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only a few steps are missing, refer to the control line truth tables (Tables 1 and 2 in A4A9
Troubleshooting) to determine which function is not operating properly. 0 V in the table
indicates that a line is active.
R E F
- 2 0 . 0
dB-n
ATTEN 1
0
1 d B /
dB
- 1 0
dBm
REF
- 2 0 .
_EVEL
) dBrn
U
A 8
dB
I
\ ,
I
X dB
N
2
dE
I
=
CENTER 2 0 . 0 0 0 0 0 0 MHz
RES BW 3 MHz
SPAN 0 Hz
SWP 2 0
s e c
VBW
1 MHr
N O T E :
C e n t e r
F r e q u e n c y
r e a d o u t
w i l l
b e
1 0 0 . 0 0 0
0 0 0
M H z f o r
an
H P 8 5 6 6 B
Figure 7. 1 dB Step Gain Test
Often several steps will appear to be slightly in error. Analysis of the truth table (A4A9
Tables 1 and 2) will indicate that no single stage could be faulty. This is caused by log fidelity
error in the log amplifier circuitry. If accurate determination of the step size is necessary, IF
substitution using a calibrated 1 dB step attenuator will be required.
Operation of the AVdB control line and the 0.1 to 1.9 dB section of the AO.O to A15dB
Attenuator on A4A9 IF Control can be tested with the following method. Press Instrument
Preset and key in the following:
(CENTER FREQUENCY)
equal to the CAL OUTPUT frequency
0 Hz
®
(SHIFT) fAUTcH (KSA)
[REFERENCE LEVEL]
20 s
. . 3 kHz
-8 dBm
Press the
(SINGLE)
sweep key and rotate the DATA control knob in a counterclockwise
direction. A stepped display similar to Figure 8 should result. If the steps are not monotonic,
the circuitry is faulty. The AVdB control line voltage for a -10 dBm Reference Level will vary
widely among instruments, although the ratio between the voltage for -11.9 dBm and -10
A4A5
7
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Table of Contents
Troubleshooting
