Techniques for Nuclear and Particle Physics Experiments A

Techniques for Nuclear and Particle Physics Experiments: A How-to Approach W. R. Leo Ch. 14: Electronics for Pulse Signal Processing Anthony Moeller 1/18/2022 1

Introduction This chapter focuses on basic functions that can be applied to signals for information processing. Will concentrate on NIM modules as opposed to more sophisticated systems such as CAMAC. All modules described are commercially available. – Some modules combine several of these functions into one unit. 1/18/2022 2

14. 1 Preamplifiers The primary function is to amplify a signal from a detector and drive it through the cable that connects to the rest of the equipment. – Try to minimize noise. – Mounted close to detector to minimize cable length. Pickup of electromagnetic fields reduced. Cable capacitance also minimized. 1/18/2022 3

Three Types of Preamplifiers Voltage Sensitive Charge Sensitive Current Sensitive – Used for low impedance signal devices; radiation detectors are generally high impedance. – Current sensitive preamps will not be discussed further. 1/18/2022 4

Voltage Sensitive Preamps Amplifies the voltage at the input. The capacitance of the detector and any stray capacitances affect the voltage. – V=Q/Ctot – The detector capacitance must be stable. 1/18/2022 5

Charge Sensitive Preamps The capacitance of semiconductor detectors vary with temperature. Charge sensitive preamps eliminate the dependence on the detector capacitance. – V 0=-Q/Cf 1/18/2022 6

14. 1. 1 Resistive vs Optical Feedback In charge sensitive preamps, the charge on the capacitor must be removed. One method is a slow discharge through a resistive feedback network. – Produces an exponential tail. – The time constant is ~40 -50 μs. An optical feedback system can be used for more precise work. – The charge is kept until a fixed limit is reached. – An opposite internal current is triggered and the capacitor is discharged. – An inhibit signal blocks the negative pulse generated in this process. 1/18/2022 7

14. 2 Main Amplifiers Serves two main purposes: – Amplify the signal from the preamplifier – Shape it to an appropriate form for processing The pulse from the preamp typically has a long exponential tail. If a second pulse arrives before the end of this tail, it’s amplitude is increased. – This is known as pileup. – The pileup effect can be reduced by reshaping. Pulse shaping is also used to optimize the signal to noise ratio. 1/18/2022 8

14. 3 Pulse Shaping Networks in Amplifiers The two most common methods of pulse shaping in commercially available amplifiers are: – Delay line shaping – RC differentiation-integration Combinations of CR differentiators, RC integrators, and delay lines are used to limit bandwidth to improve the signal to noise ratio. – This results in a change of the pulse shape. 1/18/2022 9

14. 3. 1 CR-RC Pulse Shaping The signal is sent through a cascaded CR differentiator and RC integrator. – Filtered at low frequencies for differentiation, and at high frequencies for integration. Improvement in signal to noise ratio. – The optimal signal to noise ratio is often achieved with the same time constant for both differentiation and integration. The time constant can often be adjusted with a knob on the front panel of the amplifier. 1/18/2022 10

14. 3. 2 Pole-Zero Cancellation and Baseline Restoration RC pulse shaping results in an undershoot of the shaped pulse. The undershoot is caused by the differentiation of finite length tail pulses from the preamp. – Can be corrected using a pole-zero cancellation circuit. A variable resistor is added in parallel with the capacitor in the CR stage. 1/18/2022 11

14. 3. 3 Double Differentiation or CRCR-RC Shaping A baseline shift can occur at high count rates as the undershoot of successive pulses accumulates. One solution is to add an additional CR stage to the CR-RC cascade. – Double differentiating network. – A bipolar pulse is formed, which leaves no residual charge on the coupling capacitor. 1/18/2022 The baseline shift is avoided. 12

14. 3. 4 Semi-Gaussian Shaping Giving the pulse a Gaussian shape gives a theoretical 18% improvement in signal to noise ratio. – Shaping to an ideal Gaussian is not electronically possible. – But it can be approximated by an RC differentiation followed by several RC integrations. 4 or 5 of these integrations generally gives a decent approximation. 1/18/2022 13

14. 3. 5 Delay Line Shaping Delay line shaping is an alternative to RC shaping. Uses the reflections from delay lines. The rise time is unaltered. – Ideal for fast amplifiers. Signal to noise ratio is not as good as with RC shaping. 1/18/2022 14

14. 4 Biased Amplifiers Biased amplifiers are used to expand part of a signal. – Useful for pulse height analysis. – Has a variable threshold which rejects all signal below a set value. – Also readjusts the zero position by subtracting the threshold value from the part of the signal greater than the threshold. 1/18/2022 15

14. 5 Pulse Stretchers A pulse stretcher is a module that prolongs the duration of a signal at its peak value. The rise time and width are usually readjusted to a standard value. – The pulse height is preserved. 1/18/2022 16

14. 6 Linear Transmission Gate A linear transmission gate is basically a switch which only allows an input signal to pass through if a second signal is present at the gate input. The level of the gate signal is important. – Too low, some signal is lost. – Too high, a pedestal is created. Available as individual modules, but often included as a feature on other modules. 1/18/2022 17

