Teledyne LeCroy AP033 - Manual (Page 25)

Operator’s Manual
922260-00 Rev A 19
B. Incorrect Frequency Response
Possible causes are a defective probe or oscilloscope, poor connections, or poor grounding.
1. Verify that the BW limiting of the oscilloscope is off.
2. Connect the probe to another oscilloscope. If the probe now measures properly, the problem
may be in the oscilloscope.
3. If the probe behaves as if it is ac-coupled at high frequency, check for an open input
connection.
4. Poor frequency or transient response and AC gain errors may result when one of the two input
connections is open.
5. Excessive “ring” and other transient problems can result from excessive input lead length. To
test this, shorten the input leads to less than 1 cm. If the transient response changes
significantly, the lead parasitics are the cause.
C. DC Errors
Incorrect DC gain requires recalibration or factory repair. This can be determined by completing the
gain checks in the Performance Verification Procedure.
1. Extremely high source resistance will result in DC gain errors. Check the probe accuracy with
the oscilloscope calibrator signal.
2. Verify that the probe is not being overdriven into clipping for its current gain setting.
3. Excessive offset can result from large changes in ambient temperature. Remove the input
signal from the probe and repeat the Autobalance cycle. With the Offset set to zero, did the
trace return to the center of the graticule?
D. Poor Common Mode Rejection
Use the 1-kHz calibrator signal from the oscilloscope to check common mode rejection. With both
the + input and – input connected to the calibrator signal, a flat line at zero volts should be seen on
the graticule.
1. Check the probe with the plug-on attenuator installed and removed. If excessive common
mode signal appears only when the attenuator is present, the attenuator may need to be
rematched to the probe. Use the procedure listed in this section to match the attenuator.
2. If the common mode signal appears when the probe is connected to the test circuit, but not
when it is attached to the calibrator, the problem may be caused by large mismatches in the
source impedance. Try connecting both inputs to one of the input signals in the test circuit,
then the other. If the common mode signal disappears, try probing lower impedance points
within the circuit.