ORTEC 420 - Product Description

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1-1
corresponding to 1/2, but displaced along the time axis by the one clipping time,'".
Thus, it is evident that the time 1/2 corresponds to the point at which the pulses attain
half-height. Since in linear amplifiers the rise time is constant and therefore inde
pendent of pulse height, the time at which the pulses reach half-height is also con
stant and independent of pulse height. The discriminator in the Model 420 is designed to
trigger on the leading edge of the pulse, at a selected voltage, and is forced to reset
when the pulse passes through zero volts. The discriminator output pulse is then differ
entiated, and the reset edge is used to furnish an output pulse that is relatively jitter-free
for timing. Time resolution in the order of 5 to 10 nanoseconds independent of discrimi
nator level is typical for this circuit.
1. DESCRIPTION
The Model 420 Timing Single Channel Analyzer, as indicated by its title, performs two
functions. First, it is a single channel analyzer, with,both the lower level and the window
width variable over the pulse height range. For input signals which are doubly differ
entiated, it furnishes an output which occurs at the time these signals go through zero.
This time, as explained in the preceding introduction, is a precisely known time and there
fore the output time from the Model 420 is a precise time. The unit will accept single or
double delay line differentiated signals or single or double RC differentiated signals with
pulse widths from 200 nanoseconds to 20 microseconds. When the unit is fed a unipolar
pulse, the crossover time is not fixed in time and therefore the input selector switch should
be set to the unipolar position. This will cause the discriminator to reset at the same pulse
height at which it was triggered, and therefore the time of output pulse will depend upon
the fall time of the signal input. The unit is provided with a variable delay, selectable by
a front panel control, of approximately 1000 nanoseconds to allow time balance in
coincidence circuitry and analyzer gating. The unit provides two outputs whose leading
edges contain the same timing information. One is a negative spike used for very fast
timing such as time to pulse height converters, etc., and the other is a positive 5-volt,
500-nanosecond-wide pulse for slower applications such as coincidence circuits and
analyzer gating. These output pulses are the two standard forms of logic pulses in the
ORTEC 400 Series modular instruments, and are compatible with all related input signal
requirements.