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12 Chapter 2 Operating Procedures
Calibrating the generator
About self-calibration
The generator can calibrate itself. Once calibrated, the generator does not require periodic
calibration. There are no physical controls to adjust. All calibration is electronic.
Self-calibrating generators output very accurate analog video swings; more accurate then
can be measured with most oscilloscopes. By comparing the output of these products with
a chopped precision DC voltage source, swing accuracys better than ±5mv and
component matches better than ±3mv have been routinely measured.
Given that the calibration circuit has a swing measurement accuracy of ±5mv, calibration
of a nominally 700mV video signal should yield an actual video swing of between 695mV
to 705mV. In the worst case, this would represent an error of ±0.7%.
The generator output consists of a buffer amplifier (with near 0-ohm output Z) and a
75-ohm source resistor. The resistor is inserted between a buffer amplifier output and the
output terminal of the generator. The calibration circuit measures voltage at the output of
the buffer amplifier – not at the output terminal, therefore any errors in the internal source
or external load resistors will not be calibrated and will remain.
This could potentially add an additional 1% error to the mix. So a nominally 700mV video
signal should yield an actual video swing of 688mV to 712mV at the output terminal into a
1% 75-ohm load. In the worst case, this would represent an error of ±1.7%.
Our current swing spec is ±14mV (or ±2%).
Using the Tektronix VM700T, which has a measurement uncertainty of 0.5% (or ±3.5mV at
700mV), we would expect readings between 684.5mV to 715.5mV video, when the
generator output is within 1.7% of 0.7Vp-p (in the absolute worst case). Using a typical
oscilloscope, which might have a swing measurement accuracy of ±14mV. In this case, we
should expect readings between 674mV to 726mV under the same conditions.
How to calibrate generator
To calibrate the generator:
1. Connect the generator with VGM.
2. Connect a display to the generator, and display any static image.
3. In the VGM connections pane, select the generator, and then choose Tools > CMD
Terminal.
4. In the CMD box, enter SCAL.
The generator displays the image you selected in step 2 after the calibration process is
completed.