DS90UR241_10 Datasheet, PDF (3/4 Pages) National Semiconductor (TI) – Spread Spectrum Tolerance Support Three key parameters, frequency

may be violated and potentially impact the ability to accurately
recover serialized data. Thus the quality of the SSC generat-
ed output is important in the selection of an SSC device. SSC
sources with triangle or Lexmark modulation profiles should
be used. It is recommended that the user select the minimum
spread spectrum f_dev and f_mod necessary to achieve EMC
compliance.
Appendix — Jitter Measurements
It is important to understand the high frequency jitter contri-
bution of a spread spectrum clock source. The high frequency
peak-to-peak jitter can be measured using a real time scope
combined with jitter analysis software. When analyzing the
peak-to-peak jitter, high pass and band pass filters are ap-
plied. This focuses the analysis on the specified high-fre-
quency jitter components, and ignores the intentional low
frequency modulation of the spread spectrum device. The
following section provides specific guidance to measure jitter
at the DS90UR241 (serializer) and DS90UR124 (deserializer)
inputs.
A number of platforms and tools are available for measure-
ment and analysis of jitter. This section describes the use of
Tektronix digital sampling scope and DPOJET jitter analysis
tool.
Serializer input jitter should be measured as close as possible
to the serializer's TCLK input pin. This is an LVCMOS input
signal, switching at a frequency between 10MHz and 43MHz.
A low capacitance probe with a bandwidth of 1GHz is suffi-
cient for measuring this input signal.
In DPOJET (Jitter and Eye Diagram Analysis Tools) CLICK
ON:
1. Select \ Jitter \ TJ@BER
2. Configure \ Edges \ (under Signal Type) select Clock
3. Configure \ Clock Recovery \ (under Method) select
Constant Clock – Mean and (under Auto Calc) select
Every Acq
4. Configure \ RjDj \ In the Data Signal Settings (under
Pattern Type) select Repeating, (under Pattern Length)
enter 2 for UI, and (under Jitter Target BER) enter 12 for
BER = 1E-
5. Configure \ Filters: (under Filter Spec) select 2nd
Order and (under High Pass (F1)) enter 2MHz for Freq
and (under Low Pass (F2)) select 2nd Order and enter
2.6MHz for Freq
6. To measure TJ >2.6MHz remove the Low Pass (F2)
select No Filter) and (under High Pass (F1)) enter
2.6MHz for Freq
7. Results \ Single (for Population = 1) or Run (for
Population > 1)
The deserializer input is a high speed differential serial
stream. As such, this should be measured across the dese-
rializer input termination resistor using a low capacitance,
high-bandwidth (>3GHz) differential probe. Zoom in on the
serial stream and look at the rising and falling edges. Make
sure the edges are monotonic in nature and have no reflec-
tions showing on the edges. If there is a reflection then the
jitter measurements will be larger than if there were no re-
flection.
1. In DPOJET (Jitter and Eye Diagram Analysis Tools)
CLICK ON:
2. Select \ Jitter \ TJ@BER
3. Configure \ Edges \ (under Signal Type) select Data
4. Configure \ Clock Recovery \ (under Method) select PLL
— Custom , (under PLL Model) select Type II, (under
Damping) enter 1.07 and (under Loop BW) enter 2MHz
5. Configure \ RjDj \ In the Data Signal Settings (under
Pattern Type) select Arbitrary, (under Window Length)
enter 5UI , (under Population) enter 28, and (under Jitter
Target BER) enter 12 for BER = 1E-
6. No filter(s) should be used with this measurement
7. Results \ Single (for Population = 1) or Run (for
Population > 1)
Equipment
1. Tektronix DSA71604 16GHz 50GS/s Digital Serial
Analyzer with DPOJET software
2. Tektronix P7330 3.5GHz Differential Probe
3. Tektronix P6247 1GHz Differential Probe
Measurement in “UI”
The jitter specification for the receiver input (RxINTOL) is giv-
en in terms of unit interval (“UI”). The jitter measurements
provided by the jitter analysis tools are commonly presented
in terms of time units (i.e. picoseconds). This measured time-
based value will need to be converted to UI terms for com-
parison with the 0.5UI specification. If the input clock rate
(TCLK) is known, it is simple to convert from a time-based
jitter measurement to jitter in terms of UI.
A unit interval (“UI”) is the time duration of 1 bit in the serialized
differential data stream. For every TCLK input clock period,
28 serialized bits are transferred over the differential link.
Thus, UI is defined as:UI = TCLK period (ps) / 28 bits
The conversion from measured jitter (time-based) to “UI” is:
jitter (ps) / UI (ps) = jitter (UI)
Here is an example for a system with input clock (TCLK) of
33MHz, and a measured jitter of 200ps.
• TCLK input clock period = 30303ps
• UI = 30303ps / 28 = 1082ps
• jitter (UI) = 200ps / 1082ps = 0.18UI
Revision History
November 10, 2010 : Updated to include jitter requirements
over frequency and provide detailed measurement instruc-
tions.
December 2, 2010 Updated to explain conversion to UI (unit
interval).
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