14. 7 Fan-out and Fan-in Fan-outs divide an input signal into several identical signals (height and shape). – Useful for sending one signal to several different parts of an electronics system. Fan-ins take in multiple signals, and output the algebraic sum of those signals. Both are available in linear and logic varieties. 1/18/2022 18

14. 8 Delay Lines For coincidence measurements, the time of signal propagation is important. Delay boxes consist of variable lengths of cable. – One box typically allows delays of 0 -64 ns. – By using the delay boxes, can ensure that the signals arrive at the same time. For long delays (and thus long cable lengths), signal attenuation can be a factor. From personal experience, some older delay boxes add noise that can be avoided by simply using extra lengths of signal cable instead. 1/18/2022 19

14. 9 Discriminators If the pulse height of a signal is above a certain threshold, the discriminator outputs a logic signal. If not, there is no output signal. – The threshold and width of the output signal can usually be adjusted on the front panel. Commonly used for blocking low amplitude noise. Two parameters used for measuring speed: – Double pulse resolution: Smallest time between two input pulses which will result in two output pulses. – Continuous pulse train rate: The highest frequency of equally spaced pulses which the discriminator will accept. 1/18/2022 20

14. 9. 1 Shapers A shaper accepts pulses of different widths and heights and reshapes them into logic signals of standard levels and widths. – To trigger, a minimum signal height is required. – Function is identical to that of a discriminator. 1/18/2022 21

14. 10 Single Channel Analyzer (Differential Discriminator) Sorts incoming signals by amplitude. – Has a lower threshold below which signals are blocked. – Also has an upper threshold above which signals are blocked. – The opening between these levels is called the window. 1/18/2022 22

Three Working Modes of an SCA Normal Mode (Differential Mode) – The upper and lower levels can be adjusted independently. Window Mode – The lower level and the window width are set. Integral Mode – The upper level is removed resulting in a discriminator with an adjustable lower level. 1/18/2022 23

14. 11 Analog-to-Digital Converters (ADC) Converts an analog signal to an equivalent digital form. – Fundamental link between analog and digital electronics. – The resolution of an ADC depends on its range of digitization. Two types of ADCs: – Peak sensing: the maximum voltage is digitized. – Charge sensing: the total integrated charge is digitized. 1/18/2022 24

Methods of Conversion Wilkinson method: – The input signal charges a capacitor. – Capacitor is discharged at a constant rate. – A scalar counting the pulses from a clock or oscillator is gated on at the start of the discharge. – When capacitor is fully discharged, the scalar is gated off. – The contents of the scalar is proportional to the charge on the capacitor. Successive approximation method: – The input signal is compared to a series of reference voltages to determine the pulse height. This continues one bit at a time until the required number of bits is obtained. Other methods which will not be discussed in detail include hybrid, flash, tracking, parallel ripple, and variable threshold flash ADCs. 1/18/2022 25

14. 11. 1 ADC Linearity Integral nonlinearity: – The deviation from the ideal linear correspondence between pulse height and channel number. – Less than 0. 1% for most available ADCs. Differential nonlinearity: – Measures the nonconstancy in the width of each channel. – Distortion is not significant as long as the statistical uncertainty in each channel is greater than the differential nonlinearity. 1/18/2022 26

14. 12 Multichannel Analyzers Sort out incoming pulses according to pulse height, and keep count of the number at each height. Digitizes the amplitude of the input signal using an ADC. This number is used to increment a memory channel with an address proportional to the digitized value. The pulses are thus sorted by pulse height with a number at each pulse height stored in memory. The number of channels into which the voltage range is digitized is the conversion gain. 1/18/2022 27

14. 13 Digital-to-Analog Converters The opposite of the ADC. A digital signal is converted into an analog signal that can be used for an analog device. Binary weighted resistor technique: – A binary word is passed through a network of branches, one for each bit. – Each branch has a switch which is set to a reference voltage Vr if the bit is 1; ground if the bit is 0. – The branch contains a resistance proportional to the significance of the bit. – Branches are summed to give an output voltage: a is the state (0 or 1), n is the number of bits 1/18/2022 28

Alternative Conversion Method The previous method has two problems: – The accuracy and stability depend on the precision of the resistances. For large numbers of digits, this can become significant. – Susceptible to changes in temperature since resistors of different values tend to have significantly different temperature characteristics. R-2 R ladder network – A current divider that only requires resistances of R or 2 R. (Helps take care of the temperature problem) – The stability depends only on ratios of resistances, and not on absolute values. 1/18/2022 29

14. 14 Time to Amplitude Converters (TAC) Converts a time period between two logic pulses into an output pulse whose height is proportional to the duration. – The time measurement is triggered by a START pulse and halted by a STOP pulse. One method used is a constant discharge of a capacitor beginning at the START, and to end the discharge at the STOP. – The charge collected is then proportional to the time between the START and STOP. 1/18/2022 30