Trigger Path Failure; Large Amplifier Errors; Old Calibration Constants; Failures Reported Though Calibration Achieved - Tektronix 11A52 Service Manual

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After calibrating an amplifier, the oscilloscope characterizes each trigger channel of every amplifier for gain
and offset by placing the amplifier in "cal pass-through" mode, and performing a set of measurements at
various combinations of Veal and trigger level settings. Changes are made to the oscilloscope's trigger level to
correct the combined amplifier and oscilloscope trigger gain and offset errors. If the trigger path is
dysfunctional but the display path normal, the oscilloscope labels the amplifier calibration as a failure. The
amplifier has no knowledge of the trigger signal path calibration and therefore reports no failures in Extended
Diagnostics.
Large Amplifier Errors
Gain of the analog signal path is normally within + or - 2% even without calibration. Some parts of the
calibration routine depend on a reasonable gain and signal amplitude during calibration. If measurements made
by the oscilloscope are more than 95% of full-scale, at a time when the amplifier expects its output to be well-
behaved, the amplifier is suspicious of the oscilloscope measurement accuracy and calibration fails. Normally,
these out-of-bounds measurements result in out-of-bounds calconstants, and hence Extended Diagnostics failures,
but not always. For example, if an M377 for an 11A34 is put into an 11A32, calibration will fail because the
oscilloscope sees too large a signal. The amplifier will calibrate itself correctly, but transient response will be
poor.
Old Calibration Constants
The first part of the calibration routine balances the amplifiers. Immediately after the balance routine is
finished, the amplifier calculates if there is any combination of bandwidth limit filters, fine gain, coarse gain,
and oscilloscope imbalance for which the amplifier cannot balance using the balance DAC. This is the balance
diagnostic routine. At the time the calibration process runs the balance diagnostic, the characterization of the
signal paths' gains has not been done, and previous values for the gain terms are used in the analysis. If these
gain terms are erroneous, perhaps because the amplifier had last been calibrated in a faulty oscilloscope, the
first attempt at calibration may fail because the balance DAC requirements are calculated using the old
(faulty) gain characterization. The calibration will indicate a failure which is logged during the balance
diagnostic. After calibration, when Extended Diagnostics are run, balance calculations are redone based upon
corrected gain characterization. If they now pass Extended Diagnostics, no failure is indicated and the
amplifier is working normally. A second calibration would pass normally.
Failures Reported though Calibration Achieved
There are situations in which the oscilloscope will report that some Extended Diagnostics are out of range even
though calibration is achieved. This happens because not all diagnostics are individually necessary for
calibration to succeed. For example, Extended Diagnostics will separately list Step Gain, Atten Gain, and BWL
Match, each with its own limits in percent. It is the Gain test, however, that determines if the combination of
Step Gain, Atten Gain, and BWL Match will allow that amplifier channel to calibrate at all deflection
factors. The calibration algorithm does not require each component of the amplifier path to pass
independently if it calculates that the combination will always work.
11A52 Extended Service Manual 4-21