Datasheet

÷R
Phase/
Frequency
Detector
Current
Source
Loop Filter
VCO
(~1.3545 GHz)
÷(N x 8)
÷3
÷ 4 or 5
(is125M)
÷2
PLL4_DIV
÷2
TOF4_ACLK
MUX
AFS_div
CLKout4
AFS_mode
Frame_in
TOF4
27MHz In
LMH1983
SNLS309G –APRIL 2010–REVISED JANUARY 2011 www.ti.com
Figure 8. PLL4 Block Diagram
CLOCK OUTPUT JITTER
Several of the circuits which require video clocks, such as the embedded Serializers and Deserializers found in
FPGAs are sensitive to jitter. In all real world applications, jitter has a random component, so it is best specified
in statistical terms. The SMPTE serial standards (SMPTE 259M, SMPTE 292M and SMPTE 424M) use a
frequency domain method of specifying jitter in which they refer to the peak-to-peak jitter of a signal after the jitter
has been band pass filtered. Jitter at frequencies below 10 Hz is ignored, and the jitter in a band from 10 Hz to
an intermediate frequency (1 kHz for the 270 Mbps standard, 100 kHz for the 1.5 Gbps and 3 Gbps standards) is
referred to as timing jitter, jitter from the intermediate frequency up to 1/10 of the serial rate is referred to as
alignment jitter. The limits that the SMPTE standards place are peak-to-peak limits, but especially at the higher
rates, random processes have a significant impact, and it is not possible to talk about peak-to-peak jitter without
a corresponding confidence level. The methodology used to specify the jitter on the LMH1983 was to decompose
the jitter into a deterministic component (t
DJ
) plus a random component (t
RJ
). This is the methodology used by the
jitter analysis tools supported on high bandwidth oscilloscopes and timing analysis tools from the major
instrumentation manufacturers.
To convert between RMS jitter and peak-to-peak jitter the Bit Error Rate (BER) must be specified. Without a
known BER, since jitter is a random event, the peak-to-peak jitter will be dependent upon the observation time,
and can be arbitrarily large. The equation which links peak to peak jitter to the RMS jitter is:
t
P-P
= t
DJ
+α*t
RJ
(1)
Where α is determined by the BER according to the equation:
1/2erfc(√2*α) = BER (2)
The erfc (error function) function can be found in several mathematics references, and is also a function in both
Excel and MATLAB. A fairly common BER used for these calculations is 10
-12
, which can be used to find a value
of 14 for α.
Another common method for evaluating the jitter of a clock output is to look at the phase noise, as a function of
frequency. Plots showing the phase noise for each of the four CLKout outputs can be found below.
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