Preamplifier - Datron 1062 Service Handbook

27
PREAMPLIFIER,
SCALING
AND FREQUENCY
COMPENSATION
RMS CONVERTER
To
A-D
FIG. 3.22
SIMPLIFIED
DIAGRAM
OF AC
CONVERTER
3.3
OPTION
10
AC ASSEMBLY
(Circuit
Drawing No. 430402)
(For
OPTION
12 see
page A27)
3.3.1 General Principles
The
preamplifier buffers
and
ranges the
signal in
order to present 0.9
volts
full
range to the
AC
to
DC
converter section.
Once
converted
to
an equivalent
DC
signal,
it is
applied to the analog
to digital
converter on the
main
analog assembly.
The conversion technique
is
electronic true
RMS
sensing
as
shown
in
the simplified block diagram
Fig.
3.22.
The
Datron
RMS
module
can be
best
considered
as a
functional block consisting of circuitry which accepts two
inputs,
V
and
Vf,
computes
V^/Vf and has an output
of
which
is
then
filtered so
that
all
the
AC
components
are
removed. The
output of the block
is
fed
back
to Vf, thus
closing the loop
around
the
whole
circuitry.
Mathematically: Vx
=
Vq
but Vx
=
V2/Vf
V2/Vf
=
Vo, but Vo =
Vf
V2
=
Vo2
3.3.2
Preamplifier
and
Scaling
(430402 sheet
1
Relay
RL2
is
energised
on
selection
of
AC,
directly
connecting the
Hi
terminal to the input of the
AC
assem-
bly.
If
DC
and
AC
are selected
together, the
AC
assembly
becomes
DC
coupled by energising
RL3,
causing
C57, the
AC
coupling
capacitor, to
be by
-passed.
The
signal
is
then fed to the
switched
gain inverting
preamplifier
whose
full
range
output
is
0.9 volts r.m.s.
A
simplified
diagram of
this
arrangement
is
shown
in
Fig.
3.23.
The
frequency response
is
held
flat,
to
within ±1%,
by controlling the gain defining
component
time
constants,
to
a similar
order of accuracy. Residual errors are
removed
by
the
frequency compensation
stage.
(See section
3.3.4).
The
preamplifier has
a
stable
DC
path provided by
a
dual transistor pair
Q33
and
a fast
AC
path by dual
F.E.T.'s
Q32 and Q34. Further
gain
is
provided by the
following
long-tail
pair
cascade
of
Q20, Q21,
Q22
and Q23,
which
is
loaded by
a
current
mirror,
Q24.
Q15
and
Q16
with
bias
components
Q17
and
Q18 form
a
conventional
class
AB
output
stage.
R121 compensates
for the bias
current of
Q33, whileR112
trims the offset voltage to
zero.
The
unity gain
frequency compensation
amplifier
consists of
a stable
DC
path, provide
by Mil, and
a fast
AC
path provided by
Q25
to
Q29.
The
bootstrap
circuit of
Q19
presents the varicap diode.
Dll,
with
a
high
imped-
ance, thus ensuring that the varicap
is
not shunted
to
ground.