HP 4262A - Page 236

A14 BOARD CIRCUIT DESCRIPTION.
AUTO PHASE ADJUSTMENT.
By
using
a
feedback
control
technique
in
the
Auto
Phase
Adjustment
period,
the
phase
of
the
Phase
Locked
Loop
circuit
is
automatically
adjusted
to
minimize
detection
phase
error.
This
paragraph
des-
cribes
how
the
phase
error
is
eliminated
during
APA
(Auto
Phase
Adjustment)
cycle
periods
1
and
2.
The
basics
of
the
auto
phase
adjustment
circuit
are
dia-
grammed
in
Figure
A.
In
the
APA
2
period,
the
same
signal
is
applied
to
both
the
phase
detector
and
the
phase
shifter.
The
four
phase
generator
outputs
a
90
degree
phase
shifted
pulse.
Assuming
that
the
detection
phase
is
accurate,
the
average
level
of
the
phase
detector
output
should
necessarily
be
zero.
If
any
phase
error
exists
between
the
€m
and
€ref
signal
channels,
the
phase
detector
outputs
an
E1
signal
which
is
the
integra-
tor
output
for
such
error
signal.
Because
SW3
is
open,
the
period
averaging
circuit
functions
as
an
ordinary
integrator.
APA
amplifier
(A14U5A
and
U6)
which
follow
develop
an
APAO
signal
(dc)
proportional
to
E1
and
supply
it
to
the
phase
shifter.
In
response
to
the
APAO
voltage,
the
phase
shifter
output
tends
to
lower
the
E1
level.
The
phase
error
of
detection
phase
is
thus
minimized
(refer
to
phase
shifter
circuit
description
on
service
sheet
13).
The
APA2
cycle
is
performed
by
LOOP
2
(SW1:
OFF,
SW2:
ON,
SW3:
OFF)
as
denoted
in
the
diagram.
After
the
APA2
period,
SW3
is
closed
and
SW2
is
tumed
off
to
memorize
the
dc
voltage
stored
in
capacitor
C.
The
memory
capacitor
C
maintains
the
dc
voltage
to
continuously
provide
an
effective
APAO
signal
during
the
measurement
cycle.
In
the
APA1
(Auto
Phase
Adjustment
cycle
1)
period
which
is
done
prior
to
APA2,
SW1
is
closed
and
the
current
flow
through
the
LOOP
1
charges
the
capacitors
in
the
period
averaging
circuit.
APA1
control
is
provided
to
accelerate
development
of
an
appropriate
APAO
signal
during
the
APA2
period
and
helps
to
reduce
the
time
of
the
auto
phase
adjustment
cycle.
[ sw2
5 APAL D Auto Phase Adjustment Cyclet
e ST M——
MA} APA 2 3 APA2 . Auto Phase Adjustment Cycle2
El -——J I
Figure A. Auto Phase Adjustment.
8-54