5.15.3 Discriminators and Single-Channel-Analyzers
The analogue signal from an amplifier is converted to a logic pulse by these devices.
A discriminator sends an output pulse every time its input crosses a certain threshold, while a “single-channel analyzer” (“SCA”) pulses when the input lies between a
lower and an upper level (Fig. 5.15). A typical (NIM-standard) “slow logic pulse”
output is nominally +5 V amplitude and 500-ns width. For applications where high
speed timing is important, there are also instruments that deliver “fast logic pulses”
that are negative with rise times typically on the order of a few nanoseconds.
5.15.4 Multi-Channel Analyzers
A multi-channel analyzer or “MCA” is an SCA on steroids. Instead of operating
with multiple discriminators, an MCA uses a fast ADC to digitize the amplitude of
incoming pulses. The MCA then creates a histogram for the number of pulses in
different amplitude ranges.
5.15.5 Constant Fraction Discriminators
One downside of discriminators and SCAs is that the time at which they trigger will
depend on the size of the input pulse. For applications where the time is more
important than the amplitude, spectroscopists employ constant fraction discriminators or “CFDs”. As illustrated in Fig. 5.15, for pulses that meet a certain threshold,
these devices trigger at the same time regardless of the pulse amplitude.
5.15.6 Time-to-Amplitude and Time-to-Digital Converters
In some synchrotron spectroscopy applications, such as nuclear forward scattering
and perturbed angular correlation, the arrival time of an X-ray is the important
quantity to measure. A “time-to-amplitude converter” (“TAC”) measures the time
interval between input start and stop pulses, and it then provides an analogue output
signal with a voltage proportional to that time. This output can then be processed by
an MCA to yield a histogram of events vs. time. A time-to-digital converter or
“TDC” provides a digital output corresponding to the measured time interval,
eliminating the need for an MCA.
5.15 Detector Electronics
127
The analogue signal from an amplifier is converted to a logic pulse by these devices.
A discriminator sends an output pulse every time its input crosses a certain threshold, while a “single-channel analyzer” (“SCA”) pulses when the input lies between a
lower and an upper level (Fig. 5.15). A typical (NIM-standard) “slow logic pulse”
output is nominally +5 V amplitude and 500-ns width. For applications where high
speed timing is important, there are also instruments that deliver “fast logic pulses”
that are negative with rise times typically on the order of a few nanoseconds.
5.15.4 Multi-Channel Analyzers
A multi-channel analyzer or “MCA” is an SCA on steroids. Instead of operating
with multiple discriminators, an MCA uses a fast ADC to digitize the amplitude of
incoming pulses. The MCA then creates a histogram for the number of pulses in
different amplitude ranges.
5.15.5 Constant Fraction Discriminators
One downside of discriminators and SCAs is that the time at which they trigger will
depend on the size of the input pulse. For applications where the time is more
important than the amplitude, spectroscopists employ constant fraction discriminators or “CFDs”. As illustrated in Fig. 5.15, for pulses that meet a certain threshold,
these devices trigger at the same time regardless of the pulse amplitude.
5.15.6 Time-to-Amplitude and Time-to-Digital Converters
In some synchrotron spectroscopy applications, such as nuclear forward scattering
and perturbed angular correlation, the arrival time of an X-ray is the important
quantity to measure. A “time-to-amplitude converter” (“TAC”) measures the time
interval between input start and stop pulses, and it then provides an analogue output
signal with a voltage proportional to that time. This output can then be processed by
an MCA to yield a histogram of events vs. time. A time-to-digital converter or
“TDC” provides a digital output corresponding to the measured time interval,
eliminating the need for an MCA.
5.15 Detector Electronics
127