GBS Elektronik MCA-527 - Manual (Page 34)

MCA527
3.2.8 Flat Top Time
Flattop is a parameter introduced with digital multi-channel analyzers. Basically this is
adjusted to the rise time of the preamplifier. Reason is that digitizing the amplitude can be
done with quite high accuracy, down to 0.01% error. But the input bandwidth (3MHz) is
rather high compared to the sample rate (10MHz), and so during the rise of the
preamplifier signal, the value may change by 20% within 100ns and a timing jitter in this
order may cause a significant error. Therefore values where the signal is rapidly changing
must not be used for evaluation. See Figure 9 as an example.
This is adjusted with the flattop setting. As the signal to the ADC is bandwidth limited,
there is a finite rise time of around 300ns even for infinitely fast rising signals, settling time
may be twice as long. Therefore the shortest useful flattop time may be 0.6µs, which may
be suitable for fast rise time, medium resolution detectors such as CZT. For HPGe
detectors something between 0.8µs and 1.2µs may be optimum. For a NaI with a rather
slow rise time something like 1.6µs may be better.
If flattop setting is too high, the spectrum will become more sensitive to low frequency
noise and resolution will degrade slowly. Below a certain setting, the spectrum will
degrade very rapidly. Default value for flattop is 1.2µs.
34
Figure 15: Resolution depending on flattop with a CZT detector. FWHM is here the
Gaussian, ASYM the exponential part of the peak form.
0 1 2 3 4 5
0,0
5,0
10,0
15,0
20,0
25,0
FWHM
ASYM
Flattop [µs]
F
W
H
M
@
66
2k
eV
[k
eV
]