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DMI Compliance & Sink Characterization Using DTG5000 Series Data Timing Generator
Application Note
5
www.tektronix.com/signal_sources
The jitter tolerance testing is performed in the follow-
ing broad steps:
1.
Determining Worst-case Clock-Data skew:
The skew in data is varied until the worst point is
determined. This test is performed over several
iterations as illustrated in Figure 3. The signal
generator providing the TMDS is then set to
produce this worst-case level of skew.
2.
Measuring Jitter Margins: several measurements
involve injecting a specified amount of jitter to the
clock signal path. Three measurements are
performed over two test cases. Again, these
Data and Clock components are injected only into
the system clock path. The measurements and test
cases are as follows:
– Data Jitter amplitude (D
jw
)
– Data Jitter Frequency at 500 KHz and
Clock Jitter Frequency at 10 MHz
– Data Jitter Frequency at 1 MHz and Clock Jitter
Frequency at 7 MHz.
– Worst Data Jitter Amplitude
– Data Jitter Frequency at 500 KHz and Clock
Jitter Frequency at 10 MHz
–
Data Jitter Frequency at 1 MHz and Clock
Jitter Frequency at 7 MHz.
– Worst Clock Jitter Amplitude
– Data Jitter Frequency at 500 KHz and Clock
Jitter Fr
equency at 10 MHz
– Data Jitter Frequency at 1 MHz and Clock
Jitter Fr
equency at 7 MHz.
Figure 4 explains the measurement criteria for
D_JITTER and C_JITTER mar
gins. The tests need to
be performed at all pixel clock rates supported by the
device under test. Because of the many parameters to
be adjusted and the tight mar
gins, this test can be
rather complex and time-consuming.
Minimum Differential Sensitivity
The minimum differential sensitivity test is common to
many serial standar
ds. The test confirms that the Sink
meets interoperability requirements even when it
experiences attenuated differential voltage swings.
A TMDS signal generator with the ability to change
amplitude is the proper tool for this test. The source
is used to generate a Sink-suppor
ted 27 MHz video
format that repeats the RGB gray ramp signal from
0 to 255 during each video period. The test starts at
170 mV V
DIFF
on all pairs, then the differential signal
amplitude is r
educed in steps of 20 mV until the Sink
device reports an error. If the minimum V
DIFF
to which
the Sink responds without error is less than 150 mV,
the device passes the test. The test stops when
minimum V
DIFF
reaches 70 mV. Another important
element of this test is that it is performed at two
different V
ICM
(common-mode voltage) settings,
namely 3.0 V and 3.13 V. The DTG5000 Series offers
a specific termination voltage capability that allows
the generation of the TMDS signals at the appr
opriate
levels without the requirement for external adapters
such as Bias T
ees.
Figure 3.
Determining worst-case jitter tolerance.
Figure 4.
Measurement criteria for jitter margins.
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