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

Page 9 of 25  

 

A second so-called “walk comparator” in the circuit operates in the 

same way with the bipolar signal. Its reference, the 

Walk level

, shall 

be set very near and slightly above the baseline level of the input 

signal so that it “triggers in the noise”. Then the bipolar signal 

produces a distinct series of transitions as the noise fluctuates around 

this level (see Figure 3.6b). In presence of a significant input signal a 

certain transition from “low” to “high” represents the zero-crossing 

of the bipolar signal (the “

timing transition

”) which shall serve as the 

accurate timing output of the CFD circuit. This signal is produced by 

a logic AND-gate of the two comparator outputs. 

The 

RoentDek

 

CFDs

  allow monitoring the output of the walk 

comparator via the digital monitor output socket labelled 

M

d

.

*

 

 

 

The 

Threshold level

 and the 

Walk level

 can be set via two potentiometers labelled 

Th

 and 

(Z standing for Zero Crossing Level) 

with corresponding test points to measure the actual levels with a voltmeter. A ground reference point labelled GND is 

provided on the front panel of most units. The width of the CFD timing output signals is adjustable via a potentiometer 

labelled 

W

 on the front panel. The width can be increased by a factor of 10 by placing a jumper on the dedicated position 

through a hole in the side panel (

CFD8/7x

: top lid) for each channel individually. 

Note that the test points can only give coarse information about the setting (within about ±5 mV). For optimal results and 

adjustment, the 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 3.6b also shows the 

position of the jumper terminal for increasing the timing signal output width  

 

The 

RoentDek

 CFD is useful for signals with rise/fall time > 1 ns. If the input signal source has less than 3 ns FWHM the 

CFD might not operate optimally even with small CFD delay and optimized settings of the fraction (see below). The input 

signal heights should be kept between 50 mV and -2 V (on 50 

Ω impedance) by selecting a proper gain on the gi

ven amplifier. 

The input should not be exposed to signals higher than -3 V or DC levels higher than ±1 V.  

 

The oscilloscope traces in the next chapter are shown as examples for optimal parameter settings and the effect of changing a 

certain parameter. 

3b.3

 

Adjusting the CFD parameter 

Figure 3.7b shows all relevant signals with a near-optimal adjustment of the parameters 

Threshold level

Walk level

CFD delay

 and 

CFD fraction

 for a given input signal of about 5 ns rise time (upper trace). The traces trigger on the negative slope of the lowest 

trace, which is the CFD timing output. The analogue and digital walk monitor outputs are displayed on the second and third 

trace. The 

CFD fraction

 was set to 0.35 (default) and cable produces 4 ns external delay which results in an effective 

CFD delay

 

of about 4.5 ns, i.e. sightly increased by internal delays on the circuit board). 

                                                           

*

 Note the M

d

 output has a DC offset of about -0.1 V 

Figure 3.6b: Schematics of the  

CFD digital circuit chain 

Figure 3.5b: top/side panel with fraction 

potentiometer only versions a/b) and optional 

jumper terminal for increasing the CFD output 

width by a factor of 10 (optional) 

Содержание CFD1c

Страница 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...

Страница 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...

Страница 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...

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

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

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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...

Страница 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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