HP 8340B Manual page 178

When
a
bandcrossing or end of sweep is
reached, LBX (low
bandcross)
and HSP (high sweep) are
pulled low by the A57
marker/bandcross
assembly. This causes
VSWP
to pause (stop ramping). The
instrument processor pulls LRSP (low reset
sweep) low, resetting
VSWP
to zero volts.
To
eliminate unwanted magnetic hysteresis, the instrument processor
initiates
a retrace
kick via the
YO retrace
kick pulse
generator
in
the A54 YO pretune/delay compensation assembly. This pulse
is
sent to the
ASS YO
driver
where
it is
converted to
a
60 ma current
drain on the
YO drive current. This
temporarily offsets the YO frequency by approximately —2.5
GHz,
eliminating the
magnetic
hys¬
teresis
in the YO.
Finally, the instrument processor sets PRETUNE to
a
value proportional to the
next
lock frequency.
When the
loop is
locked, the
sweep sequence
is
repeated.
In
sweeps where the
YO
is
to
sweep
5
MHz
or less, the
20-30
loop
is
swept
and the
YO
loop remains
phase-locked.
Since the
YO
loop
is
phase-locked to the 20-30 loop output, the
YO frequency will
follow that of the 20-30
loop.
The 20-30 loop is swept by
a
voltage ramp, 20-30 SWP, generated by the
ASS
sweep generator
assembly. For YO sweeps <500
kHz,
the
sweep
width
range attenuator
on the
ASS
sweep generator
assembly is open-circuited so that
VSWP
remains at zero volts.
START/STOP Sweep Operation. Starting and stopping the
sweep ramp
is
controlled primarily by
high
sweep (HSP),
which is generated on A57 marker/bandcross assembly
(located
in the controller
section). When HSP
is
high, the ASS
sweep generator
is free to sweep. When
HSP
is
low, the ASS
sweep
generator stops, but does
not reset
sweep voltage, VSWP.
The
AS7 marker/bandcross assembly
pulls
HSP
low, stopping
the sweep whenever any one of the
following three events takes place.
•
LBX (low bandcross) goes
low.
The
AS7
marker/bandcross assembly
pulls LBX and HSP low at
bandcrossings and at the end of
the
sweep. The
ASS
sweep generator
pulls LBX
low
if
there
is an
instrument malfunction that allows
VSWP
to go beyond
its
normal limits.
•
LSSP (low stop sweep)
on the rear panel is
pulled low. Note: this line
is meant for
special,
dedicated applications and is not designed to be
a
general purpose stop sweep
input.
•
The
instrument microprocessor issues
a
STOP SWEEP.
The AS7 marker/bandcross assembly
drives
HSP
high, starting
a
sweep whenever any one of the
following
three events takes place.
•
A line
trigger input is
received after line trigger mode has been selected.
• An external trigger input
is received after external trigger mode has been selected.
•
The
instrument processor issues
GO SWEEP IMMEDIATE.
•
LSSP
on the rear panel
goes
high after the instrument processor has completed
its
tasks and
is
waiting for the input.
The
AS7 marker/bandcross assembly controls
the timing of the
various sweep events (marker on/off,
bandcross, and end of sweep) by monitoring
MKR RMP.
This
is a zero
to ten
volt
ramp generated
by
the ASS sweep generator assembly. A
buffered version of
this,
SWEEP OUTPUT,
is
available on the
front and rear panels.
MKR RMP
pauses for bandcrossings.
It starts at zero volts at the beginning of
each sweep.
It
reaches
five volts at the center frequency of the
sweep
and
goes
to ten volts at the end
of the sweep.
D-8
Sweep Generator
-
YO
Loop Theory
of Operation
HP 8340B/41B