Section III
GENERAL
OPERATING
INFORMATION
Model3580A
Figure 3-17. Large Signal Hides Small Signal.
the only bandwidth on which the noise sidebands can
be
resolved.
On
the 1
Hz
bandwidth the noise sidebands are
more than 70
dB
below the peak
of
a
CW
response ± 10 Hz
away from the center frequency, f
0
(Figure 3-19). In some
isolated cases, the noise sidebands may slightly degrade the
resolution on the 1
Hz
bandwidth. For the most part,
however, noise sidebands can be ignored.
Table 3-3. Frequency Resolution.
AMPL
MAX.
DIFFERENCE
RESOLUTION
OdB
2 X
BW
10
dB
2 X
BW
20dB
5 X
BW
30dB
5 X
BW
40dB
5 X
BW
50dB
10 X
BW
60dB
10 X
BW
70dB
10
X
BW
BW
=
BANDWIDTH
setting
3-90. Low Frequency Limit.
To
utilize the
full
dynamic
range
of
the instrument at low frequencies, the lowest
frequency to
be
resolved must
be
at least 5 times the
selected
BANDWIDTH.
This low frequency limit
is
due
to
the zero response described in the following paragraphs.
Figure 3-18. Small Signal Resolved.
3-91. As the 3580A frequency
is
tuned toward 0 Hz, the
VTO
frequency approaches the 100 kHz IF. Although the
VTO
signal
is suppressed by the
use
of
a double balanced
mixer, part
of
the VTO signal feeds through the 100 kHz IF
Filter and appears on the display.
The
response produced
by the
VTO
signal peaks
at
0
Hz
and
is
appropriately called
the "zero response".
As
with any other
CW
signal, the zero
3-14
response on the display
is
an amplitude
vs.
frequency plot
of
the IF Filter (Figure 3-20). The wider the bandwidth,
the wider the zero response.
3-92.
The
amplitude and bandwidth
of
the zero response
determines the lowest frequency that can
be
resolved. On
any
BANDWIDTH
setting, the peak amplitude
of
the zero
response
is
more than 30
dB
below the full
scale
reference
set by the INPUT SENSITIVITY and amplitude VERNIER
controls (AMPLlTUDE REF LEVEL switch in
NORMAL
position).
With
the zero response more than 30
dB
below
full
scale
and a dynamic display range
of
80 dB, the
maximum difference between the peak
of
the zero response
and any measureable input
signal
is between 40
dB
and
50
dB.
Table
3-3
indicates that the maximum resolution
between two
signals
whose relative amplitude is between
40
dB
and 50
dB
is
5 times the
BANDWIDTH
setting.
Figure 3-19. Noise Sidebands
(1
Hz
BW)
3-93. Response Time. Generally, when making swept fre-
quency measurements, it is desirable to
have
good resolu-
tion and, at the same time, sweep
as
rapidly
as
possible.
This involves a definite trade off since the narrower
bandwidths provide the greatest resolution but require
slower
sweep
rates. As the bandwidth
is
narrowed, the IF
Filter takes longer to respond
to
electrical changes taking
place at its input. Consequently, the sweep rate must
be
slow
so
that the signal remains in the passband long enough
for the filter to fully respond. Optimum sweep rate
is
discussed in Paragraph 3-135.
3-94. For applications where narrow bandwidths and slow
sweep
rates
are
required, the 3580A Adaptive Sweep
feature
~an
often
be
used to substantially reduce the
measurement time. Adaptive Sweep
is
discussed in Para-
graph 3-147.
J;.95.
Noise Rejection. The maximum sensitivity
of
the
analyzer
is
limited by its own internally generated noise.
As
outlined in Paragraph
344,
internal noise
is
a function
of
bandwidth, input resistance and tuned frequency.
The
narrower bandwidths provide the greatest noise rejection.
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