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Section
lll
capacitarrce
on both
channels.
The effects
of
source
impedance
can
be
nrininlized.
horvever.
by thc
use
of
10
I{egohnr
l0:l
divider
probr.'r
(+p-
I000.1A) which
reduce
the efl'ective
shunl
eapacilance
lo
approxirnately
l0
pF.
-l-58.
Table -l-3
can
be used
to detcmlite
thc
rcactance
of
the 10pF
probe capacitalce
ovcr the cntire
lrequency
spectrurn.
A
gelleral
"rulc
of thuntb"
is
that
the
capacitive
rcactarcc
oI the I0
pF
probe should
be
at
least 100
tines
the
di.lJcrurc
bctweerl
the
two
source
irnpedances
at
thc
highest
operali
g
frcquency.
If
this rule is
applied.
the
n&ximuDl
or
"worst-case"
eruor
will
be
less
that
0.6
degrees.
If
10
pF
divider
probes are
used
and the
1wo
source
ittpcdarces
are
primarily
resistive,
the
phase error
can
be
approxirnated
using
the hrliorving fornrula:
Tun.
P=
lR'o
- Rsb
I
xc
=
phase
error in
dcgrees
=
sourcc resistance
of
channel
A
=
source resistanoc
ofchannel
B
=
reactance
of l0
pF divider
probe
at
the
op-
erati
g
Irequency
3-59.
Using
10:1 Divider
Probes.
3-(r0. 'l'he 3575A input chalnels
are designed
to
pernrit
the
use
o1 l0llegohm,
I0pF
Voltage Divider
Probes
(Jrp-
1000.1A). Ilrese probes can
be
used
to
extcnd the
maxi-
munl input
levels
to
l(X)
V
nlls
and the
overlll
operatjng
rargc
to
120
dB.
Voltage
divider
probes also reducc
the
effective
shulll
capacitancc
to
approxintately
l0pF-
al-Id
thereby
rninirDize
errors due
to
source
impedance
(Para-
graph 3-55).
3-61.
When
using
l0:l
divider probes, the
following
guideliltes should
be
observed:
Model
3575A
a.
Use
l0
l\{egohnr.
l0 pF
Divider
Probcs
(-hp-
1000-lA
or
equivirlent)
only.
Other
probcs can create
errors
partie-
ularly
u,hen
measuring
phase
at high Irequencies,
b.
Before using
l0:1
divider
probes, compensate
the
probes
ar
outlincd in
Paragraph J-rr2.
c.
Do not
use
a
l0:l
divider
probe on one channel and
a
dircct
input
to
the
other
channel.
d.
Use the lowest voltage
range
that
provides
the
required measuretrent
capability.
e.
Notc
that whcn 10:1
divider
probes are
used.
tlie
single
channel
arrplitude
readings
(Log A
and
l.og
B) rvill
bc offset
by
-
20dBV
e.g.,
I
Vrnrs
applied
to
probe
\r
l
measure
- 20
dBV rather than
0
dBV.
Relative
readings
(B/A)
will
not
be
olfset
if
identical
probes
are used.
3-62.
Probe
Compensation.
Before
using
10:l
divider
probes
it
is
necessary
to
adjust the
probes
for
optimunr
flatness
and identical pluse
characteristics.
Once
the
probss
are properly
adjusted
they
should
not
require
furrher
attention
unless
they are
interclutrged
or
inadvertentl\
misadjustcd.
It
is
good
practice, however,
to
perfornr
periodic
verificatiorl
checks
to
ensure
that
opthturr
adjusr.
lnent
is
maintained.
-l
6-1.
'[hc
following probe
adjustments
s]rould
be
per-
formed
with
a
I
V
nns
sine wave
applied to
both
probes:
RIQUIRED EQUIPMENT:
Test
Osrillator
(-hp-
651B)
50
Ohm
Feedthru Terminatiorl
thp-
I1048B)
a.
Connect
50
ohnr
output of
test
oscillator, ierminnted
in
50
olun load,
to
both 3575A input
probes.
Wrerc:
p
Rsa
Rrb
\
Table
3.3.
Reactance
of
10
pF
Probe.
Frequency
Beactance
Frequency
Reactance
Freq,-rency
Reactance
10
Hz
1591
M
20
Hz
1951
M
30
H2
530.5
M
40
Hz
397 9
lvl
50
Hz
318 3
l\4
60Hz
26s3M
10
Hz
221.4
M
80
Hz
198.9
M
90
Hz
1768M
100
Hz
159.1
M
2OO
Hz
19.51
M
300
Hz
53.05
l\,4
400
Hz
39.79
N4
500
Hz
31.83
M
600
Hz
26
531\,4
TOO
Hz
2214
M
800
H2
19
89
M
900
Hz
17
68
M
1
kHz
15.91
lvl
2kHz
7.96
M
3
kHz
5.30
lvl
4
kHz
3.98
M
5
kHz
3.18lvl
6
kHz
2.65
M
I
kHz
2.21
M
I kH,
1.99
M
9
kHz
1
.11
M
10
kHz
1.59
M
2A
kHz
796 K
30
kHz
530
K
40
k{z
398 K
50
kHz
318 K
60
kHz
265 K
70
kHz
22t
K
80
kHz
199
K
90
kHz
171 K
1O0
kHz
159
K
2OOkHz
79.6
K
3o0
kHz
53.0 K
4oo
kHz
39.8
K
5OO
kHz
31.8
K
600
kHz
26.5 K
7Oo
kH,
2?
t
K
800
kHz
19.9
K
900
kHz
11.1
K
'l
MHz
15.9 K
2
MHz
7.96 K
3
MHz
5.30 K
4
MHz
3.98
K
5
[4Hz
3.18 K
6ltrHz
2.65 K
1
MHz
2.21 K
I [4Hz
1.99 K
9
MHz
1.17
K
10
MHz
1.59 K
13
MHz
1.22 K
3.1
0
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