Section 4: Theory of Operation
4.3 FPS Theory of Operation
0101-8292-0, Rev. C
4-11
CV-12SLX User Manual
Figure 4-10 Emission Current Ramp Circuit
Auto Bias Circuit
Figure 4-11 shows the Auto Bias circuit, which is designed to maintain the filament current at a
level just below the zero emission current point. To do so, the Auto Bias circuit gradually
increases the filament until emission current begins to flow. When this is detected, the emission
current is scaled back to a point just below emission threshold. The process begins with Gun ON
and HV ON asserted and with Auto Bias invoked. As noted above, switches A104A and A104B
form an SBDT switch that selects either the filament current request or the Auto Bias signal as
the input to the filament current loop compensation summing point. Selection by this SBDT
switch of the Auto Bias signal also removes the clear from counter U101 and clears the stop
from clock oscillator U203, allowing it to clock the counter. The counter outputs drive the 8-bit
D/A converter U202. The D/A output is Ref times the count divided by 256 volts. This count-up
process continues until Emission Current is above the threshold, as determined by comparator
U102C. The comparator output sets the latch U16B Auto Bias Set. The latch output stops clock
U203 and slightly reduces the gain of scaling amplifier U105B via analog switch A104C, setting
the autobias level just below the emission point.
Clock
Osc
HV
In
Reg
Emission
Request
HV
Is
ON
Gun
ON
In
Buffer
AND
8 Bit
D/A
AND
OR
INV
Buffer
8 Bit
Up
Counter
To emission current
loop compensation
summing junction
Remove Clear
Stop
Clock
Emission Request Signal
Enable
