ORTEC 473 - Operation; Theory of Operation; General

4. OPERATION
After the 473 has been installed and interconnected as described in Section 3, the only operating functions that are required
are the setting of the detector selector switch, adjustment of the Disc Level control, and selection of the desired operating
mode.
f Mi ially the detector selector switch can be set at the switch position that identifies the type of detector being used,
hovvever, the only differences between the three switch positions marked for detector types are the effective delay time that
will be used for constant fraction pickoff and the controlled dead time duration in the output circuit. If the switch is set at
Nal, the dead time for each output pulse will be 1 fis regardless of the mode by which the output was generated. The
controlled dead time is 50 ns for any of the other three switch settings. For reference, the selectable delay times are 14 ns for
Ge(Li), 1.3 ns for Scint, and 1.9 ns for Nal. Shaping that is provided by the preamplifier may affect the relation between the
optimum switch selection and the actual class of detector that is being used. If the switch is set at Ext, the delay time can be
any amount greater than the 3 ns built-in delay that is a part of the circuit leading to and from the rear panel connectors. One
additional function is selectable when the Ext setting is used, and this is the effective fraction; the basic setting of a jumper on
the printed circuit selects 30%, but the jumper can be moved to select 20% or 10% if desired. See information in Section 5,
Theory of Operation, for changing the jumper selection of the effective fraction.
The function of the Disc Level adjustment is to permit selection of signals with amplitudes of interest and to eliminate
response to signals with smaller amplitudes. The proper setting of this control depends on the range of signal amplitudes that
are furnished into the input. The control range is from 50 mV to 5 V, with the 10-turn potentiometer used for precise setting
and excellent repeatability.
For most applications, the mode selector will be set at C.F., for Constant Fraction operation. If the input signal rise times
tend to vary through a wide range, such as the signals that can be furnished from a large-volume coaxial germanium detector,
the switch can be turned to Slo R.T. Rej to operate as a Constant Fraction discriminator but to also reject input pulses with
too slow a rise time so that the timing spectrum has a better resolution.
The mode switch can be turned to L.E. to select conventional leading edge discriminator operation. If the detector selector
switch is set at Nal, the internal dead time is held at 1 (is so the maximum input count rate is limited to 1 MHz. But if the
detector selector switch is set in any other position, the dead time is 50 ns, permitting response to an input count rate up to
20 MHz. The other functions selected by the detector switch have no effect during L.E. operation because the constant
fraction portion of the 473 circuits is bypassed.
5. THEORY OF OPERATION
5.1. GENERAL
The circuits of die 473 are shown in block diagram 473-0101-81 and, in more detail, in schematic 473-0101-S1. Both of
t '.Gse drawip'"^ are included at the back of the manual.
The input is furnished simultaneously to three internal circuits. One circuit is a lower level leading edge discriminator that
defines the onset of a signal of interest and arms internal logic circuits in the module. A second circuit is an upper level
leading edge discriminator; each input signal must exceed this level in order that an output will be generated. The third circuit
is a constant fraction discriminator that provides a precise timing recognition for the input signal.
A front panel Disc level control effectively adjusts the response threshold for both the lower level and upper level
discriminators. The nominal range for this control is 50 mV to 5 V, associated with the upper level threshold range; the
threshold for the lower level discriminator is automatically adjusted at about 50% of the upper level so that it is triggered
earlier on the leading edge of the input pulse.
When an output is generated, it furnishes two independent NIM-standard fast negative signals and one NIM-standard slow
positive signal. The leading edges of the three output signals are coincident. A feedback in the output trigger circuit ensures
the generation of only one output pulse in each of the three output signal paths.
The Constant Fraction mode of operation can be used either with or without the Slow Risetime Reject feature. When Slo
R.T. Rej is selected with the front panel mode switch, the C.F. timing signal is not generated if the rise time of an input pulse
is too slow. The Leading Edge mode of operation does not use the slow risetime reject feature.