High - Datron 1062 Service Handbook

29
3.3.4
High
Frequency Compensation
During the calibration cycle,
the
microprocessor
notes
and
stores
the high
frequency (HF) error of each
range.
When
AC
volts
is
selected
the
compensation inform-
ation for a particular
range
is
recalled
by the micropro-
cessor,
transferred across the
isolation barrier
and latched
on
to
M13,
M14
(Drawing No.
430328
sheet
5),
see Fig.
3.26.
As
in
the case of the Input Current
Compensation
(section 3.2.
2. 5),
the
output from the latches
is
applied to
a
digital-to-analog
converter,
AN2. The
voltage
produced
is
fed to the
AC
converter
via
connector
J1
pin 11
and
applied to varicap
Dll.
The
varicap
is
thus adjusted
to give
the amplifier chain
a flat
frequency response.
The
calibration
is
carried
out
at
one H.F. frequency
but
since
it
flattens
the
AC
amplifier response, the correc-
tion
is
valid
for
all
specified frequencies.
It
should be noted
that the calibration routine
is
iterative since
the varicap
is
non-linear.
FIG. 3.26
SIMPLIFIED
DIAGRAM OF AC HIGH
FREQUENCY COMPENSATION
3.3.5
Frequency Detection
(430402 sheet 2}
The
signal
frequency
is
monitored by
M10
which
is
set
so that
a signal
frequency greater than
5kHz
causes a
logic
'V, (0 volts)
on
M10
-
4.
This
signal
indicates to the
Digital
Board
via
M18,
M2
(Drawing No.
430328
sheet
5)
which one
of the
two
sets
of specifications
should be
used
for calculating the
measurement uncertainty
when
the
Spec key
is
depressed.
3.3.6
Test
During
the self-test
routine (actuated
from
the front
panel or
remotely
programmed) the
AC
assembly
is
checked for correct operation.
The
circuitry
is
placed into
the .IV range
as
described
in
Section
3.2. 1.3.
Filter
is
selected
and
F.E.T.
Q31
is
'closed'
from
M5
-
13
causing
a signal of
0.08
volts
DC
to be injected into
the preampli-
fier.
Thus
a signal
of
approximately 3.14 volts
is
output
from
the
RMS
section
and applied to the
A
-
D
converter
situated
on the
Analog assembly. This
signal
is
then meas-
ured
and
compared
with
a
stored value.
If
the
measured
signal
is
within
±6%
of the stored value, the
test
continues
with
a
IV
range check.
Range
.1
1
Output from
RMS
section
-1-3.14
volts
-tO.314 volts