Datasheet

TOF1
H
IN
F
IN
LMH1983
SNLS309G –APRIL 2010–REVISED JANUARY 2011 www.ti.com
Figure 15. CLKout4 Jitter Histogram Figure 16. CLKout4 Phase Noise
Horizontal scale: 7 ps / div
OUTPUT DRIVER ADJUSTMENTS
The LVDS output drivers can be adjusted via the I
2
C interface to change the differential output voltage swing, the
common mode voltage and to apply pre-emphasis to the LVDS output
Register 0x3A MSB (bit 7) turns on the pre-emphasis which may be used to extend the reach between the
LMH1983 and the load. It is recommended that the trace length be kept short, as longer traces have more
opportunity to couple with hostile signals and degrade jitter performance.
The differential output swing of CLKout pins is adjusted through bits [6:4] of register 0x3A a larger value loaded
into this portion of the register will increase the output swing.
The common mode output voltage is also able to be adjusted via register 0x3A, using register bits [3:0]
TOF1 ALIGNMENT
Each of the four clock outputs has a corresponding Top Of Frame (TOF) output signal. The LMH1983 is
programmed with a video format for each of the three video clocks, and the TOF signal will provide a digital
indication of when the start of a new frame occurs for that particular format. As an example, if PLL1 is
programmed with a video format corresponding to NTSC, CLK1 will be 27 MHz, and TOF1 will output a pulse
once per frame, or once every 900,900 clock cycles. The default state is for the LMH1983 to detect the input
reference format, and to program this format as the output format for CLK1, so if the input reference is an NTSC
reference, then TOF1 will default to a 29.97 Hz signal.
If the H
IN
, V
IN
, and F
IN
inputs to the LMH1983 are coming from the LMH1981 Sync Separator, then the rising
edge of the F
IN
input will come in the middle of a line (between H
IN
pulses. The TOF pulse, if aligned, will be a
pulse with a width of 1 H period, with transitions aligned with the leading edges of the H
IN
pulses, and when set
for a 0 offset, will be high during the H period in which the F
IN
input transitions, as can be seed in the “TOF1
Timing” scope shot.
Figure 17. TOF1 Timing
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