievable resolution. 

3b.3.1

 

Threshold level 

The 

Threshold level 

shall be set just above the input signal line’s noise level. This can be verified by observing the CFD output 

simultaneously with the analogue walk output on an oscilloscope as shown in Figure 3.8b. 

 

If the threshold is set too high, small (but valid) signals are not registered (see Figure 3.8b, left picture), if it is set too low (right 

picture), the CFD output will also trigger on spurious noise components and produce background counts. 

Summary of Contents for CFD1c

Page 1: ...Dek Handels GmbH Supersonic Gas Jets Detection Techniques Data Acquisition Systems Multifragment Imaging Systems The RoentDek Constant Fraction Discriminators CFD8c CFD7x CFD4c CFD1c and CFD1x 11 0 17...

Page 2: ...Kernphysik Max von Laue Str 1 D 60438 Frankfurt am Main Germany Web Site www roentdek com WEEE DE48573152 Product names used in this publication are for identification purposes only and may be tradema...

Page 3: ...3 CFD fraction 12 3b 3 4 Walk level 13 3b 3 5 Output signal width 14 3B 4 THE VETO OPTION 14 3b 4 1 Accessing the jumpers in CFD1c CFD1x 16 3b 4 2 Accessing the jumpers in CFD8c and CFD7x 16 3b 4 3 A...

Page 4: ...Page 4 of 25 The Constant Fraction Discriminators CFD8c CFD7x CFD4c CFD1c and CFD1x 11 0 1701 1...

Page 5: ...Constant Fraction DiscriminatorsCFD8b CFD4b and CFD1b 11 0 1701 1 Page 5 of 25...

Page 6: ...odules containing such boards please review first Chapters 3b 1 to 3b 3 and then refer to Chapter 3b 7 to learn about the difference between the standard CFD and the RoentDek bCFD settings 3b 1 Genera...

Page 7: ...weight 0 8 kg with an external 12 V DC mains adapter for use with 100 250 V AC sockets The power consumption is max 0 5 A at 12 V DC 6 W or max 0 7 A at 12 V DC 10 W for the CFD1x The CFD4c has the s...

Page 8: ...fraction is determined by the choice of fix resistors or a potentiometer as in case of the RoentDek CFDs While one of the signals is inverted later on the other experiences a certain delay CFD delay...

Page 9: ...he signals have to be verified on an oscilloscope The CFD Fraction can be adjusted by another potentiometer labelled Fr on the front panel CFD4b CFD1b side panel CFD8b top lid see Figure 3 6b Figure 3...

Page 10: ...s damped by a factor of 10 For achieving the ideal settings it is first of all important to note that the CFD operates best in all respects and yields the optimal temporal resolution if the input sign...

Page 11: ...delay D of 5 5 ns Note that the RoentDek CFDs will not operate without connecting an external delay cable bridge If the pulse rise time RT is defined as the time from reaching 10 to 90 of the signal m...

Page 12: ...livers the CFD with a standard CFD fraction setting of about 0 35 The CFD fraction can be varied between 0 15 and 1 via a potentiometer In order to observe and quantify the CFD fraction ratio on the o...

Page 13: ...during the bipolar signal s crossover while fluctuating almost equally between high and low states before and long after the signal due to electronic noise on the line not visible in the pictures If...

Page 14: ...idth bridges of the internal circuit see Figure 3 5b With the jumper in place the maximum width is increased to 2000 ns Please contact RoentDek if you are in need of the optional jumpers Obviously a s...

Page 15: ...15b Trace of the input signal to the Veto socket upper trace and signal traces of CFD timing signals lower trace integrated over many events for Veto left and Gating mode right In order to work reliab...

Page 16: ...he front panel 3b 4 2 Accessing the jumpers in CFD8c and CFD7x I Disconnect the mains power supply II On the rear panel remove all five screws red circles Please note that different types and sizes of...

Page 17: ...ear front panel and fix them with all screws and hex bolts 3b 4 3 Accessing the jumpers in CFD4c I Remove all six screws from the right side panel marked in red see Figure 3 20b Figure 3 20b Side pane...

Page 18: ...oscope see Figure 3 21b If a CFD is used for operating a RoentDek delay line detector or similar device these pre trigger events result in a distinct structure in the time sum spectrum see Figure 3 21...

Page 19: ...neighbours 3b 6 The CFDx pulse height determination option The CFDx function is an add on circuit that allows measuring the signal s pulse height The pulse height information is coded as a time delay...

Page 20: ...er trace ramp monitor output the Ramp switch is turned off here Signals are triggered on the CFD timing output signal trace not shown Left and right pictures were obtained with two different cable len...

Page 21: ...Dx signal This is indicated in Figure 3 26b The ramp slope is about 1 V per 12 ns giving an almost linear correspondence between pulse height and time delay with a typically 10 to 20 ns offset for pul...

Page 22: ...iming signal with a RoentDek TDC8HP The same spectrum could be recorded by plotting the time difference between trailing and leading transitions of the CFDx output It is advisable to mark the pulse he...

Page 23: ...se heights will not be registered Please contact RoentDek for advice unless you have received a properly matched system with bFAMP for a certain delay line read out anode Timing signals off the MCP co...

Page 24: ...3 12B SIGNAL TRACES AS IN FIGURE 3 8B LEFT WITH THE WALK LEVEL SET TO THE BASELINE OF THE INPUT SIGNALS LEFT PICTURE AND SLIGHTLY TOWARDS THE POSITIVE SIDE WHERE THE NOISE TRIGGERS BEGIN TO VANISH RIG...

Page 25: ...DTH POTENTIOMETER SETTING LOWER TRACE RAMP MONITOR ALL TRACES TRIGGERED ON THE STOP SIGNAL THE STOP SIGNAL SHOULD BE USED IF THERE IS A SPARE TDC CHANNEL AVAILABLE 21 FIGURE 3 27B AS FIGURE 3 25B RIGH...

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