Datron 1062 - Page 37

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