Probe Gain - Tektronix 11A52 Service Manual

Diagnostics
A failure in any of these diagnostics is a good indication that the amplifier will not be able to accurately
attain some or all of its possible settings. Failures in these diagnostics do not indicate specific parts of the
amplifier hardware, however, they can give clues to the troubleshooter about where amplifier signal path
problems may be. Failures in any calconstant routine may be caused by ACVS problems (M382, Sample
&
Hold
Farms), faulty output amplifier [M377] !Cs, interconnect problems (coaxial cables, the metal-on-elastomer
[MOE] attenuator connectors, Hypcon connectors), and improper component installation.
The following diagnostics are included in this section because their success or failure depends strictly upon
observations of calconstants generated by the last amplifier calibration cycle. The name of the diagnostic as
displayed by the oscilloscope diagnostic interface is shown in the following table, first in upper case letters,
then followed by an expanded description of the diagnostic.
•
PROBE GAIN-Probe Gain
•
A TIEN GAIN-Attenuator Gain
•
STEP GAIN-Step Gain
•
BWL MATCH-Bandwidth Gain Match
•
GAIN-Overall Gain
•
OUTPUT ERR-Output Error
•
BALANCE-Balance
•
COARSE DAC-Coarse Offset
• FINE DAC-Fine Offset
•
SP
ARE
GAIN-Spare Gain .
•
PRB COARSE-Probe Coarse Offset
•
PRB FINE-Probe Fine Offset
•
PRB IMBAL-Probe Imbalance
•
BIAS COMP-Bias Compensation
•
BIAS ERROR-Bias Error
The individual calconstant diagnostic routines are described here.
PROBE GAIN
This test checks the gain error of the currently installed probe. In order for the test to report results that
accurately reflect the probe gain error, a probe calibration cycle must have first been initiated on the probe
being tested. If a probe is installed and a probe calibration cycle has not been run on the probe, the test will
always give a very small error (the error occurs as a result of the input BNC-to-attenuator connection resistance
which is outside the calibration systems feedback loop). This is because the nominal gain of the probe (read
from the probe-coding resistor or from the EEPROM in a Level 2 probe) is used as the probe gain calconstant until
a probe calibration cycle is run. After a probe calibration cycle, the probe gain calconstant is replaced with the
value generated by the probe calibration. If the test fails, the probe's gain error is not within specifications.
The test reports its results as a percentage gain error, with an allowable probe gain error of
±2%.
Test Failure
If this test fails with no probe installed, it indicates that the probe code signal for the channel may be shorted
to ground. If
so, the amplifier will never recognize a probe on this channel because it is seeing a continuous
PROBE ID signal. As a result, the amplifier will never install a default nominal probe gain calconstant.
Because the default probe gain is never initialized when the amplifier receives a continuous Probe ID signal,
an indeterminate value will be sent to the diagnostic, with unproductive results.
4-12 l 1A52 Extended Service Manual
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