HP 3575A Manual page 37

Section
IV
a
few
degrees.
This
is a
vast improvement over
conventional
phase meters
that give
totally
efioneous readings
in
the
presence
of
noise.
For further iniormation
concerning
the
effects
of
noise
on 35754
phase readings,
refcr
to
Para-
glaph 3-54.
4-57. Function Switching Circuits
(A8
Schematic No.
5).
4-58.
The
Function
Switching
Assernbly
(A8)
is
comprised
of
four
indiyidual
sectiolrs-
Eaoh
section
performs
a
particular switching or logic function in
respollse
to
the
front
panel
control
settings.
4-59.
Compensating
Circuits
(A8lCl, lC2,
O1
through
Q6).
The logarithmic
outputs frotn the
Synchronous Recti-
fiers
in
channels
A
and B
are
applied
to
operational
amplifiers
A8lC2
and
A8ICI,
respectively. Note
that
the
gain
of
these
operational
amplifiers
is
controlled
by
thermistors
R56 and R57 in the
feedback networks.
The
purpose
of
the
thermistors
is
to
change
the gain
of
the
operational amplifiers
to
cornpensate
for
dight gain valia-
tions
that
occur
in the
log
Arnpliliers due to temperature.
Under normal
conditions,
the gain
of ICI
and IC2
is
hetween
I
and
l.l.
4-60.
Transistors
Ql through
Q3
and Q4
tfuough
Q6
at the
outputs
of ICI
and IC2
are
controlled
by
the corresponding
voltage range switch
on
the
front panel.
When the channel
B range switch is set
to
the 2
mV to 20 V position,
Ql and
Q3 are
conducting and Q2
is
cut
off.
When
conducting,
Ql
supplies
a
I
mA
current which produces a
200
mVdc
drop
across
R12. This offsets
the
Log B anrplitude reading by
+ 20
dB
to
compensate
for the
20
dB
sigrBl attenuation
introduced by the lnput
Attenuator.
When the 0.2
mV
to
2V
range
is
selected,
Ql
and Q3
are
cut
off
and Q2
is
switched
on.
Since
the Input Attenuator
does
not
attenuate
the
signal
on the lower
range, an
offset
is
not required.
The
purpose
of
Q2 is
to
attenuate
the
signal
slightly to maintain
equal
toading on both
ranges. Transistors
Q4
through
Q6
at
the
output
of
IC2 operate
in
the
same
manner
to
corcct
the
lrg
A amplitude
reading.
4-61. Amditude
Summing
and
Function
Switching
Cir-
cuits {A8O7 through
014,
lc3).
The
outputs
of A8lCl
and
A8lC2 are appled dirertly
to
the
function
switching
Model 35754
circuits consisting
of
AtlQT through A8Q14.
This
circuitry
is
controlled by two iines,
Ap (A8
pnrs
8 and
Jland
Bp
(AE pin 9
and
K)
rvhich
are
connected
to
the
front
panel
AMPLITUDE
FUN-CTION
switch through the
lnterface
Substitution Board
(Al6A)
or
the
lnterface
Board
(Al6B
.
Optior
001. 00-l).
442.
Table
4-4
is a
truth
table
which indtates
the
condition
of
the
AF and BF control lines
and
the
resulting
states
of
t}le
yarious
switching
transistors
for
each
ol
the
three
amplitude functions. Whcn Log A or
Log
B is selected.
the
srmming amplifier
(lC3) is
operated
with
one
inpur
referenced to circuit ground tfuough
A8R47
and
junctiorl
FET Ql.l.
Since
R47
and feedback
resistor R46 are of
equal value,
the closedJoop
gain
of
the sumnring anrplifier
is
two (from the +input, pin
3). When l,og
A
is selected.
the signal
from
IC2
is
coupled through FET
Ql0 and
lU1
to
pir
3
of
IC3.
When l,og
B is selected, the signal
from
ICI
is
coupled
though
Q8 and R29
to pin
3
of
IC3. Note that
the
signa.l
at thejunction
of
R29 and
R3l
is
divided by two
due
to
the
series resistance
to gound
provided
by
R31.
Since
the gain
of
the srnrlling
arnplifier
is
1wo.
the
amplitude
of
the
output
signal
(lC3 pin 6)
is equal to
rhat
of the
Log A
or
Log
B
input from ICI or
IC2.
4-63.
When
l,r.lg
B/A
is selected, QlO and Ql4
are
cut
oIf
and Q8 and
Qll
are switched on. Thc
Log
A
signal
fronr
lC2
is then applied
to pin
2
ot
lC3 through Ql
I
and
R.17
and the
ll]g
B signal is applied
to pin
3
of
lC3 through
QE
and R29. This
makes
the output
of
lC3
equal
to
the
differerpe
betweer
the
Irg
A
and
Log B
inputs
e.8..
Log
B
minus
log
A = Log B/A.
zl8l.
Display Switching (A8O15
through 018).
The display
switching
circuit
consisting
of
A8Q15 through
Ql8
is
controlled by
a single
line, Cp (A8
pins
l0
and L) which
is
connected to the
front
panel DISPLAY switch through
the
lnterface Substitution Board, A16A.
When AMPLITUDE
djsplay
is
selected,
ctucuit ground is
applied
to
the
CF
control line
causing Q15 and Ql6 to conduct
and Ql7
and
Qt8 to cut
off.
The
signal
at
the output
of
summing
amplifier
IC3 is then coupled
through
Ql6
and
out A8 pin
U
to
the Output Filter
Assembly. When PHASE
display
is
selccted, +
5
Vdc
is
applied
to
Cp, Q15 and Ql6
are
cut
ofl
and Ql7 and Ql 8
are switched
on.
The dc
output
fron
the
Phase Detecto(.is then coupled
from A8 pin
V tluough
Ql8
and
out A8 pin U to the
Output
Filter.
Table
4"4.
Amplitude Function Switching.
Amplitude
AF BF A7 Oa
Og OIO
O11 A12
O13 O14
O19
Log
A
Log
B
Los B/A
111
110
ooo
01001'l 01
't0110000
00110011
Conrrol Lines:
1=+5Vdc
O-GND
1 =
conducting
4-10