Signal processing

The output signals from the photomultipliers are images of the intensity profile of the focal volume with the addition of noise in varying amounts: accurate signal processing techniques are needed in order to achieve a good resolution of flight time.

The signal is sent to an amplifier at fixed gain and then to an adjustable low-pass filter, which allows the elimination of high frequency noise and thus increases the signal to noise ratio, an additional amplifier sends the signal to a constant fraction discriminator (CFD) unit. This emits a pulse output when the signal exceeds a predetermined fraction of its maximum value, the output signal is in this way independent of the size of the input signal. If this amplitude exceeds a trigger level, which may occur when a particle is too large and therefore not indicative of the stream velocity, the CFD discards the measure.

The start impulse enables a precise current to charge a capacitor, whose voltage rises according to a characteristic ramp law, the stop impulse interrupts the charge, so a voltage signal is generated proportional to the time interval between the two pulses (time of flight).

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