LeCroy DDA 7 Zi series, Руководство оператора

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Operator's Manual
WP700Zi-OM-E-RevA
516
In this example, you can see that the application of signals with the same risetime, but different from that applied
by the deskew fixture produces a deskew error, unless compensated. In the above figure, the green trace is the
actual signal applied to the probes that has a risetime of twice that employed by the fixture. The red and blue
traces represent the trace acquired through the probes and channel due to this edge. Because of the different
risetime applied, there is a small error.
A possible solution to this would be to vary the risetime of the edge applied to the probe by the fixture, a very
difficult design. But even despite this difficulty, with a variable risetime solution, you would be required to know
ahead of time what the measured risetimes would be, or to measure the risetimes first.
The TF-DSQ fixture, in accordance with the philosophy of requiring only one calibration in the fixture, handles this
in a special manner. The user simply enters the measured risetime of the signals after the probe is connected to
the circuit. Since the scope software saves the edge acquired during the deskew calibration process, it applies
this saved edge to a variable filter using digital signal processing until the measured risetime is arrived at. At that
point, the software calculates the difference in the time of the 50% crossing and calculates an additional skew
correction to be applied. In this manner, the risetime is compensated for in the deskew calibration without the
requirement of recalibration.
The above figure shows the traces realigned as a result of the skew correction applied after the measured
risetime has been entered.
DC Calibration Theory
DC calibration involves the calculation of two constants to be applied to waveforms to correct for voltage
measurement inaccuracy. The two constants are the gain, applied multiplicatively, and the offset, applied
additively. It is important to distinguish the gain and offset correction from the channel gain determined by the
sensitivity control (volts/division selection) or the offset control. The sensitivity and offset controls change the
absolute gain and offset of the front end amplifier, but cannot correct for inaccuracy. This is true also with the
vertical gain and offset controls in zooms. The way to visualize this is to place a cursor at a point on a waveform
and read the voltage. Adjusting volts/div or offset, or adjusting the gain or offset of a zoom, will affect the size of
the waveform on the screen, but will not affect the voltage measured at the cursor position. The gain and offset
correction applied during DC calibration will affect the voltage measured according to the following formula:
where V is the voltage measured prior to calibration.
Probes are calibrated for each fixed gain setting of the scope, meaning they are calibrated at 10, 20, 50, 100, 200,
500 mV and 1V per division. A unique gain and offset calculation is made for each range.
The calibration of the probe is performed utilizing 5 DC levels. The DC levels are applied such that the voltages
ideally appear on the scope screen at -3, -1.5, 0, 1.5 and 3 vertical divisions. The best fit line is calculated, and
the appropriate gain and offset that would make the line fit the actual voltages applied is also calculated. The
gain and offset for each range is the gain and offset correction displayed in the gain and offset fields.
In all cases, the DC levels applied to the probe are measured by an ADC on the fixture placed near the probing
points. In this way, the absolute voltage at the probe tips are known precisely and any DC probe loading effects
are accounted for.