The maximum trapezoid output level at the Monitor Output is 8 volts. However, be-
cause of the extended range of the 9660’s digital filter, the digital-to-analog converter
which generates the monitor output can be over driven past its 12-bit range. When this
happens, the monitor output will be seen to “wrap around” or jump negative before it
continues displaying the trapezoidal waveform. In Figure 17, the 9660’s gain was in-
creased slightly to demonstrate the wrapping effect. As long as the MCA memory or
spectral range is not exceeded, the monitor output “wrapping” will not affect the MCA
spectrum, since the dynamic range of the digital filter is significantly larger than that
of the Monitor Output.
The range of the monitor output is also affected by detector noise, gain and selection
of shaping time. When the 9660 senses increased detector noise, it automatically in-
creases the signal baseline to maintain the signal and noise within the dynamic range
of the fast digitizing ADC. The DC gain of the digital filter is proportional to the pro-
cessing time and the baseline (DC) component and can become proportionately large
for long filter rise times. Since the monitor output circuit includes a baseline
subtractor, the range of the filter appears to shift in the negative direction when signifi-
cant noise is present, especially at long shaping times.
Figure 18 shows the type of waveform generated at the Monitor Output when the 9660
is connected to a detector driven by a low energy
55
Fe source. The Filter Rise Time
and Flat Top are set relatively long, 8.0
µ
s and 2.4
µ
s respectively.
Chapter 5 - Monitor Output
30
Model 9660-9660A ICB Digital Signal Processor
Figure 17 Monitor Output Showing "Wrap
Around"
Scope:
Vertical: 2 V/div
Horizontal 2.5
µ
s/div
