VCXO-Based Clock Jitter Attenuator and Translator
MDS 2069-02 G
15
Revision 050203
Integrated Circuit Systems, Inc. l 525 Race Street, San Jose, CA 95126 l tel (408) 295-9800 l
MK2069-02
Circuit Troubleshooting
1) IF TCLK or VCLK does not lock to ICLK
First check VCLK to ICLK. It is best to display and
trigger the scope with RCLK, especially if a non-integer
VCXO PLL multiplication ratio is used.
If VCLK is not locked to ICLK:
1.1) Ensure the proper ICLK input is selected.
1.2) Check PV, SV, FV Divider settings
1.3) Ensure ICLK is within lock range (within about 100
ppm of the nominal input frequency, limited by pull
range of the external crystal). If in doubt, tweak the
ICLK frequency up and down to see if VCLK locks.
1.4) Ensure ICLK jitter is not excessive. If ICLK jitter is
excessive device may not lock. Also see item 2.1
below.
1.5) Clean the PCB. The VCXO PLL loop filter is very
sensitive to board leakage, especially when the VCXO
PLL phase detector frequency is in the low kHz. If
organic solder flux is used (most common today) scrub
the PCB board with detergent and water and then blow
and bake dry. Inorganic solder flux (Rosen core)
requires solvent. See also section 3 below.
2) If There is Excessive Jitter on VCLK or
TCLK
2.1) The problem may be an unstable input reference
clock. An unstable ICLK will not appear to jitter when
ICLK is used as the oscilloscope trigger source. In this
condition, VCLK and TCLK may appear to be unstable
since the jitter from ICLK (the trigger source) has been
removed by the trigger circuit of the scope.
2.2) The instability may be caused by VCXO PLL loop
filter leakage. Refer to item 1.5 above.
2.3) VCLK and TCLK jitter can also be caused by poor
power supply decoupling. Ensure a bulk decoupling
capacitor is in place.
2.4) Ensure that the VCXO PLL loop bandwidth is
sufficiently low. It should be at least 1/20th of the phase
detector frequency.
2.5) Ensure that the VCXO PLL loop damping is
sufficient. If should be at least 0.7, preferably 1.0 or
higher.
2.6) Ensure that the 2nd pole in the VCXO PLL loop
filter is set sufficiently. In general, CP should be equal
to CS/20. If CP is too high, passband peaking will occur
and loop instability may occur. If CP is set too low,
excessive VCXO modulation by the charge correction
pulses may occur.
3) If There is Excessive Input to Output Skew
3.1) TCLK should track VCLK. The rising edge of TCLK
should be within a few nanoseconds of VCLK.
3.1) VCLK should track RCLK. The rising edge of
VCLK should be within 5-10 nsec of RCLK (VCLK
leads).
3.3) The biggest cause of input to output skew is VCXO
PLL loop filter leakage. Skew is best observed by
comparing ICLK to RCLK. When no leakage is present
the rising edge of RCLK should lag the rising edge of
ICLK by about 10
sec. Loop filter leakage can greatly
increase this lag time or cause the loop to not lock.
Refer to item 1.5, above.
3.4) Another way to view the loop filter leakage is to
observe LDR pin. Use RCLK as the scope trigger. LDR
will produce a negative pulse equal in length to the
charge pump pulse.
3.5) Filter leakage can also be caused by the use of
improper loop capacitors. Refer to the section titled
‘Loop Filter Capacitor Type’ on
page 8.