Amplitude Modes; Absolute/Relative Measurements - HP 3580A Operating And Service Manual

Section III
GENERAL OPERATING INFORMATION
Model 3580A
frequency and, in effect, is the Fourier spectrum of the
input signal. Some of the more common measurements that
can be made using the open-loop configuration include
harmonic distortion, intermodulation distortion, spurious,
square-wave symmetry and noise.
3-31. Closed Loop.
In the closed-loop configuration, the
3580A functions as a network analyzer for characterizing
two-port devices such as amplifiers, attenuators and filters.
For closed-loop measurements the network to be tested is
inserted between the rear panel TRACKING OSC OUT and
the front panel INPUT. the tracking oscillator supplies the
stimulus to the network and the 3580A measures the
response. As the frequency is swept over the band of
interest, the instrument responds to the amplitude varia-
tions introduced by the network. The resulting display is an
amplitude vs. frequency plot of the network.
3-32. Amplitude Modes.
3-33. The front panel AMPLITUDE MODE switch permits
selection of three amplitude modes: Linear (LIN), Log
10 dB and Log 1 dB. When the Linear mode is selected and
the amplitude VERNIER is in the CAL position, the
vertic.al axis of the display is calibrated in rms volts (average
responding). The bottom line of the display graticule
represents 0 volts while the top line represents the full scale
input voltage determined by the INPUT SENSITIVITY and
AMPLITUDE REF LEVEL control settings (Paragraph
3-53). When either of the Log modes is selected, the vertical
axis of the display is calibrated in dBV (1 V rms
=
0 dBV)
or dBm/600 ohms, depending on the position of the
dBv/LIN - dBm slide switch. In the Log 10 dB mode, the
vertical scale is 10 dB per division and the maximum
display range is greater than 80 dB (Paragraph 3-67). In the
Log 1 dB mode, the vertical scale is expanded to 1 dB per
division with a maximum display range of 10 dB. Any
SPECTRUM ANALYZER
hp 3580A
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3580A-B-3541
10 dB portion of the 80 dB display range can be displayed
by changing the AMPLITUDE REF LEVEL setting (Para-
graph 3- 71 ).
3-34. Absolute/Relative Measurements.
3-35. Absolute Measurements. Absolute measurements re-
veal the actual amplitude of responses appearing on the
CRT display. The 3580A can be calibrated for absolute
measurements in rms volts, dBV (I V rms = 0 dBV) or
dBm/600 ohms. For absolute measurements with the
3580A, the front panel amplitude VERNIER control must
be set to the CAL (fully clockwise) position and the
instrument must be calibrated as outlined in Paragraph
3-199.
3-36. Relative Measurements. In signal analysis, relative
measurements are used for comparing the amplitudes of
two or more frequency components of a signal.
In
network
analysis, relative measurements are used to compare the
amplitude variations of a response curve at two or more
frequencies. Relative measurements do not require a cali-
brated scale. That is, using the amplitude VERNIER and
other amplitude controls, the gain of the analyzer can be
adjusted so that any input level within the range of 100 V
rms to 0.1 µV rms will produce full scale deflection on the
CRT display. This arbitrary full scale input level then serves
as a reference for measuring signals that are lower in
amplitude. In the Linear mode with the VERNIER not in
the CAL position, the vertical scale on the CRT is no longer
calibrated in volts per division. Thus, the unit of measure
becomes "percent of full scale" where the reference is
100% and one vertical division is 10%. In the Log modes
the vertical scale is always 10 dB per division or 1 dB per
division even though the full scale reference is arbitrary.
For relative measurements in the Log 10 dB mode, the top
line of the display graticule (full scale) represents 0 dB and
signals are measured
in
dB below the 0 dB reference level.:
SPECTRUM ANALYZER
hp 3580A
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Figure 3-6. Measurement Configurations.
3-6