HP 8340A Service Manual page 277

The Sweep Time OAC
(Block I) function together as a current source that produces a
current proportional to the selected sweep time. The higher the
current, the shorter the sweep time. The Sweep Time OAC
produces a voltage which is applied to the sweep Time Scaling
Resistors (Block I).
Resistor block is held at ground,
converted to a current which then goes through the Virtual Ground
Amplifier (Block
which corresponds to the selected sweep time,
Scaling Resistors
and the sweep Time OAC
the range.
Virtual Ground Amplifier
Figure 80-19 is a simplified schematic of the Ramp Generator
(Block K) The input to the Ramp Generator is a current source
•
formed by the sweep Time OAC
Resistors (Block I),
Since the non-inverting input of U8 is connected to ground,
op-amp wil� kee � its inverting input at ground also. That is,
will create a virtual ground at the virtual Ground test point
(VGNO, TP3). It does this by varying the voltage at the gate of
Q5. This varies the resistance of that FET and therefore varies
the voltage drop across it. The voltage across C30
Generator, Block
current flow is always into the source and out of the drain of
Q5.
CR2 and CR3 are low leakage diodes used for current steering.
Ramp Generator
The Marker ramp signal is generated by feeding a constant current
into C30. The voltage across the capacitor is buffered and offset
to produce the 0 to +10 volt Marker Ramp.
simplified schematic showing the ramp generating scheme. These
ramp generating circuits operate
or modes, depending upon the positions of S l and S2.
current source formed by the sweep Time Scaling Resistors
I) and the sweep Time OAC
of S l and S2 determine where the current will go.
The first mode is the sweep mode.
open, and the current flows into C30. The second mode is the
pause or hold mode.
on the Current Shunt and diverts the current through through CR3,
a current steering diode. The third mode is the reset mode.
S l is closed and S2 is open,
diverted by switch S l into the output of Ul4.
8-364
Model 8340A - Service
(Block G) and the Sweep Time Scaling Resistors
Since the output of the sweep Time Scaling
J) and then into C30.
(Block I) select the appropriate decade range,
(Block G)
J
(Block
and the Virtual Ground Amplifier
K) is always negative,
K
(Block
Here S l is open and S2 is closed. This turns
and the current through CR2 is
Scans by HB9HCA and HB9FSX
the voltage from the OAC is
In deriving the current,
the sweep Time
does the interpolation within
the sweep Time scaling
G),
and the conventional
Figure 80-19 is a
in one of three distinct states
is always on,
G)
In this case both Sl and S2 are
(Block
G)
(Block J).
the
it
(Ramp
Since the
(Block
the positions
Here