BNC PB-5 User Manual
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edges of each channel and calculating the width of each channel individually. This
method is subject to error since it is difficult to accurately find the edges of each channel
particularly due to system noise. DNL may be more conveniently and quickly
determined by using the sliding pulser method, where a constant frequency pulse is
swept in amplitude at a constant rate. When the channel widths are identical, the pulses
will fall in each channel for an equal length of time and the number of counts
accumulated in each channel will be equal. The MCA display for zero DNL would then
be a horizontal straight line.
DNL measurements on a MCA are typically made as follows:
1)
Connect the pulse generator OUTPUT to the analyzer input.
2)
Set the pulse top to FLAT, rise time to 50 ns, pulse width to 1 µs, fall time to 0.5 µs,
polarity to positive (for most commercial MCAs), and the frequency to 50 kHz or
more (depending upon the amount of MCA dead time and the time required to test a
large number of channels).
3)
Select RAMP SETTINGS from the main menu and set the start/stop to cover the
required number of channels (a small or large number of channels can be effectively
tested in volts or keV).
4)
To lessen the possibility of drift set the ramp time to its minimum setting (30
seconds). To assure adequate statistical data set the number of ramp cycles
appropriately (999, when testing a large number of channels). An 8.3-hour test can
be conveniently run with a 0 to 10 volt ramp (see Section 1.5.3).
5)
Clear the memory in the MCA and place the analyzer in the acquire mode.
6)
Select ENTER TO START from the ramp menu to execute the ramp. The ramp is
executed by any one of three ways: Pushing the spinner knob, pushing the ENTER
key, or pushing any of the numbered keys. The ramp is stopped by repeating any of
these three choices. The changing amplitude value can be observed on the LCD
during the ramp.
7)
When sufficient counts have been accumulated for the statistical accuracy desired,
stop the ramp by any of the three methods above. Alternatively, let the program stop
the ramp.
The amount of noise in the system and whether it is statistical or non-statistical, will
affect the time required to smooth out irregularities in the DNL display. The maximum
error of the DNL measurement will be inversely proportional to the square root of the
number of counts accumulated in each channel plus the error in the sliding pulse train.
The accuracy of the sliding pulse train is better than ±150 µV (< 1 part in 64,000).
Therefore, the contribution to DNL from the pulser will be minimal when testing
analyzers up to 8K channels.
The DNL of the analyzer may be computed by: DNL = 100 {1
Nx/Nav } %
Where Nx = number of counts in channel x; And Nav = average number of counts in all
channels
Nx is generally taken as the worst case deviation from the average. Occasionally there
is a "dropped" channel or an odd-even effect. This type of analyzer defect is easily
observed with the PB-5 DNL test but this type of anomaly should not be used for the
calculation of DNL.