Model
8754A
Using an External Source
USING AN EXTERNAL SOURCE
When
the test device
magnitude
or
phase response changes rapidly with frequency, measurement un-
certainty (predominantly seen as noise
on
the trace)
may
be caused by residual
FM
characteristics
of
the
source. Using a stabilized source in place
of
the 8754A built-in source can improve accuracy
and
resolution
when testing
narrowband
devices. These figures show transmission magnitude measurements for a 20
MHz
crystal
bandpass
filter using the
standard
8754Asource (A)
and
the
HP
Model 8640B Signal
Generator
(B).
All
performance
characteristics
of
the 8754A receiver remain the same
but
a crisp, easy
to
read trace
and
improved frequency resolution result
from
the reduced residual
FM
characteristics
of
the 8640B.
..
Frequency
MHz
(A)
8754A
Source
I'+j1+'
Frequency
MHz
1
........
..-----140
kHz
---~~~I
(B) 8640B Source
In
order
to
achieve
proper
receiver phase-lock and absolute power calibration when using an external
source, the
8754A
FREQUENCY
MHz
display should be adjusted
to
agree with the Source's START,
or
lower, frequency. Also, the SWEEP WIDTH MHz control should be adjusted
to
agree with the Source's
sweep width,
or
set
to
EXT RF for sweeps less
than
one and one-half octaves.
There are two types
of
external sources which can be used with
the
8754A. First, there are stabilized signal
generators with FM capabilities such
as
the
HP
8660
or
HP
8640 which achieve a swept display using
the
8754A
-5V
to
+5V sweep
output
as
their
FM
input. Second, there are sources such
as
the
HP
8662,
3325,
and 8601 which contain their own sweep capabilities and achieve a swept display on
the
8754A
by
using
their sweep
output
as
the
8754A
external sweep
input
(OV
to
+1
OV,
adjustable from +6V
to
+12V maxi-
mum
using
the
8754A
front panel Horizontal Gain control).
Using a source with FM capability (swept by
the
8754A
SWEEP OUT)
is
preferred, since retrace blanking
and phase lock control are internal and automatic. When using an externally swept source
(8754A
hori-
zontal axis driven externally),
then
only one channel can be displayed (no alternate, 2 channel display
is
allowed), and receiver relock points are
not
blanked.
33
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