HP 3575A Manual page 13

Model 3575A
SECTI
PERATING
!
3.1. INTBOD
UCTIO N.
l-2.
This
section contains completc operating instructions
for the
Nlodel .1575A
Gain"Phase
Meter.
Included
is
a
description
of
controls. general
operating information,
a
basic operatins procedure
ard
information
concerning
Optior) 001
thrrrueh ()Oi instrunreDts.
3.3.
CONTRO LS
ANO INDICATORS.
-3-,1.
Figurcs
-i-
I lnd -l'l
illuslrate and describe the
Iunction
of all
front
and rear panei controls,
connectors
and
indicators. The
dey:ription ()f
eacir item is keyed
to
the
drawing
within
the figure.
3.5.
GENEBAL OPERATING INFORMATION.
3-6. lnput
Connections.
3-7.
The
input
si8nals can be applied
to
the 3575A through
twisted
pairs,
shielded cables
equipped
with
BNC
connec-
tors (-hp-
l050iA ('ablc
Asscmbly ),
or 10:l
divider probes
(hp-
1000.1A
Voltagc Divider Probe).
Test leads should
be
kept
as short
as possiblc to n)ininize
extraneous pickup
and/or
loss due
to
cable capacitancc. When
using
l0:l
divider prr.)bes.
it
is first
.ecesMrv
to
compeuate
the
probes
as
outlined in
Paragraph 3-62.
3'8.
G
rou
nding and
Termination.
3"9.
ln
the
design
of
the 3575A,
extra
care has been
talen
to control
internal
gound
currents that could
degrade
the
accuracy
of
anplitude and
phase readilgs. Due to
it's
wide
dynamic
range
and high
sensitivity, however, the
3575A
can be affocted
by
external
ground currents
or
"ground
loops" that are
caused
by poor
grounding
or
incorrect
tennination. This
nrcans
that
when
usirg the.l575A,
particularly
at
low
levels and
low
frequencies. the operator
must be extremely
"ground
conscious
' if
accurate.
repeat-
able readings are to
be
obtained.
3-10.
There are basically
two
types
of ground
loops that
can
cause
measurernent errors in the 3575A.
The
flrst
type,
commonly knorvn
as
a powcr-line ground loop,
is encoun-
tered
at
frequencies trelow
the power,line frequency
or
at
Section UI
0N
ilt
NSTRUCTIO
NS
integral multiples
of
the
poweriine
frequency
where either
or
both input
signals
are below
50mV.
Il1e power-line
ground loop
is caused by extraneous
curents
that
ci0ulate
between
the
signal
source
and the 3575A
by
way of
power-line ground. The other type
of
ground
loop
is
introduced
by the
signal source
and is generally
encoun'
tered
at
frequencies
below
I
MIJZ
where the
signal
3pptied
to
one channel
is
greater
than I
V
nns and the
signal
applied
to
the other channel
is less
than
l0
mV rms.
These
two types
of
ground loops
are
illustrated in
Figures 3--l and
-l-4 and arc discussed
in the
following
paragraphs.
311.
Power'Line Ground
Loop.
Figure
3-3A
shows
the
hput
arrangement
for
a
simple grounded measurement. Ein
represents the
source being measured along
with
any
noise
associated
with
it
and
is generally called the "nornal"mode
source".
Rs
represents
the
source resistance
and
the
resistance
of
the high leadl
Rg represents the resistance
of
the ground lead. Current
from
Ein (normal-mode
current)
flows tluough
Rs,
Z;
ald
Rg and the instrument
responds
to
the drop
across
21.
As
long
as
the grounds
on
both
sides
of
Rg are identical, extraneous currents cannot ciculate
between the
source
ground and the instrument ggound. If.
howevcr. the grounds are differcnt
due
to
voltage drops
in
the ground
lead
or
currents induced
into it.
a
new
source
is
developed and the measurement
appears
as
shown
in
Figure
3-38. The new
source,
Ecm (the dilference
between
grounds)
is called the "common-mode source"
be.cause
it
is
'comflron
to
both
the high and ground iines. Common'mode
current can flow through
Rg
or through
fu
and
21.
Since
Zl
is
usually much larger
than
Rs
and since
they
are
both
in
parallcl
with
Rg,
most
of
the voltage across Rg
will
appear
acrr.rss
Z1
causing
an error
in
the
amplitude
or
phase
reading.
3-12. To
mininTe
power-line
ground loops. the following
guidelines
rhould be observed:
a.
Keep input
leads
as
short
as
possible.
b.
Provide
good ground
connections
to
minimizc
the
resistance
of
Rg.
c.
Connect the sigrul
source
and the 3575A
to
the
same
power
bus.
d.
lf
a removable
ground strap
is
provided on the
signal
source,
float thc
source
to
brcak thc
comnlonrnode current
p3th.
0
-3,1