Fluke PM6690 Service Manual page 100

Move the 10 MHz sinewave signal back to input A. Change
the measurement function to Period Single A. Now the S/R
flip-flop should not be used, check the control signal at R623,
it should be -1.6 V (on is -1.0 V). Select statistics. The std de-
viation should be less than 100 ps.
Change the measurement function to Time Interval A - A. Se-
lect Statistics Mode. Check that the standard deviation is less
than 100 ps. Measure at pin 8 of the ADCs U23, U22, U21 and
U20. See Figure 6-100 for a typical timing diagram. Check
the upper (TP3) and lower (TP4) voltage limits of the ADCs.
They should be approximately 3.5 - 3.6 V and 1.0 - 1.4 V. The
important thing is that the lowest voltage pulse on any pin 8 of
the ADCs (U23, U22, U21, U20) should be at least 0.2 V
above the lower limit and that the highest voltage pulse on any
pin 8 of the ADCs should be at least 0.3 V below the upper
limit. If an interpolator has a voltage pulse outside the limits
the measurement result will be wrong. Figure 6-101 shows the
signals on an ADC.
The signal from the prescaler is connected to pin 22 (sin-
gle-ended) of U11. It comes via a level converter. Check the
input signal to the converter at R335 (PECL levels).
If the FPGA or a part in the interpolators has been changed or
repaired, a calibration of internals must be performed after-
wards. See Chapter 7.
X3
Figure 6-99
Important locations for the measuring logic.
6-56 Troubleshooting
R623 X4
R335 U11
Figure 6-100
U23
U20 U21 U22
ADC 10461 behavior. Time A-A Smart, 10 MHz
in, block measurement.
U20:8
U21:8
U22:8
U23:8