
dL
HP
3048A Phase Noise Floor
and
Region
of
Validity
of £(f)
50
r—■ r-n-i 1 r-i~ri 1 r-r-ri 1 r-r-ri 1 i-n-j 1 r-rn ,
■ ■
| i-n-
40 '
________
__ . . __
30
:
— — ■ —
20
'■
——
10
'. _
.01
.1 1 10 100 1K 10K 100K 1M 10M 40M
£(f) [dBc/Hz]
vs. f[Hz]
Description
Caution must
be
exercised when
£(f) is
calculated from
the
spectral den-
sity
of the
phase fluctuations, S^(f), because
of the
small angle criterion.
The
—10
dB/decade line
is
drawn
on the
plot
for an
instantaneous phase
deviation
of 0.2
radians integrated over
any one
decade
of
offset frequency.
At approximately
0.2
radians
the
power
in the
higher order sidebands
of
the phase modulation
is
still insignificant compared
to the
power
in the
first order sideband which ensures
the
calculation
of £(f) is
still valid.
Be-
low
the
line
the
plot
of £,({) is
correct; above
the
line
£(f) is
increasingly
invalid
and
S^
(f)
must
be
used
to
represent
the
phase noise
of the
signal.
(S^(f)
is
valid both above
and
below
the
line.) When using
the £(f)
graph
to compute
S^f), add 3 dB to the
offset Level.
S»(/)= 2 (£(/))
7-4 Quick Reference
_
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