HP 3580A Operating And Service Manual page 29

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.