HP 4262A - Page 237

A14
BOARD
CIRCUIT
DESCRIPTION.
PERIOD AVERAGING CIRCUIT.
A
period
averaging
technique
was
adopted
to
get
pure
dc
voltage
at
high
speed
from
a
rectified
ac
signal
having
a
large
ripple
component.
Generally,
a
filtering
circuit
has
a
long
transient
response
time
in
converting
a low
frequency
burst
input
signal
to
a
pure
dc
voltage.
The
period
averaging
technique
enables
a
dc
output
voltage
to
be
produced
which
is
almost
equal,
(in
a
precise
fashion)
to
the
final
value
in
only
several
periods
of
the
input
ac
signal.
The
4262A
employs
the
period
averaging
circuit
for
smoothing
the
phase
detector
output
to
a
dc
and
for
combining
specified
measurement
accura-
cies
and
provides
an
improved
measurement
speed
at
the
120Hz
test
frequency.
Figure
B
shows
the
full-wave
rectified
current
input
signal
of
this
circuit.
During
the
first
T
(time)
period,
the
input
current
charges
the
integrator
capacitor
C1
(A14C8
in
the
actual
circuit.
In
a
120Hz
measurement,
Q18
conducts
to
add
C7
in
parallel
with
C8).
At
the
end
of
this
period,
the
integrator
output
El
is
pro-
portional
to
the
dc
current
of
the
input
signal
(since
T
is
equal
to
one
period
of
the
input
ac
signal).
After
the
first
T
period,
voltage
El
is
memorized
as
a
charge
on
C2
(A14C6)
when
switch
SW
(A14Q5)
is
momentarily
closed,
and
EO
(period
averaging
circuit
output)
becomes
a
step
function.
As
the
feedback
current
(Ir)
from
EO
to
the
integrator
input
is
designed
to
be
almost
equal
to
Ic
(input
current
to
the
period
averaging
circuit)
in
magnitude,
the
difference
between
Inc
and
IF
is
integrated
during
the
next
(T)
integrating
period
so
that
output
voltage
EO
becomes
exactly
proportional
to
Inc.
After
four
or
five
periods,
EO
will
be
a
pure
dc
signal
having
no
ac
component
and
be
precisely
proportional
to
(Inc)
the
input
current. ®
-— R2
Ci b
Rl
O——Ar—d
sw
o
Eo
—
toc €
l c2
INTEGRATOR
ac
saan/\/v\
Reclifiod’/\/\/\/\/\
Input
Signal
Figure
B.
Period
Averaging
Circuit.
8-55