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Generating of short pulses with high amplitudes by using of standard Current-Feedback-Amplifier

Technischer Bericht 2013 11 Seiten

Elektrotechnik

Leseprobe

Generating of short pulses with high amplitudes by using of standard Current-Feedback-Amplifier.

Franz Peter Zantis, Dipl.-Ing.(TU)

RWTH Aachen University, Physical Institut IIIa, Electronic Workshop

The traditional method to generate pulses in the range of some nanoseconds with high amplitudes, is the using of the avalanche-effect of transistors. However the problem with this method is the poor possibility for parameterizing.

That was the trigger to look for a different way: under using of modern electronic components, a pulse generator, which is parameterizable, was developed. The parameters can be set by using a Windows- or Linux-PC. The pulse width can be adjusted in the range of 3 80 ns; the pulse amplitude in the range of 18...84 V and the pulse repetition rate in the range of 1 Hz to 50 kHz.

1 Introduction

In physical experiments of particle physics, often short pulses with relatively high energy are necessary. In the current application, a pulser, which is to be developed, must feed an antenna to generate electromagnetic pulses. In other applications it is used to generate ultrashort light pulses with LEDs.

To generate this pulses, the avalanche effect of a transistor can be used. As described e.g. in1. But, with this solution, parameterization is not, or only possible with great effort.

A different way is the use of the ECL-technique (details see9 ). With this, parameterization is possible. However, the pulses which comes from an ECL-circuit have very small amplitudes. This means, that an amplifier is mandatory. This is an additional expenditure and an amplifier has maybe not a good influence to the signal shape. But that is not all: the ECLtechniques is also outmoded and the components are not easy to get.

The new upcoming CF-Amplifiers (CFA = Current Feedback Amplifier) with slew rates higher than 6V/ns was the trigger for the idea to generate the pulse inside of the output circuit. This makes the use of an amplification of a generated raw-pulse needless.

2 The Idea

The basic idea of this application is to use a Current Feedback Amplifier as comparator. This is unusual, because of the fact, that at minimum one of both inputs of an CFA is a current input. So it must be compared a voltage with a current - which is impossible on the first view.

The concept of the pulse-shaper can be seen in figure 1. A standard microcontroller

generates a trigger pulse with individual repetition rate. As described, the CFA works as comparator. The incoming pulse is connected to the positive (voltage-) input (+IN). At this time, the output of the VFA is at low level. So the positive potential at the input +IN of the CFA is higher than the equivalent positive potential at the current input -IN. As result the output of the CFA goes to high-level with its maximum slew-rate and T1 opens. The pulse appears at R7. It is feeded by C2.

The trigger pulse from the microcontroller with the amplitude U ˆ circuit R1 and C1.

t

Abbildung in dieser Leseprobe nicht enthalten

For small t (t < RC), the voltage at C is approximated a linear function (see2 )

Abbildung in dieser Leseprobe nicht enthalten

The "slew rate" of the RC-circuit is in this case:

Abbildung in dieser Leseprobe nicht enthalten

The VFA works also as a comparator. It compares the constant voltage of [Abbildung in dieser Leseprobe nicht enthalten] with the voltage at the condenser C1, which increases with a short delay from the pulse (according to Eq. 2.2). If the amplitude at C1 is higher than [Abbildung in dieser Leseprobe nicht enthalten], the output of the VFA goes to high and changes the voltage level at -IN of the CFA. Here, the slew-rate of the VFA must be considered. Of course, also the slope of the pulse, which comes from the microcontroller, has influence of the delay. So, there are three variables, which have influence to the delay, with which the CFA can be switched.

- the slope of the incoming trigger pulse from the microcontroller [Abbildung in dieser Leseprobe nicht enthalten]
- the "slew rate" of the RC-circuit [Abbildung in dieser Leseprobe nicht enthalten]
- the slew rate of the VFA [Abbildung in dieser Leseprobe nicht enthalten]

This makes the calculation complicate. Additional, the layout of the circuit board is not free of parasitic capacitive and inductive components. This has e.g. influence on the value of C1.

A very fast VFA needs a bigger capacity of C1. In the best case, if and the slew rate of the VFA can be unconsidered.

Abbildung in dieser Leseprobe nicht enthalten

On the other hand: with a sufficient slow VFA, the other two variables can be ignored and C1 can be dispensed. However, the question is, if in this case the delay, to toggle the CFA is may be too long to meet the needed pulse width.

If the output of the VFA changes to high, the output of the CFA changes to low level and T1 closes.

Apart from the discussed influence of the slew rates, the pulse width depends on the value of the voltage [Abbildung in dieser Leseprobe nicht enthalten]. The higher the value of [Abbildung in dieser Leseprobe nicht enthalten], the wider is the pulse at the output. The values of [Abbildung in dieser Leseprobe nicht enthalten] can be varied in a linear way, between

Abbildung in dieser Leseprobe nicht enthalten

Not easy to dimension are the resistors R2, R3 and R5. The resistors form an electrical charge balance for "-IN" as a counterbalance to the potential at "+IN".

Abbildung in dieser Leseprobe nicht enthalten

[...]

Details

Seiten
11
Jahr
2013
ISBN (eBook)
9783656553915
ISBN (Buch)
9783656583615
Dateigröße
857 KB
Sprache
Englisch
Katalognummer
v265723
Institution / Hochschule
Rheinisch-Westfälische Technische Hochschule Aachen – Physikalisches Institut IIIa
Note
Schlagworte
generating current-feedback-amplifier short puls generator

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Titel: Generating of short pulses with high amplitudes by using of standard Current-Feedback-Amplifier