Internal Cal. Signal; Cal Signal - HP 3580A Operating And Service Manual

Section III
GENERAL OPERATING INFORMATION
Model3580A
3-74. The Amplitude Accuracy specification is broken
down so that portions of the specification that do not
apply to a particular measurement can be eliminated. All
applicable portions of the specification must be added
together to obtain the overall accuracy specification.
It
should be noted that the overall accuracy specification
reflects the absolute worst-case error that could possibly
be
encountered. Typically, all parameters are well within their
specified tolerances and the probability of having a worst-
case condition is very slight.
As
more parameters are added
to the specification, the magnitude of the possible worst-
case error increases but the probability of having a
worst-case condition greatly decreases.
3-75. The Frequency Response, Amplitude Display and
Input Attenuator specifications must always be taken into
account when calculating the overall accuracy specification.
Excluding the Switching Between Bandwidths and Ampli-
tude Ref. Level specifications, the worst case error is
± 2.8 dB in the Log mode or ± 10% of reading in the
Linear mode.
3-76. The Switching Between Bandwidths specification can
be
disregarded as long as the Amplitude Calibration
Procedure is performed on the BANDWIDTH setting that is
used for measurements. If the BANDWIDTH setting is
changed, the Switching Between Bandwidths specification
must be added to the overall accuracy specification.
Similarly, the Amplitude Ref. Level specification can be
disregarded as long as the AMPLITUDE REF LEVEL
control is in the NORMAL position. If the AMPLITUDE
REF LEVEL setting is changed, the Amplitude Ref. Level
specification must also be added to the overall accuracy
specification.
3-77. Internal Cal. Signal.
3-78. With the INPUT SENSITIVITY switch set to the
CAL position, the high INPUT terminal on the front panel
is
disconnected and an internally generated calibration
signal is applied to the Input Amplifier. The calibration
signal is a highly accurate 15/85 duty cycle pulse train
which provides a 10 kHz fundamental frequency compo-
nent along with odd and even harmonic components spaced
at 10 kHz intervals (Figure 3-12). The magnitude of the
pulse is such that the fundamental frequency component
produces full scale deflection when the instrument is
properly calibrated. The amplitudes of the harmonic
components are not meaningful. The calibration signal can
be
used for amplitude calibration or to verify the frequency
accuracy of the instrument.
3-79. In the Amplitude Calibration Procedure (Paragraph
3-199), the front panel 10 kHz CAL potentiometer is
adjusted so that the 10 kHz fundamental frequency compo-
nen t of the cal. signal produces full scale deflection. this
calibrates all circuitry following the input attenuator to a
full scale accuracy of ± 1.5% at 10
kHz.
3-12
Figure 3-12. Cal Signal.
3-80. Bandwidth Setting.
3-81. Refer to Figure 3-13 for the following discussion.
The 3580A uses a hetrodyne technique where the 0 Hz to
50 kHz input signal is mixed with a 100 kHz to 150 kHz
signal from a Voltage-Tuned Local Oscillator (VTO). To
select a given frequency present at the input of the Mixer,
the VTO frequency is tuned so that the difference between
it and the frequency of interest is 100 kHz. The 100 kHz
intermediate frequency (IF) is fed through the IF Filter,
detected and applied to the vertical axis of the CRT
display. Signals outside the passband of the IF Filter are
rejected. The BANDWIDTH setting determines the band-
width of the IF Filter and thus, the selectivity of the
instrument.
---1IVERNIERI
INPUT
SENSITIVITY
Q
Q __ \
I ,
.
--
ilNPUTl
,....v_·--~
OVERLOAD
DETECTOR
I BANDWIDTH!
Q
I
I
I
IOOKHz
INPUT
CIRCUITS
IF FILTER .IF
JOOKHz to 150KHz
FROM VTO
(IOOKHz)
Figure 3-13. Frequency Tuning.
3-82. For operating purposes, the 3580A input channel
can be pictured as a bandpass filter than can be manually
tuned or swept over the 0 Hz to 50 kHz frequency range.
The instrument responds only to signals passing through the
filter and thereby sorts out the various frequency com-
ponents present at the input. The BANDWIDTH setting
determines the width of the filter skirts at the - 3 dB points
above and below the tuned frequency:
.Lower 3 dB Point
=
f
0
-
BW
2
Upper 3 dB Point
=
f
0
+
BW
2
Where:
f
0
=
Tuned Frequency (0 Hz to 50 kHz)
BW
=
BANDWIDTH Setting (1 Hz-300 Hz