HP 3580A Operating And Service Manual page 25

Section III
GENERAL OPERATING INFORMATION
Model 3580A
OVERWAD indication. The AMPLITUDE REF LEVEL
switch is then set so that the low-level signals of interest can
be measured. Signals greater than the full-scale level
indicated by the white window on the INPUT SENSITIV-
ITY switch dial will peak the display but will not damage
the instrument or introduce harmonic or spurious re-
sponses.
3-59. Scale Factor. The blue markings on the AMPLI-
TUDE REF LEVEL switch dial indicate the scale factor
which, for absolute measurements is the factor by which
the INPUT SENSITIVITY (Max. Input) setting must be
multiplied to determine the full-scale sensitivity. For
example, if the INPUT SENSITIVITY switch is set to the
2 V range and the AMPLITUDE REF LEVEL switch is set
to the X0.01 position, the full-scale sensitivity is: 2 V X
0.01=0.02 Vor 20 mV.
3-60. For absolute measurements the full-scale sensitivity
is
conveniently indicated by the white window on the INPUT
SENSITIVITY switch dilll and the scale factor can generally
be ignored. If, for some reason, the scale factor is to be
used, note that the eveh numbered positions on the
AMPLITUDE REF LEVEL dial are not marked. This is
because the scale factor in these positions depends on the
INPUT SENSITIVITY switch setting. If the INPUT SENSI-
TIVITY switch is set to the 20 V, 2 V, 0.2 V, etc. position,
the unmarked positions on the AMPLITUDE REF LEVEL
switch dial represent X0.5, XO.OS, X0.005 and X0.0005. If
the INPUT SENSITIVITY switch is set to 10 V, 1V,0.1 V,
etc., the unmarked positions represertt X0.2, X0.02,
X0.002 and X0.0002. This applies only when the amplitude
VERNIER
is
in the CAL position.
3-61. For relative measurements where the amplitude
VERNIER is not in the CAL position, the full-scale
markings on the INPUT SENSITIVITY switch dial do not
apply and, for expanded-scale measurements, a scale factor
must be used. In relative measurements the scale factor is
the factor by which a relative amplitude reading must be
multiplied to obtain the correct reading in percent of full
scale.
3-62. When making relative measurements it is important
to remember that any time the VERNIER is not in the
CAL position, the relationship between the marked and
unmarked positions of . the AMPLITUDE REF LEVEL
switch varies as a function of both the INPUT SENSITIV·
ITY and amplitude VERNIER settings. There is always a
Xl, XO.I, X0.01, XO.OOI relationship between the marked
positions and this same relationship exists between the
unmarked positions. However, there is no longer a XI,
X0.5, XO.I or Xl, X0.2, X0.1 relationship between the
marked and unmarked ·positions. To obtain the correct
scale factor the following guidelines must be observed:
a. If the full-scale reference is set with the AMPLITUDE
REF LEVEL switch in a marked position, all measurements
must be made using marked positions.
3-10
b. If the full-scale reference is set with the AMPLI-
TUDE REF LEVEL switch in an unmarked position, all
measurements must be made using unmarked positions.
c. The AMPLITUDE REF LEVEL setting on which the
full-scale reference level is established becomes the XI
setting. If the XI setting is a marked position, the scale
factors for the remaining marked positions become XO.I,
XO.OI, etc. Similarly, if the XI setting is an unmarked
position the remaining unmarked positions become XO.I,
XO.OI, etc.
3-63. Examples. Consider the case where the fundamental
frequency component of an input signal is 0.75 V and it is
necessary to measure the second harmonic component
whose relative amplitude is I%. With the AMPLITUDE
REF LEVEL control initially set to the NORMAL (XI)
position arid the amplitude VERNIER fully counter-
clockwise, the INPUT SENSITIVITY switch can be set to
the 0.2 V position without overloading the instrument. The
amplitude VERNIER can then be adjusted so that the
amplitude of the fundamental frequency component is
100% of full scale. The 1 % second harmonic will perhaps be
visible on the display but an expanded scale will be required
to measure it accurately. In this case, the full-scale
reference was established with the AMPLITUDE REF
LEVEL switch in the XI position. Thus, the unmarked
positions cannot be used and the scale factors of the
marked positions are as indicated on the switch dial. By
setting the AMPLITUDE REF LEVEL control to the XO.O 1
position, the 1 % second harmonic can be expanded to
100% of full scale. It will then be necessary to multiply the
100% reading by the X0.01 scale factor to obtain the
correct reading: 100 x 0.01 = 1 %.
3-64. Next, consider the case where the amplitude of the
fundamental frequency component is 1.8 mV and it is
necessary to measure a harmonic component whose relative
amplitude is 4%. With the AMPLITUDE REF LEVEL
switch in the NORMAL (XI) position and the amplitude
VERNIER fully counterclockwise, the INPUT SENSITI-
VITY switch can be set to the 0.2 mV (lowest) range. With
a fundamental frequency component ofless than 0.2 mV, a
full-scale reference cannot be obtained on the 0.2 m V
range. It is, therefore, necessary to go to the 0.1 mV range
using the AMPLITUDE REF LEVEL switch. In this case,
the full-scale reference will be established with the AMPLI-
TUDE REF LEVEL switch in an unmarked position. This
unmarked position becomes the Xl position. To expand
the harmonic to a measurable level, it will be necessary to
rotate the AMPLITUDE REF LEVEL control clockwise to
the n.ext unmarked position. This unmarked position has a
scale factor of XO.I and will expand the 4% harmonic to
40% of full scale. The correct reading can then be obtained
by multiplying the 40% reading by the X0.1 scale factor:
40XO.I = 4%.
3-65. Alternative Method. An alternative method for de-
termining the relative amplitude of two signals is to first
measure the absolute voltage levels and then calculate their
relative amplitude using the following formula: