HP 4262A User Manual

Section VIII
Model 4262A
PRINCIPLES OF OPERATION
The following outlines 4262A measurement
principles
using
some
equations
to
aid
and
ac-
quaint
you
with
the
basic
concepts
of
the
unit.
To
simplify
explanation
in
general,
only
the
principles
for
C-D
(capacitance
and
dissipation
factor)
measurements
are
discussed
here.
The
measurement
principles
for
other
impedance
paramters
can
be
deduced
by
a
similar
course
of
reasoning.
In
Cp
-
D
measurements,
since
a
constant
test
voltage
is
applied
to
the
unknown,
the
DUT
generally
presents
a
high
impedance
to
the
test
signal.
The
following
equation
shows
the
rela-
tionship
beteen
voltage
E1
at
the
‘“‘H”
terminal
(voltage
across
the
DUT)
and
range
resistor
am-
plifier
output
voltage
E2
(voltage
across
range
resistor):
E2=(Gp+jwCp)Rr-E1
.........
eq.
8-1
where,
Gp
is
parallel
conductance
Cp
is
unknown
capacitance
Rr
is value
of
range
resistor
w
is
angular
frequency
of
test
signal
The
phase
detector
separately
extracts
the
real
and
the
imaginary
voltage
components
of
E2
(re-
presented
by
formulas
GpRrE1
and
jw
CpRrE1,
respectively).
Figure
A
is
a
vector
diagram
of
phase
detector
output
voltage.
During
the
charging
cycle
T1,
the
phase
detector
detects
the
90
degree
phase
component
of
the
E2
signal.
Thus,
the
integrator
output
voltage
becomes:
klwCpRrE1T1
..................
eq.
8-2
where,
k1
is
a
constant
value
determined
by
4262A
circuitry.
Following
the
E2
signal,
the
E1
signal
is
applied
to
the
phase
detector
and
the
discharge
cycle
begins.
The
phase
detector
detects
a
signal
whose
magnitude
is
E1/10
(
that
is,
the
E1
signal
is
attenuated
to
1/10
to
develop
the
appropriate
time
T2
for
discharging
the
integrator)
by
phase
detection
of
the
signal
in
phase
with
E1.
The
resulting
change
in
integrator
output
voltage
developed
by
the
E1/10
signal
is:
The
integrator
output
eventually
reaches
zero
volts
(as
a
result
of
the
charge
and
discharge
cycle).
Thus,
the
sum
of
the
voltages
given
in
equations
8-2
and
8-3
is
zero.
And,
kwCpRrE1T1
=
k1
11*];
T2........
eq.
8-4
Cp
is
derived
from
equation
8-4
as
follows:
T2
Cp=——""
...
eq.
8-5
P~
T0wReT1
4
(w
=
2nfm)
To
eliminate
w
from
equation
8-5,
the
4262A
establishes
a
constant
charging
time
T1
as
follows:
where
k2
is
a
constant
value
(for
each
test
signal
frequency).
Equation 8-5 then becomes:
T2
Cp=m
..................
This
is
how
the
measurement
frequency
is
can-
celled
out
of
the
equation
for
the
measured
capa-
citance
value.
The
discharge
period,
T2,
is
meas-
ured
by
counting
clock
fc
whose
frequency
is
constant
at
31.83kHz
(its
period
is
31.4usec
=
107
x
10-6
sec).
/2
jwCpRpE1 -E2
GpRrE1l
E2 3n/2
Figure A.
Thus,
if
n
is
the
number
of
counts
for
fc,
T2
can
be expressed as follows:
?
(Sheet 1 of 2)

HP 4262A Specifications

General

Brand	HP
Model	4262A
Category	Measuring Instruments
Language	English